UNIVERSITY OF CALIFORNIA • COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA HERBICIDAL USE OF CARBON DISULFIDE H. A. HANNESSON, R. N. RAYNOR, and A. S. CRAFTS BULLETIN 693 October, 1945 UNIVERSITY OF CALIFORNIA ■ BERKELEY, CALIFORNIA CONTENTS PAGE Introduction 3 Historical review 3 Toxicity studies 8 Movement of carbon disulfide vapor in soils 10 Effect of soil factors on carbon disulfide movement 11 Laboratory studies ■. 11 Temperature 12 Soil moisture 14 Soil type 15 Texture 15 Soil series 16 Compaction 16 Adsorption 20 Permeability studies on field cores 23 Preparation of cores 23 Determination of apparent density 23 Measurement of air flow 24 Calculation of resistance factor 24 Interpretation of air-flow data 24 Effect of soil moisture on resistance 26 Summarization of air-flow data 29 Air-flow and plot studies on several California soils 29 Discussion of results 44 Practical use of carbon disulfide 45 Season of application 45 Dosage in relation to soils and soil type 45 Dosage in relation to soil moisture 47 Dosage in relation to compaction 47 Effect of wind on carbon disulfide treatment 47 Response of different perennial weeds 48 Problems of reinf estation 48 Economics 49 Effects upon the soil 49 Brands of carbon disulfide 51 Methods of application 51 Precaution 53 Summary 54 Literature cited 56 HERBICIDAL USE OF CARBON DISULFIDE 28 H. A. HANNESSON, 4 R. N. RAYNOR, 5 and A. S. CRAFTS 8 INTRODUCTION Carbon disulfide is recognized as an effective herbicide for controlling deep- rooted perennial weeds. A clear, volatile liquid, it readily vaporizes to produce a gas highly toxic to plant tissues. When it is injected beneath the soil surface, its vapors diffuse through the air spaces, destroying all plant structures they may contact within the soil. The chief herbicidal value of carbon disulfide lies in its use against such weeds as wild morning-glory (Convolvulus arvensis L.), hoary cress, or white top (Lepidium and Hymenophysa spp.), Russian knapweed (Centaur ea repens L.), poverty weed (Iva spp.), and burweed (Franseria) . It has also- been successfully employed in the control of such shallow-rooted perennials as Bermuda grass (Cynodon Dactylon [L.] Pers.) and quackgrass (Agro- pyron repens [L.] Beauv.). It has occasionally been used against Johnson grass (Sorghum halepense [L.] Pers.), ragweed, willows, and many other less noxious species. Carbon disulfide, as a weed killer, has no deleterious effect upon the soil, a distinct advantage on intensively cropped land. Some is lost by diffusion; the remainder is eventually decomposed and oxidized to sulfate. Stimulated crop production often follows the treatments. This fact indicates that partial soil sterilization may result from the vapors. If straight carbon disulfide is applied, the soil may be safely seeded within 6 to 8 weeks. This chemical is classed, therefore, as a temporary soil sterilant, in contrast to arsenic com- pounds, which affect the soil for several years. Sodium chlorate, ammonium sulfamate (Ammate), ammonium thiocyanate, and boron compounds are also temporary sterilants ; but, in comparison with carbon disulfide, they have more lasting effects upon the soil. HISTORICAL REVIEW Carbon disulfide was used as an insecticide as early as 1854 by Garreau (Simmons and Ellington, 1926 ) 7 for fumigating stored grain. As a soil fumi- 1 Received for publication April 2, 1945. 2 As used in this publication, the term carbon disulfide refers to the technical product supplied by chemical manufacturers for use in fumigation and in weed and rodent control. In California this product is obtainable from Wheeler, Reynolds & Stauffer under the designation of "Anchor Brand." This company also manufactures "Activated Carbon Bi- sulfide," composed of the technical grade of carbon disulfide plus an amendment that lowers its vapor pressure. This material is sold exclusively for weed killing. The added amendment may have a fairly prolonged sterilizing effect upon the soil under certain soil and climatic conditions. Anchor Brand does not have this residual effect. (For further discussion of this subject see p. 51.) 3 The studies described herein were inaugurated and directed by the College of Agricul- ture, Division of Botany. The work was supported by funds contributed to the California Agricultural Experiment Station by Wheeler, Reynolds & Stauffer. ' Formerly Research Assistant in Botany; resigned August, 1941. 5 Formerly Assistant Botanist in the Experiment Station; resigned July 1, 1944. 6 Associate Professor of Botany and Associate Botanist in the Experiment Station. 7 See "Literature Cited" for complete data on citations, referred to in the text by name of author and date of publication. [3] 4 California Experiment Station Bulletin 693 gant it was recommended by Thenard in 1869 (Fleming and Baker, 1935). Controlled dosages applied in vineyards during the dormant season were found to kill phylloxera without undue injury to the vines. Many later investigations proved the value of carbon disulfide as a general soil fumigant for controlling soil-inhabiting insects, fungi, and nematodes. Incidental to these studies and to many commercial treatments have been observations of the effects of the toxic vapors upon the roots of higher plants. Marion (1878) and other workers on phylloxera found that injections must be kept a certain distance from the vine ; direct contact of the liquid with roots is usually fatal. Carbon disulfide emulsions, which are less toxic than straight carbon disulfide, are now commonly employed against insects, garden centi- pedes, and nematodes in planted areas. Though known to be toxic to plants, carbon disulfide was first used as a herbicide by Walker (1906), who poured the liquid around the stems of sas- safras and killed them in an apple orchard. Wilcox (1909) subsequently used the same method on sassafras in Maryland ; later, on guava, lantana, prickly pear, and other weeds in Hawaii. Control of wild morning-glory by carbon disulfide in November, 1913, was reported by Horticultural Commissioner Thomas Beers (1913) of Santa Barbara County, California. The following month J. C. Loomis (1913), writing in the Pacific Rural Press, reported the success of treatments on morning-glory and other weeds at Irvington, Cali- fornia. Beers advised using a prod to make holes for injection, whereas Loomis employed a short-handled hoe to dig a hole for application to each individual plant. Injections were closely spaced and shallow. In 1919 Thomas Mayhew initiated experiments in Monterey County. His method (Barnum, 1923) involved treating dry soil by making holes 3 feet apart each way and applying 4 ounces of the liquid in each hole to a depth of 18 inches. Later, Hickman (1922) recommended 2-ounce injections in holes 18 inches apart each way and 16 inches deep. In an experiment on Russian knapweed, reported by Johnson (1923), part of the infestation was treated with 4 ounces per injection in holes 18 inches apart each way and 18 inches deep ; and part was injected with 4 ounces per hole in holes spaced 30 inches apart in 18-inch rows and 12 inches deep. When both treatments proved suc- cessful, Johnson concluded that the lower dosage was sufficient and that soil conditions as well as the habits of the weed should be considered. Apparently, therefore, the method of application was not critical, provided a proper dosage was applied. By 1925 the procedure in California seemed fairly standardized, for John- son (1925) described the common method as application by hand in holes 18 inches apart and 12 to 18 inches deep at a rate of 2 ounces per hole. Three years later, the holes were being made 18 to 24 inches apart and 6 to 8 inches deep (Johnson, 1928). On packed roadside soils the distance between holes was reduced to 12 to 15 inches. These values for dosage, spacing, and depth have since persisted in California. Thousands of carloads of carbon disulfide have been applied in California and throughout the western states with gen- erally good results. The chief questions concern the occasional failures after the use of a uniform dosage in soils of widely varying properties and against weeds of differing character. Some of the results that brought up these ques- tions will be listed. Herbicidal Use of Carbon Disulfide 5 Frost (1932) , using a mechanical injector (a metering pump on a subsoiler ) , obtained 95 to 100 per cent control with dosages equivalent to 3 ounces spaced 20 by 27.4 inches and 4% ounces spaced 30 by 27.4 inches. An Idaho recom- mendation in 1926 (Ayres, Hulbert, and Ahlson, 1926) states (in contrast to Mayhew's opinion) that the soil should be moist. Excellent control was ob- tained on some areas treated during a summer rain and already saturated with moisture. Two-ounce injections were made in holes 18 inches deep and 24 inches apart. By 1934 the method in Idaho had changed to a 6- to 8-inch depth with 2 ounces per hole at 18- to 20-inch spacings. Spence and Hulbert (1935) pro- posed a 6- to 8-inch depth for white top (hoary cress) and quackgrass; 8 to 10 inches for other weeds. In 1937, Spence recommended a 6-inch depth for all weeds except morning-glory, which was to be treated 8 inches deep. In Utah (Peterson and Tingey, 1928), treatments in moist soil were best, with failures occurring in dry soils. Rogers (1928) in Colorado, however, ob- tained his best results in rather dry soil. Rogers and Hatfield (1929) used 2-ounce doses in holes 2 feet apart each way and 18 inches deep in heavy soils, 12 inches in sandy ones. Most of the early work with carbon disulfide was done in spring, summer, and early fall, failures being reported from winter treatments. In 1937, how- ever, treatments made through frozen soil in late 1936 were observed in Idaho. Complete kills were obtained. Recent reports from eastern Oregon indicate similar results. Furthermore, injections to only 4 inches have proved success- ful on shallow-rooted perennials in moist soil where the surface was well sealed by tamping or rolling. Bermuda grass has been killed by surface sprinkling, followed by covering with waterproof paper or wet canvas; also by covering quickly with 2 to 4 inches of soil. Since treatments have succeeded where dosage, depth, spacing, soil mois- ture, and season have varied so widely, evidently not one but many factors determine the effectiveness of a given treatment. Moreover, any blanket dosage recommendation yielding comparable results under all variations in these factors will necessarily be much above the minimum practical dosage. Money could be saved and failures averted if the relations of these various factors to successful treatment were understood. Carbon disulfide was soon brought into use against wild morning-glory, one of the most serious weed pests on western farms. In irrigated fields of beans, corn, melons, cotton, and similar summer row crops, however, many morning- glory plants survived the treatment by making new growth from the crown. After several years of empirical testing by farmers, weed-control super- visors, and sales agents, the general practice was to apply 2-ounce 8 doses in holes spaced 18 inches apart each way and 6 inches deep (see pp. 32 and 53). In well-moistened soils, however, a depth of 4 inches was preferred if the treatment could be followed by careful packing of the topsoil by tamping or rolling. This practice was successful in Idaho, especially in the Snake River Valley. During the 1930's many carloads of carbon disulfide were used in Idaho. 8 Ounce, as used here and in most of the references, refers to fluid ounce. One fluid ounce equals 29.57 ml. An avoirdupois ounce equals 22.5 ml of carbon disulfide. 6 California Experiment Station Bulletin 693 But, whereas the standard dosage and spacing just described gave satis- factory control under a wide range of moisture and temperature conditions in Idaho, they sometimes failed in California and in certain other regions out- side the Snake River Valley. Furthermore, this standard method required application of over 3,000 pounds per acre, at a cost prohibitive for extensive control on any but the most productive soils. When the present project was undertaken, three questions were uppermost : Why does the standard carbon disulfide treatment fail in certain isolated cases? How can this trouble be remedied? Are there any conditions under which this chemical will control perennial weeds at lesser dosage rates ? A survey of the irrigated farming region in the Snake River Valley revealed one very interesting point — namely, that this whole region was occupied by only one soil series, consisting mainly of loam or fine sandy loam. This soil, a loess, is made up of gritty particles fairly uniform in size, and has a low con- tent of clay. Carbon disulfide gives effective results in this soil at any time of year, even when injected in winter. The only requirement seems to be that the surface be firmly packed to seal in the vapors after application. Since sealing is most easily accomplished when the soil is moist, the usual practice has been to irrigate a few days in advance and to treat as soon as the soil is in the proper condition for seedbed preparation. The first plot experiments conducted at Davis were designed to test the effects of soil moisture on the results of carbon disulfide treatment, Plots having a heavy stand of wild morning-glory on Yolo loam were scalped 9 in midsummer of 1935, mulched, and treated with a hand applicator. One series was then sprinkled immediately with water to provide about 2 inches of soaked soil on the surface ; the remaining plots were tamped at the points of injection, but not otherwise treated. The subsoil in all the plots at the time of the appli- cation was near the permanent wilting percentage, and the topsoil was air-dry. Although the sprinkling brought up many seedlings 10 that had to be hoed repeatedly, control on the sprinkled plots was practically perfect the follow- ing spring, whereas that on the dry plots was not above 80 per cent. Evidently, although the sealing of the surface by sprinkling gave excellent results, recent irrigation as by flooding was not a prerequisite to successful treatment. Plot tests the next year (1936) gave fair control on dry plots, but failed on recently irrigated ones. The irrigated soil had been repeatedly disked for several years, but not deeply plowed. Its wet cultivator sole 11 was apparently almost completely impervious to carbon disulfide vapor. The following winter (1936-37), the plots were plowed to a depth of 12 inches, to break up the cultivator sole. Repetition of the plot tests the following summer gave different results. Applications were made in September, 1937; readings taken August 16, 1938, are visual estimates of resprouting expressed as a percentage of stand on untreated control plots. The results (table 1) show that satisfactory control may be had on wet soil provided there is no 9 The term scalped, as used in weed control, refers to a removal of top growth, as by hoe- ing or by the use of a power-drawn weed knife. 10 Morning-glory seedlings can be distinguished from young resprouts of old roots by the presence of typical cotyledons. Resprouts did not appear upon the sprinkled plots during the year of treatment. 11 See footnotes 14 and 15 for definitions of sole and pan formation in the soil. Herbicidal Use op Carbon Disulfide 7 compacted subsurface layer to prevent the vapor from diffusing, and pro- vided the surface layer is well sealed. The poor results on the dry plots of table 1 are attributed to a cloddy condition that made the topsoil practically impossible to seal against loss of vapor. A similar series of dry plots on a Madera soil that had been deep plowed and disked down to a dust mulch gave almost perfect control of wild morning- glory. Injections were made to a depth of 12 inches. A year after the applica- TABLE 1 Results of Carbon Disulfide Applications on Wild Morning-Glory Plots at Davis, California (Treated in September, 1937; read in August, 1938) Wet (recent y irrigated) Spacing and plot no. Dry (no irrigation) Spacing and plot no. Application rate per square rod* Hosprouts, percentage of controls Application rate per square rod* Resprouts, percentage of controls 16^2 inches: pounds per cent I6J/2 inches: pounds per cent 1 9 13H 1 2 13 14 9 13H SO 2 30 3 18 1 15 • 24% inches: 18 2 24% inches: 4 9 13H 2 1 16 17 9 13H 80 5 50 6 18 3 18 18 12 33 inches: 33 inches: 7 9 2 19 9 80 8 13^ 1 20 13M 75 9 18 5 21 18 60 40 inches: 40 inches: 10 9 5 22 9 80 11 13H 2 23 13H 60 12 18 2 24 18 25 * The standard dosage of 2 ounces per injection at 18-inch spacings is equivalent to approximately 2 gallons, or 20 pounds, per square rod. tions only 3 plants survived on the 12 plots, these being on the plots receiving the 9-pound application. These results indicate that (1) high soil moisture is not essential to success with carbon disulfide, (2) soil moisture at or below field capacity will not prevent successful treatment, (3) plow or cultivator sole definitely restricts diffusion of the vapor in the soil, (4) a wet sole may completely prevent diffusion, and (5) sealing of the surface against vapor loss is essential to success. Experiments similar to those reported in table 1, but applied to cold wet soil in midwinter, showed that the lower dosages and wider spacings were not satisfactory. Evidently dosages as low as one half the standard, however, could be used in warm, light-textured soils, free from impervious layers and properly sealed on the surface. Dosages up to or above standard were needed where these requirements could not be met ; and any treatment might fail if the surface sealing was not satisfactory. 8 California Experiment Station Bulletin 693 TOXICITY STUDIES The herbicidal properties of carbon disulfide were studied in an attempt to find general principles upon which to base interpretations of field work. Some of the questions arising in the field are as follows : Does carbon disulfide kill roots through the vapor phase of the soil, or is it dissolved in the soil moisture and absorbed through young roots ? Is the vapor absorbed in the upper root system and translocated downward ; or does it move down through the soil, Fig. 1. — Toxicity of carbon disulfide to oats. When the plants were 26 days old, liquid carbon disulfide was injected into the bottoms of the cans. From left to right, the concentrations were 0, 3,640, 2,860, 2,080, 1,560, 1,040, 780, 520, 390, 260, 120, and p. p.m. on the basis of the air-dry soil weight. Toxicity is highest in A, Yolo fine sandy loam ; intermediate in B, Arbuckle clay loam ; and low in C, Stockton adobe clay. (From Bobbins, Crafts, and Raynor, Weed Con- trol, by permission of McGraw-Hill Book Company, copyright owners.) killing as it goes ? How rapidly is the root killed, and is the vapor concentra- tion critical ? These and other problems were attacked by means of toxicity studies performed in the laboratory and greenhouse. Figure 1 depicts three series of pot cultures of oat plants that were treated by injecting carbon disulfide into the bottoms of the cans and sealing the soil by wetting on top. As shown in the figure, the tops of the plants receiving heavy dosages soon wilted and died, particularly in the sandy soil. Plants only slightly injured by the carbon disulfide vapor recovered ; only those whose roots were actually killed showed permanent injury. Wilting and death of the tops followed injury to the roots and were merely a secondary reaction to the poison. Carbon disulfide kills roots only by contact. Herbicidal Use of Carbon Disulfide 9 Cultures in the adobe soil had just as much carbon disulfide applied as did those in the loam and fine sandy loam. The toxicant was apparently adsorbed on the colloids and in that form was less available to the plants. />/'/? c/? c/amp Af#/7(?/r?efer Fig. 2. — Apparatus for treating wild morning-glory roots with carbon disulfide vapor. Each root, suspended through the cork, was exposed to a known concentration of vapor for a given period at a temperature of 20° C. In later experiments, roots of wild morning-glory were excavated and taken to the laboratory, where they were placed with their lower ends in carboys, their upper ends extending into the air (fig. 2) . Each plant was sealed in place in the carboy; the carboy was evacuated until it showed a predetermined subatmospheric pressure ; and then enough liquid carbon disulfide was intro- duced to bring the pressure back to atmospheric as it vaporized. The experi- ments were conducted at 20° C (approximately 68° F). At varying times 10 California Experiment Station Bulletin 693 the roots were removed, cut into 3-inch segments, and placed in moist sand in the greenhouse. Readings were made on survival and resprouting. In other experiments, time of exposure to carbon disulfide vapor and con- centration of the vapor were the variables. Table 2 summarizes the results. These data bring out the important fact that toxicity depends upon both time and concentration. The product of the two gives a constant percentage kill; a product of approximately 240 is necessary for complete killing under the experimental conditions. Another important fact established was that the carbon disulfide vapor acts strictly as a contact poison. Upper portions of the exposed roots that extended out through the corks into the air survived the treatments at all TABLE 2 Wild Morning-Glory Root Segments Killed by Exposure to Carbon Disulfide Vapor at Three Different Concentrations in Atmosphere Surrounding Eoots 1 per cent concentration 5 per cent concentration 10 per cent concentration Exposure time Kill Exposure time Kill Exposure time Kill hours 80 120 160 240 300 per cent 40 50 80 100 100 hours 16 24 32 48 60 per cent 5 75 50 100 100 hours 8 12 16 24 30 per cent 40 20 50 100 100 exposure times and all concentrations. There was no evidence that the toxic principle or any secondary products of the killing process were moved out of the region that was in direct contact with the toxic vapor. MOVEMENT OF CARBON DISULFIDE VAPOR IN SOILS When liquid carbon disulfide is injected into the soil, it immediately per- meates the air spaces in the soil adjacent to the point of injection. At the same time it starts to volatilize, producing a vapor about two and a half times as heavy as air. One might expect this heavy vapor to move downward more rapidly than laterally and upward. Apparently, however, convection is re- stricted because of soil resistance. For this reason, upward movement almost equals downward, and the pure vapor continues to expand until all the liquid has disappeared. Diffusion occurs only at the outer edges of the pure-vapor phase. When the liquid is all gone, diffusion becomes the chief mode of move- ment, and the gas spreads at practically the same rate in all directions. Because of the type of movement that occurs, much carbon disulfide vapor is lost unless the soil surface is well sealed. Especially is this true where the depth of injection is less than 12 inches. Fleming (1923), studying soil fumigation with carbon disulfide, described asymmetric movement of the vapor in his experiments on prepared soil in a fumigation box. Bywaters and Pollard (1937) and Higgins and Pollard (1937) found similar behavior; but they concluded that vapor movement is largely a simple diffusion process, convection occurring only in coarse-tex- tured or loosely packed soils. O'Kane (1922) reached a similar conclusion. Herbicidal Use of Carbon Disulfide 11 The fact that the vapor concentration is less above the level of injection than below it must be explained by the rapid surface losses rather than by gravi- tational or convection flow. Effect of Soil Factors on Carbon Disulfide Movement. — Soil permeability, involving soil compaction, texture, and moisture, has been recognized as one factor limiting carbon disulfide movement. Gastine, Couanon, and Gastine (1884) pointed out that the physical condition of the soil greatly affects the diffusion of the vapor. In very permeable soils diffusion is rapid, and loss may be excessive. In compact soils diffusion is slow, but the vapors may persist too long for early planting of a crop. The most favorable condition is to have a permeable subsoil, but impervious surface layers. Excess moisture renders clays impermeable, and dryness fosters excessive losses by diffusion from the surface. A rain during or immediately after treatment was found to seal the surface. Temperature effects also have received attention. According to Fleming (1923), Marion tried to correlate the distance of lateral diffusion with soil temperature, but the variations were so small that he could find no correla- tion. He did find, however, increased diffusion at high air temperatures. He pointed out the effects of barometric pressure and air temperature upon gas movements in the soil. Leach (1920) studied the effects of soil temperature under field conditions, but could discover no distinct correlation in spring, summer, and fall. Laboratory Studies. — The field observations and empirical experiments re- viewed in the preceding pages do not indicate the quantitative effects of tem- perature, moisture, compaction, and textural grade upon carbon disulfide vapor movement in soils. Any evaluation of the relative importance of these factors must rest on fundamental studies under controlled conditions. Such studies have been made by By waters and Pollard (1937) and Penman (1940) in England, and by Hagan (1941) and Hannesson (1945) at this Station. The conclusions reached by the English and American workers substantially agree. Only the results on California soils will be reviewed, since they are of immediate interest. Hagan's (1941) work consisted mainly of the design and use of apparatus for studying gaseous movement through prepared laboratory soils. He em- ployed a constant total pressure and a measured partial-pressure gradient of carbon disulfide vapor through the soil. The apparatus (figs. 3 and 4) con- sisted of a tube to hold the soil being studied ; a shallow dish sealed to the lower end, containing liquid carbon disulfide, from the surface of which the vapor could diffuse ; and a manifold fitted to the top, through which air could be slowly drawn to carry away carbon disulfide vapor as it diffused upward through the soil. The air was drawn through absorber columns, where the carbon disulfide was taken out in alcoholic potash solution. The whole apparatus was set up in an insulated room with accurate tem- perature control so that experiments could be run at any temperature from 5° to 45° C. Air-dry, pulverized soils were packed into tubes with a compact- ing machine to a known density and at a measured moisture content. Soils to be run at a moisture content near field capacity were packed air-dry, irri- gated, and allowed to stand till the moisture was uniformly distributed. Soils 12 California Experiment Station Bulletin 693 to be studied at intermediate moisture contents were packed by hand in small increments, each of which was wetted with sufficient water by means of an atomizer. Some intermediate moisture samples were packed dry, irrigated, Fig. 3. — General view of apparatus for determining flow of carbon disulfide vapor through prepared soil columns. This apparatus was housed in an insulated chamber with accurate temperature control. (From Hilgardia vol. 14, no. 2.) and then dried by forcing air through the columns until the desired moisture content was attained. Apparatus and methods are described in detail by Hagan (1941). Temperature. — From a mathematical analysis of his data on diffusion rates in prepared soil columns, Hagan (1941) concluded that the quantity of car- 14 California Experiment Station Bulletin 693 bon disulfide vapor which will flow through a given volume of soil per unit time is inversely proportional to the viscosity of the vapor, and directly pro- portional to the difference in carbon disulfide vapor pressure at the two ends of the column. Temperature affects this relation in two ways : First, as a gas is heated, the activity of its molecules is speeded; this decreases its viscosity and raises its vapor pressure. Second, in a closed system, increasing the tem- perature would simply increase the concentration of vapor ; but in a system £6 3-2 Vi 28 2.4 I \ 2.0 I /■Z 0-8 < / « » / i ,< / / > • < s / f/ &'' / $s y y y y > f c y y y /O /5 20 25 30 35 40 7~e/r?per<7ft/re ; rfeyrees Ce/?f/yr<7c/e 45 50 T?ig. 5. — Effect of temperature on the flow of carbon disulfide vapor through Yolo fine sandy loam (run AFZ) and Yolo clay (run AGA). open to a moving air stream at one end, like Hagan's apparatus, increasing the temperature raised the partial pressure of vapor at the bottom end, steepening the vapor-pressure gradient, besides decreasing the viscosity. Hence the rate of vapor flow increased along a rising curve, as is shown in figure 5, prepared from Hagan's data. From the standpoint of practical soil fumigation, a rise in soil temperature has two effects. First, the rate of both volatilization and diffusion of carbon disulfide in the soil is increased, tending to bring about a rapid and thorough distribution in the soil. Second, losses of carbon disulfide vapor from the soil surface become greater and, unless prevented by surface sealing, may attain serious proportions. Soil Moisture. — The water content of the soil also influences its permeabil- ity to gas movement. The addition of water makes the soil much less porous ; at a moisture content somewhat above field capacity, a soil may become almost impervious to gas. Figure 6, taken from Hagan's work, illustrates this point Herbicidal Use of Carbon Disulfide 15 for Yolo fine sandy loam. As this graph shows, permeability is directly re- lated to moisture, and zero permeability or complete restriction of gaseous diffusion through this soil is attained at a moisture content of about 19 per cent. These findings explain why a shallow layer of moist soil, such as results from rain or sprinkling, will restrict loss of vapor from the soil surface. They also explain the ineffectiveness of attempts to kill weeds with carbon disulfide in waterlogged soils. Soil Type. — Among the characteristics involved in the soil type are the fol- lowing : nature of parent rocks; processes of soil formation ; extent and nature 1 I /o g \ \ s \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ I i i \ i i fO /5 £0 25 Moist e/re co/?£e/?£ fP#), per cent 30 JS Fig. 6. — Influence of soil moisture content on the permeability of the soil for carbon disul- fide vapor in Yolo fine sandy loam, calculated to an apparent density of 1.258 grams per cubic centimeter. (From HilgardiavoL 14, no. 2.) of the soil profile; particle size distribution (textural grade); and organic- matter content. Intimately concerned with these and often controlled by them are moisture-retaining properties, colloid content, soil color, and soil structure. Closely related to all these characters are the properties of soil biology and, in turn, soil fertility, which are more difficult to define. Because of the complexity of soil types, any attempt at correlation between them and carbon disulfide toxicity is difficult. By reducing the number of variable factors to a minimum, however, and controlling temperature, com- paction, and moisture, it has been possible to extend somewhat the knowledge of carbon disulfide behavior in soils. In order to broaden the scope of appli- cation of the studies, soil types representative of important soil groups and agricultural areas were selected. Texture. — Using the methods of soil preparation described, samples of Yolo fine sandy loam, Yolo loam, and Yolo clay were prepared to an apparent density of approximately 1.20 grams per ml, their air-dry moisture contents being 3.1, 3.6, and 5.8 per cent, respectively. Table 3 gives the permeabilities of these samples, both when air-dry and when moistened to approximately field capacity. 16 California Experiment Station Bulletin 693 Similar determinations on two Salinas soil samples and two Hanford soils are also presented. According to these results on pulverized soils, texture affects permeability, the fine-textured soils having relatively low values. This situation may be ex- pected from the particle size distribution in such soils, the clays having chiefly small particles, the loams and sands coarser grains. The Yolo and Hanford series represent two important groups showing contrasting permeabilities. The former shows relatively high permeability in the dry condition and low in the moist. Hanford has some of the lowest values in the dry condition, and high permeability when moist. These results TABLE 3 Influence of Soil Texture upon Permeability of the Soil to Carbon Disulfide Vapor Soil series and texture Permeability units* Dry (air-dry) Yolo series: Fine sandy loam Loam Clay Salinas series: Fine sandy loam Clay Hanford series: Fine sandy loam Loam K X J0" 9.70 8.10 6.70 8.40 6.95 7.60 7.40 Wet (approximate field capacity) K X J0» 1.20 1.10 0.80 1.89 1.51 3.75 2.78 * Permeability units are defined by Hagan as "the gram-poises per cm 2 per second per millimeter of mercury difference in partial pressure per cm." in the dry soils may possibly be explained by differences in particle shape and size distribution. In the wet condition the character of the colloids is probably the predominating factor in determining permeability. Soil Series. — Hannesson (1945) has presented data on permeability of sur- face soils of 6 series, representing 3 different textural grades (table 4). These data show, again, that the coarse-textured soils have the higher per- meability and that permeability is much reduced by the addition of moisture. Among the various soils within a given texture, the differences are not great, but certain soils stand out. Among the clays the Stockton has a low per- meability, correlated possibly with the very high colloid content. Compaction. — The degree of compaction of a soil (that is, its density) de- termines the size and number of the minute passageways between the soil particles. When force is applied to a dry soil at right angles to the soil surface, the particles are pressed together with a minimum of disturbance of their spatial relations. Figure 7 illustrates how such compaction affects three Yolo soils. These curves show, as stated above, that textural grade affects permeability, Herbicidal Use of Carbon Disulfide 17 the coarser textures being the more permeable. They show, further, that per- meability decreases with increasing compaction and that, in these three textures within a single series, the three curves appear to diverge from a common point on the base line. Apparently, therefore, if these three soils could be compacted to an apparent density of about 2.0 grams per ml, they would all be impervious to carbon disulfide vapor. TABLE 4 Permeability of Soils of Six Soil Series and Three Textures to Carbon Disulfide Vapor at Two Moisture Contents Dry soil Moist soil Soil texture and series Moisture content Permeability units* Moisture content Permeability units* per cent K X W 12 per cent K X W n Fine sandy loam : Yolo ( 3.10 \ 3.00 8.22 8.23 19.9 18.8 0.23 0.70 Salinas 3.10 1.70 8.42 7.58 16.9 17.0 1.89 Hanf or d 3.75 Loam: Egbert 7.70 7.90 32.2 2.25 ( 3.45 7.94 22.0 1.60 Yolo I 3.90 I 4.20 8.19 7.73 22.6 22.4 0.64 0.74 Fresno ( 3.20 \ 3.20 8.43 8.28 15.9 2.18 Hanf ord ( 3.60 \ 3.60 7.30 7.50 18.5 2.78 Clay: Salinas 8.90 [ 6.10 6.95 7.11 30.5 31.3 1.51 0.67 Yolo < 6.10 7.26 32.3 31.4 0.31 0.90 Stockton 7.10 5.60 27.1 0.60 * Permeability units are denned by Hagan as "the gram-poises per cm 2 per second per millimeter of mercury difference in partial pressure per cm." According to Hagan (1941) an average value for Yolo soils in the field is 1.32 grams per ml. In dense plow soles, however, apparent-density values may be as high as 1.63 grams per ml in Yolo soils, and higher in others. 12 Where soils are subject to heavy compaction when wet, as on roadways and corrals or in the thin, slick layers beneath a dull plowshare, values of 2.0 grams per ml or more may be obtained (Shaw and Bodman, 1928). The parent rocks that enter into the composition of most of these soils have an approximate density of 2.65 grams per ml. If the soils could be sufficiently 12 Bodman, G. B., and N. E. Edlef son. Soil permeability to water. California Agr. Exp. Sta. Project 968:43. 1934. (Unpublished manuscript.) 18 California Experiment Station Bulletin 693 compacted so that no pore space was left, they would approach this value. Since they become impervious to carbon disulfide vapor at a density of about 2.0, apparently many of the pores in the soil are blind or noncontinuous and do not permit ready diffusion of gases. Supplementing these studies, Hannesson (1945) tried the effects of com- paction on moist soils, applying his forces both at right angles to the surface i I 10 1.00 1. 10 1.20 1.30 1.40 Compacf/on {apparent de/is/fy) //? grams per m/ Fig. 7. — Effect of compaction (apparent density) on permeability of air-dry soils to carbon disulfide. Curves were drawn in by approximation. Density values are grams per ml. Moisture contents of these samples are comparable with those reported in table 3. (From Hilgardia vol. 16, no. 10.) (compression) and parallel to the surface (shearing) , thus imitating somewhat the effects of certain cultivation implements. Below are given the values that he obtained. Dominant compaction force Compression Shearing Permeability when air-dry (moisture content about 2 per cent), K X 10 1 * 7.50 0.95 Permeability when moist (moisture content about 12 per cent),£" X 10" 1.70 0.15 According to these results, a shearing force is much more effective in com- pacting a soil than a strictly compressional force. This fact should be ap- parent from the way in which such forces are applied. A compressional force, when applied, acts to bring the particles closer together with a minimum dis- turbance of their spatial relations. A shearing force, on the other hand, shifts the soil particles into a more stable configuration in which voids are largely eliminated. The slicking action resulting from plowing or otherwise manipu- lating the soil while in a wet condition consists of both of these types of compac- Herbicidal Use of Carbon Disulfide 19 tion. A practical example is the pulling of a plow. If the shares are dull and the soil very moist, both soil layers separated by the plowshare are highly com- pacted ; the result is almost complete loss of permeability to gas. This slicking 5 ?o /s /6 /4 /2 /O 4 4 - / <\,/o°cJ ,ZO^C/ 25°C 4 S /2 /6 20 CS 2 /> res sure Cc/77 //{?) 24 Fig. 8. — Adsorption of carbon disulfide by oven-dry Yolo fine sandy loam at three tem- peratures (10° C, 20° C, and 25° C). Adsorption is expressed in milliliters of carbon disul- fide vapor at standard temperature (0° C) and pressure (1 atmosphere) taken up per gram of oven-dry soil. Pressure is expressed in centimeters of mercury (cm Hg). action at the bottom of the plow furrow may initiate plow-sole formation. It may also immediately render the subsoil inaccessible to carbon disulfide vapors from liquid placed above it. Thus, although virgin soils are usually 20 California Experiment Station Bulletin 693 highly pervious, tilled soils may be relatively impervious to gases as a result of various tillage operations to which they have been subjected. Adsorption. — Because soils contain colloids they tend to adsorb gases and this adsorption process affects any carbon disulfide injected for herbicidal •V ^ 0.4 4 <9 /2 /6 20 CS 2 pressure (cm //g) Fig. 9. — Adsorption of carbon disulfide by three textures of Yolo soil. The experiments were performed on soils moistened to field capacity. Adsorption is expressed in milliliters of carbon disulfide vapor at standard temperature and pressure taken up per gram of soil on the oven-dry basis. purposes. Studies of the effects of temperature, pressure, soil moisture, and texture on carbon disulfide adsorption in a number of soils have been made. Surface soils of the Yolo series were used in the experiments. For each de- termination 4 kilograms of soil was placed in a carboy, the carboy was evacu- ated to a predetermined pressure, and liquid carbon disulfide measured into the carboy. As the carbon disulfide vaporized the pressure increased, finally Herbicidal Use of Carbon Disulfide 21 reaching approximately that of the outside atmosphere as determined by a mercury manometer in the cork closing the carboy. The total quantity of carbon disulfide added minus the equilibrium amount held in the atmosphere of the carboy was taken as the amount adsorbed. The final concentration in the system was determined by the initial pressure established in the carboy by evacuation. Curve A of figure 8 shows the effect of pressure upon carbon disulfide ad- sorption by Yolo fine sandy loam at 10° C. Up to a pressure equivalent to about 10 cm of mercury the curve shows a typical adsorption equilibrium, the amount taken up decreasing at higher concentrations (pressures). The curve then inflects, turning rapidly upward. This probably represents capillary condensation of the carbon disulfide in the intermicellar pore spaces of the soil colloids. Curves B and C show adsorption at 20° C and 25° C respectively. Less car- bon disulfide is adsorbed as the temperature is increased. This is an important relation, for it indicates that at high soil temperatures the carbon disulfide molecules will remain more free in the soil atmosphere and exert a maximum killing effect. Figure 9 shows the relation of soil texture to carbon disulfide adsorption. The fine-textured soil (Yolo clay loam) took up more of the vapor than did the coarse one. Experiments with even finer-textured soils show greater uptake. It should be noted that the soils used in the experiments re- ported in figure 9 were moist and that the vertical scale has been much expanded to show detail in the low absorption region. The effect of soil texture is reflected in field practice where higher dosages are more often required in heavy clay soils than in light sandy ones. In figure 10 the vertical scale has been somewhat contracted but it is still five times that of figure 8. This has been set to steepen the curve for oven-dry Yolo fine sandy loam in order that the curves for moist soils can be included. Curve A shows adsorption by oven-dry soil which takes up large quantities of carbon disulfide vapor at relatively low concentrations. The pore spaces in this soil are unoccupied by water and have a maximum capacity for adsorp- tion. At 8.4 per cent moisture content (curve B), or about the permanent wilting percentage, for this soil much less vapor is absorbed at the low con- centrations, and only when carbon disulfide vapor pressures of 14 to 18 cm of mercury are reached does the soil start to adsorb rapidly. Curve C showing adsorption at approximately field capacity indicates even lower uptake at the low pressures. Consideration of these relations brings out a number of points of great im- portance in the use of carbon disulfide in the field. In the soil the partial pressure of carbon disulfide vapor may reach a value as high as 20 cm of mer- cury. Assuming an average value of 10 cm throughout the soil at 10° C, when volatilization of the liquid is complete, a Yolo soil of coarse texture may take up around 20 per cent of the carbon disulfide added in a standard application. At the highest summer temperatures somewhat less would be adsorbed. Fine- textured soils would hold more ; as high as 75 per cent being indicated by the results in a cool clay soil at the permanent wilting percentage or below. Moisture in the soil tends to lower the soil's capacity to adsorb carbon disulfide but, on the other hand, restricts the ready diffusion of the gas. For this reason 22 California Experiment Station Bulletin 693 carbon disulfide remains in high concentration in localized regions longer in wet soils than in dry. The fact that successful control of weeds may be had under a fairly wide range of soil moisture conditions indicates that these two 3.6 3.2 2.0 8i 24 2.0 A6 /2 0-0 0.4 /A , 01/ 'e/7 c/rt/ /■*~3, P„&4 -c, P„ ?o J 4 o 4 0/2/6 20 C5 2 pressure (cm //g) 24 Fig. 10. — Adsorption of carbon disulfide by Yolo fine sandy loam at three moisture con- tents. Adsorption is expressed in milliliters of carbon disulfide vapor at standard tempera- ture and pressure taken up per gram of soil on the oven-dry basis. effects tend to compensate each other and make soil moisture less important than some other factors in weed control. On the other hand, high soil temperatures tend to lower the adsorptive capacity of the soil and at the same time speed up diffusion. While both of these tend to stimulate surface losses from the soil, if the surface is properly sealed they cause high toxicity and effective results. Soil texture is also im- portant and must be taken into account because the effects upon adsorption and diffusion are cumulative as in the case of temperature. Fine-textured soils Herbicidal Use of Carbon Disulfide 2.'3 tend to adsorb much carbon disulfide ; they also hinder diffusion if structure is not well developed. One would expect from this that coarse soils might be successfully treated at almost any time; fine-textured soils of low per- meability should be treated during summer. Field experience would seem to bear out this deduction. Cold clay soils offer the greatest obstacles to success- ful treatment with carbon disulfide, and this is particularly true when they are excessively moist and lacking in structure. All of the phenomena so far described take place in laboratory samples of soils so prepared that soil structure is largely eliminated as a variable. In the field, structural characteristics are very pronounced and may in many in- stances overshadow the factors so far discussed. In the following section studies involving structure will be described. PERMEABILITY STUDIES ON FIELD CORES Since soil columns used in the studies just described were prepared from dried pulverized soil, all characteristics resulting from natural structures or from the activities of earthworms, insects, plant roots, and other organisms were largely obliterated. Mechanical difficulties were encountered in deter- mining carbon disulfide diffusion through field cores, because of a lack of suitable impermeable materials for preventing leakage of this gas along the cylinder walls. Buehrer's (1932) method of characterizing soil structure by low-pressure air flow was adapted to the study of permeability in field cores by the use of a rubber-lined sleeve 13 to surround the core and hold it in position during measurement. Preparation of Cores. — The field cores used were taken near plots estab- lished to study the effects of soil type, spacing, depth of application, and dosage on the killing of perennial weeds by carbon disulfide. Cores from soils at moisture contents near field capacity were found most suitable, since drier ones crumbled in handling. Accordingly, small plots were irrigated enough to moisten them to 4 feet, and cores were taken 1 week afterward. In winter, plots were sampled directly, since they were already near field capacity. Cores were taken with a cylindrical cutter, about 4 inches in diameter, forced into the soil by an anchored jackscrew. They were removed in successive segments either 4 or 6 inches long, placed intact into metal cans, sealed, and transported to the laboratory. Smaller cores, 4.7 cm in diameter and 5.0 cm in length, were taken from the field cores for determination of permeability. Natural structure was maintained by carefully paring the outer soil away with a sharp knife in advance of the leading edge of the cylindrical core cutter. To avoid slicking of the end surfaces, the knife was held at a high angle in trimming the cores to length. Moisture was determined on the soil trimmed from the large cores in reducing them to the final diameter. Determination of Apparent Density. — Each small core was next placed in a cylindrical metal sleeve 5.0 cm in diameter and 5.2 cm in length, lined with a thin rubber tube cemented to the sleeve at the ends. The rubber liner (pre- pared from dental dam material) was held in close contact with the soil core 13 Originally devised by Ross E. Moore, formerly Instructor in Soil Technology and Junior Soil Technologist in the Experiment Station. 24 California Experiment Station Bulletin 693 by water introduced between the sleeve and the liner through a side tube in the sleeve. A head of 50.0 cm of water pressure was used to press the liner around the core. To give the volume of the core, the volume of water and the unoccupied volume at the ends of the core were measured and subtracted from the total volume of the sleeve. Because it accounts for surface irregularities, this method provides a satisfactory measure of core volume. Each core was immediately weighed; and from volume and weight, cor- rected to the air-dry basis, the apparent density was computed. This gives a measure of relative compaction at various depths, provided the texture is uni- form within the depth sampled. Measurement of Air Flow. — With the core in place within the rubber liner of the metal sleeve, the sleeve was clamped between two end plates, each pro- vided with an exit tube. Rubber gaskets between the sleeve and plates pre- vented the air from entering or escaping except through the tubes. One tube was then connected to a source of constant air pressure equivalent to 5 mm of 95 per cent ethyl alcohol. The volume of air moving through in unit time was measured. Calculation of Resistance Factor. — The rate of air flow through the soil column, in cubic centimeters per second, was used in computing the resistance factor. Free flow through the unoccupied apparatus was 6.7 ml per second. Using this as zero resistance, and no flow as 100 per cent resistance, the factor was calculated as follows : Taking a measured flow of 3.0 ml per second, the factor would be ' ,, ' x 100 = 44.8 per cent. Because resistance parallels 6. 1 compaction in a general way, air flows were computed to resistance, thus mak- ing comparisons more graphic. The figures that follow show the resistances, apparent densities, and mois- ture contents of the successive fractions of soil profiles studied. The values are usually averages of two samples. Those for the three Yolo soils from Davis are based on three samples. Where resistances at field capacity and in the air-dry condition are compared, cores from only one of the duplicate samplings were measured in the air-dry condition. Depths plotted in the graphs are distances at which the median points of the cores lie below the soil surface. Interpretation of Air-Flow Data. — The Yolo series of soils was studied in greater detail than any other. Figure 11 illustrates the results obtained on a Yolo loam. The curves are the average of three runs. Curve R records the re- sistance of the cores to air flow ; C is the compaction or apparent density ; and Piv is the moisture content. The scale at the left is percentage and is used in reading both resistance and moisture content. The scale at the right is density in grams per ml and has been arbitrarily adjusted to make curves R and C coincide in range. It must be noted that this scale is different in different figures. Since most of the soils studied gave results that fall in a general pattern not essentially different from that of Yolo loam, these curves (fig. 11 ) will be dis- cussed in detail. Starting in the topsoil, resistance is high in the first core, probably because of compaction by the beating rain. The compaction curve shows this to be the soil fraction with the highest density. Resistance and Herbicidal Use of Carbon Disulfide 25 1 /oo 30 60 40 20 <^ C^ /v> /o 20 30 40 £0 60 70 30 Depf/7 in cenf/me/ers 90 /oo //o /20 /■SO /■40 A. /30\ I /■/O tf" /■OO Fig. 11. — Changes in resistance to air flow (B), in compaction (C), and in moisture content (Pw) with depth in Yolo loam. All values are averages of measurements on three separate columns. Eesistance is expressed in percentage and is read on the left-hand scale. Compaction is expressed as apparent density, in grams per ml, on the right-hand scale. /oo so V 60 \ ^w? \c p#y __ / \ v. - - _ . - - /50 /40 /30 /20 20 — =— /O 20 30 40 50 60 70 SO Oepf/? //? ceflh Meters 90 /oo //o /20 I/O /oo \ ! Fig. 12. — Changes in resistance to air flow (-E), in compaction (C), and in moisture content (Pw) with depth in Yolo fine sandy loam. Average of four columns. /oo /■SO SO 60 70 SO De/>ffy //? ce/7//'rf?efers Fig. 13. — Changes in resistance to air flow (-E), in compaction (C), and in moisture content (Pw) with depth in Yolo clay loam. Average of four columns. compaction both drop in the second soil core, where the structure is somewhat looser and considerable straw and trash have been incorporated by previous plowing. Resistance then goes through a maximum in the next three cores, representing soil in the 6- to 10-inch (15- to 25-cm) horizon. This, the plow- sole region, is characterized by thin layers of slick or highly compacted soil formed by mechanical action of the plowshare. Since the compaction falls 26 California Experiment Station Bulletin 693 through this region, evidently the impervious regions are of limited extent and do not occupy the complete soil mass. At a depth of about 10 inches (25 cm) the resistance starts down. It decreases rapidly to 16 inches (40 cm) and then levels off with little change, until from 30 inches (75 cm) down- ward it increases slightly again. Compaction likewise drops through the 10- to 20-inch (25- to 50-cm) layers, where it reverses and rises steadily to a value of 1.25 grams per ml at 36 inches (92 cm) . /oo 80 \ 60 «$» 40 20 N / \ \ \ * \ we/ fi (fry s ■ /V n \ ^*' •^ s Pw c ty\ O /O 20 SO 40 SO 60 70 SO SO /OO //O /30 Oepf/? //? ce/7///7?e/ers Fig. 14. — Changes in resistance to air flow in Yolo loam at field capacity (.R-wet) and air-dry (E-dry), and in moisture content (Pw-wet, Piv-drj) with depth. Average of three columns. /oo 60 M 40 ^ zo y/?* 9/ 1 /■*■ / ■v. -" " N \ \ 1 1 \ \ 1 \ 1 1 1 \ \ \ / I / ^Ptv , vef \ i y* • dry X /o 20 SO 40 50 60 70 SO SO /OO //O /20 Pep/A /// cenf/zne/ers Fig. 15. — Changes in resistance to air flow, and in moisture content with depth, in Yolo fine sandy loam at two moisture contents (field capacity and air-dry). One column. The region of maximum decrease in resistance is that in which biological activities are important and where cultural practices have little effect on structure. In this region usually occurs a large proportion of the storage-root system of perennial weeds ; and here gaseous diffusion becomes increasingly free as the f umigant vapor moves downward and spreads through the soil. The small increase in resistance below 30 inches (75 cm) may represent a change in texture, or it may reflect reduced biological activity and a denser structure. These are encountered in Yolo soils. Figures 12 and 13 present similar studies on two additional Yolo soils. Effect of Soil Moisture on Resistance. — Resistances of all field cores were measured at approximately their field capacity. A notable feature is the large change in permeability, with little variation in moisture, in different sections Herbicidal. Use of Carbon Disulfide 27 /oo JO • <*■/? we. 4 / c^ 40 ~ ^ ^' Pdrc/ ^-/V /:'* 20 n Pw 4 ■frc/-^ " -^...-"V y *"••».—• *■■» li \ / * Jl ~~ fPv dry Vw wef 20 30 40 SO 60 /0 20 30 40 SO 60 Depfft in ce/7f//ve/ers Fig. 19. — Changes in resistance to air flow, and in moisture content with depth, in two California soils at two moisture contents: A, Gridley loam; B, Sacramento clay. A single column was measured for each soil. /0O JO 20 SO 40 £0 60 70 SO SO /O0 //0 /20 £>epf/r /'/? ce/?///77e/ers Fig. 20. — Changes in resistance to air flow, and in moisture content with depth, in Han- ford fine sandy loam at two moisture contents (field capacity and air-dry). Average of two columns. /oo do X 60 \ k ^ 40 20 1- - . ^*v // — >\ ll 1 \\ -/? ive/ \< ^fidrt/ \ A \\ \/ / / v^ . - ^ — ' / *ph ' ivef — — ^ y r ph 'dry O /O 20 30 40 SO 60 70 SO SO /00 //O /20 Oep//? //? ce/?f//77e/ers Fig. 21. — Changes in resistance to air flow, and in moisture content with depth, in Honcut loam at two moisture contents (field capacity and air-dry). One column. air-dry. An attempt was also made to obtain moisture contents between the air-dry condition and field capacity by exposing cores to known concentra- tions of sulfuric acid in desiccators. Since fluctuations in moisture within the range encountered in the field prove less important than might be expected from studies on artificially packed samples, work on intermediate moisture contents was limited. Herbicidal Use of Carbon Disulfide 29 Figures 14 to 21 give the results of the moisture studies. Although lowering the moisture content increases permeability, the percentage of increase is usually less than the Variations normally existing in different cores from a profile. Absolute increases in permeability from drying are greatest in soils already rather permeable when wet. Relative increase in permeability is greatest in impervious regions such as plow sole. Highly impermeable layers may not become permeable upon drying, but permeable layers may become even more permeable upon drying. Summarization of Air-Flow Data. — Detailed interpretations of air-flow data on other soil types are presented in the section that follows (pp. 29 to 44) , where results of plot tests are correlated. From study of many profiles, repre- senting a variety of soils, conclusions may be summarized in the following statements: (1) In general field cores of the finer-textured soil types are more permeable than those of the coarser soil types, at least below plow depth. This is at variance with results obtained upon laboratory-packed soils and indicates that a high colloid content is conducive to permeable soil structure. In the absence of well-developed structure, cores of coarser-textured soils are more permeable. (2) Within a given profile, permeability is usually correlated with apparent density. Exceptions usually involve dense or slick layers of limited thickness. (3) Plow sole was characteristic of most soils sampled, and the plow sole was the least permeable portion of the profile. A few soils not recently cultivated had denser surface layers. These samples usually came from pastured fields where the surface was packed. Judging from these results, structure as determined by the sum total of natural agencies, such as rodents, earthworms, plant roots, insects, fungi, bacteria, and colloidal hydration, becomes dominant in determining the per- meability of soils as they occur in the field. Soil layers below the reach of tillage implements are permeable to gas unless they contain dense alluvial strata, claypan, 14 hardpan, or standing water. Within the tilled portion, plow sole 15 usually constitutes a relatively impervious layer which, because of its high resistance, may have its permeability increased twofold or more by dry- ing. This moisture relation is important where treatment with carbon disul- fide must be made through such a plow sole. Moisture is also of primary importance in surface sealing. Where, because of high water-table conditions or frequent irrigation practice, perennial weeds have feeder roots within 6 inches of the soil surface, shallow injection followed by sprinkling, or injection performed immediately after rainfall, may be resorted to. AIR-FLOW AND PLOT STUDIES ON SEVERAL CALIFORNIA SOILS To define more accurately the volume and spacing of carbon disulfide injec- tions, plots involving several conditions of soil moisture and temperature were established at Davis. Midsummer treatments on dry soils were also made in several other localities, including those from which samples for air-flow measurement had been taken. 14 The term claypan as used here refers to a dense, highly colloidal layer in the soil whereas hardpan represents a cemented impervious layer. Both occur below the soil surface at vary- ing depths and are formed by natural agencies during the development of the soil. 15 Plow sole is a relatively impermeable layer resulting from plowing or other tillage operations and is usually present within the limits of the 4- to 12-inch depths. 30 California Experiment Station Bulletin 693 TABLE 5 Spacings between Injection Points and Volume of Liquid per Injection to Give Indicated Numbers of Points and Pounds of Carbon Disulfide per Square-Rod Plot Spacing Pounds of carbon disulfide per square rod Injection points Between rows In rows 5 10 15 20 25 Volume of liquid per injection number 30 inches 33.6 23.8 19.3 16.8 15.0 inches 38.8 27.5 22.3 19.4 17.3 ml 60 30 20 15 12 ml 120 60 40 30 24 ml 180 90 60 45 36 ml 240 120 80 60 48 ml 300 60 150 90 100 120 75 150 60 TABLE 6 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide (Anchor Brand*) at Different Soil Moisture Contents on Wild Morning-Glory in Yolo Loam (Plots Treated August 8 to September 28, 1940) Holes per square rod Dosage per hole Percentage kill Depth, and pounds per square rod Dry soil (at permanent wilting per- centage) Moist soil (recently irrigated) 6 inches: 5 number 90 60 30 90 60 30 90 60 30 90 60 30 90 60 30 90 60 30 ml 20 30 60 40 60 120 60 90 180 20 30 60 40 60 120 60 90 180 per cent 72 35 64 87 92 93 100 96 100 78 45 68 97 97 99 100 96 100 per cent 45 5 72 5 37 10 97 10 95 10 97 15 98 15 97 15 83 10 inches: 5 91 5 40 5 55 10 95 10 58 10 61 15 94 15 54 15 92 * For description of brands of carbon disulfide see footnote 2, page 3. TABLE 7 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide at Different' Soil Moistures and Soil Temperatures (as Determined by Season) on Wild Morning- Glory in Yolo Loam Date of treatment and soil moisture Method of treatment Percentage kill Amount per square rod Holes per square rod Dosage per hole Depth Anchor Brand Activated pounds number ml inches per cent per cent 20.0 90 80.0 6 80 . , 20.0 120 60.0 6 60 . . Feb. 27, 1941, soil at field capacity. . , 20.0 150 48.0 6 98 25.0 90 100.0 6 90 93 25.0 120 75.0 6 80 93 , 25.0 150 60.0 6 100 97 ' 15.0 30 180.0 6 94 15.0 60 90.0 6 99 June 11, 1941, soil at field capacity.. « 15.0 90 60.0 6 99 99 20.0 30 240.0 6 94 96 20.0 60 120.0 6 100 100 20.0 120 60.0 6 100 •• ' 10 30 120.0 6 94 87 10.0 120 30.0 6 96 87 12.5 30 150.0 6 97 96 12.5 120 37.5 6 98 96 • 15.0 30 180.0 6 100 15.0 120 45.0 6 97 10.0 30 120.0 10 93 95 k 10.0 120 30.0 10 97 97 ' 15.0 60 90.0 6 90 15.0 90 60.0 6 98 . . Aug. 20, 1941, soil at field capacity. . < 15.0 120 45.0 6 94 87 20.0 60 120.0 6 99 80 20.0 120 60.0 6 100 98 , 22.5 90 90.0 6 98 •• ' 15.0 30 180.0 6 75 15.0 120 45.0 6 90 90 Sept. 29, and Oct. 6, 1941, soil - 20.0 30 240.0 6 95 . # 20.0 120 60.0 6 97 95 25.0 30 300:0 6 90 k 25.0 120 75.0 6 99 98 ' 15.0 30 180.0 6 80 15.0 120 45.0 6 90 75 Sept. 22 and Oct. 2, 1941, soil < 20.0 30 240.0 6 85 20.0 120 60.0 6 95 95 25.0 30 300.0 6 90 k 25.0 120 75.0 6 98 95 ' 15.0 120 45.0 6 99 20.0 30 240.0 6 99 20.0 60 120.0 6 99 Nov. 11, 1941, top 6 inches wet, 20.0 120 60.0 6 100 - 25.0 120 75.0 6 100 15.0 120 45.0 8 99 99 20.0 120 60.0 8 100 100 25.0 60 150.0 8 99 . . ; 25.0 120 75.0 8 100 100 ' 20.0 90 80.0 6 99 .. 20.0 120 60.0 6 100 99 Jan. 21, 1942, soil at field capacity < 20.0 150 48.0 6 99 25.0 90 100.0 6 99 99 25.0 120 75.0 6 99 25.0 150 60.0 6 99 99 32 California Experiment Station Bulletin G93 In setting up the schedule of treatments, the exact values of the standard dosage, 19.88 pounds per square rod, and the number of injection points per square rod, 121, were for convenience rounded off to 20 pounds and 120 points. A schedule of treatments was then set up on the arbitrarily chosen basis of %, / 2 , %, 1, and l 1 /^ times 20 pounds, and 120 points. Although in TABLE 8 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide at Different Soil Moistures and Soil Temperatures (as Determined by Season) on Wild Morning-Glory in Yolo Fine Sandy Loam Date of treatment, and soil moisture Method of treatment Percentage kill Amount per square rod Holes per square rod Dosage per hole Depth Anchor Brand Activated pounds number ml inches per cent per cent 20.0 90 80.0 6 99 20.0 120 60.0 6 98 99' Feb. 17, 1941, soil at field capacity. . < 20.0 150 48.0 6 97 25.0 90 100.0 6 99 99 25.0 120 75.0 6 99 25.0 150 60.0 6 99 99 r 7.5 30 90.0 6 75 75 7.5 120 22.5 6 65 25 10.0 30 120.0 6 90 80 Aug. 1, 1941, soil dry 10.0 120 30.0 6 75 90 < 12.5 30 150.0 6 95 12.5 120 37.5 6 95 7.5 30 90.0 10 60 25 7.5 120 22.5 10 50 30 ' 15.0 120 45.0 6 99 20.0 30 240.0 6 99 Nov. 14, 1941, top 12 inches moist, < 20.0 60 120.0 6 99 dry below 20.0 120 60.0 6 98 15.0 120 45.0 8 99 99 , 20.0 120 60.0 8 90 99 ' 20.0 90 80.0 6 99 20.0 120 60.0 6 99 99 Feb. 7 to Feb. 11, 1942, soil at I 20.0 150 48.0 6 99 field capacity 25.0 90 100.0 6 100 99 25. J 120 75.0 6 99 , 25.0 150 60.0 6 99 100 commercial applications, and in our previous plot-test work also, the spacings between rows are the same as between injection points in the row, the system proposed by Taylor (1939) was adopted as providing for the complete ex- posure of the soil mass to vapors of a fumigant with the minimum amount of overlap between adjacent diffusion circles. In this system the distance between points in the rows is 1.15 times the distance between rows. Table 5 shows the calculated spacings between points and between rows, and also the milliliters of carbon disulfide liquid per injection to give 5, 10, 15, 20, and 25 pounds per square rod at 30, 60, 90, 120, and 150 injection points. The most complete series of tests was made on Yolo loam. As figure 11 shows, there was a close correlation between resistance to air flow and apparent Herbicidal Use of Carbon Disulfide 33 density of the soil, both being high in the top 10 inches (25 cm) and much less below. The effects of dosage rate, soil moisture, and depth of injection of carbon disulfide on wild morning-glory were tested on plots of Yolo loam in 1940. Table 6 presents the results. Evidently 15.0 pounds was necessary for a prac- tical kill, and certain plots at this rate were not satisfactory. Treatments were somewhat more effective on dry plots than on wet ones. Whereas on the dry plots the 10-inch depth w r as slightly better than the 6-inch (15 cm), on the wet plots the reverse was true, probably because of crown survival. In 1941 a more extensive series was used (table 7). Soil moisture and temperature were the main variables in these plots. Anchor Brand and "Activated" carbon disulfide 1 '' (a product containing compounds to reduce the vapor pressure) were compared. One series was treated in windy weather and one on still days, to determine how moving air affects soil ventilation. Dosage was also varied. Plots treated on February 27, 1941, were at the minimum seasonal soil tem- perature. The moisture content was at field capacity. Both 20.0 and 25.0 pounds were effective at 150 injections per square rod. Plots treated June 11 were at intermediate soil temperature and field capacity. Only the widest spacing (30 holes) reduced effectiveness at this temperature. Both wet and dry plots were treated during maximum soil temperatures in August. Under these conditions spacing, within the limits tested, had no sig- nificant effect. Dosage as low as 10.0 pounds per square rod proved quite successful. Moisture seemed to have no bearing on the results. Plots treated in windy and still weather in September and October showed no significant differences. This soil had a well-compacted surface and was very permeable below (fig. 11). Under less favorable conditions, wind might have had more influence. Plots treated November 11, 1941, had 6 inches (15 cm) of moist soil on the surface, with the subsoil less moist. Soil moistures were as follows : Depth of soil, in inches to 6 6 to 12 12 to 24 24 to 36 36 to 48 Depth of soil, Soil moisture, in cm per cent to 15 23.0 15 to 30 13.6 30 to 61 13.7 61 to 92 15.5 92 to 122 14.4 With this excellent surface seal in a soil still fairly warm, the toxic vapors were well distributed, and even 15.0 pounds gave good results. Fine controls were also obtained on January 21, 1942, at minimum seasonal soil temperature and with moisture at field capacity. Throughout the series on Yolo loam, results with Anchor Brand and "Activated" carbon disulfide were not different. Yolo fine sandy loam was tested by air-flow and plot studies. As shown in figure 12, resistance was fairly high throughout, there being less evidence of plow sole than in most soils tested. This soil is characterized by numerous narrow strata and lenses of dense material. The maintained resistance despite 18 See footnote 2. 34 California Experiment Station Bulletin 693 TABLE 9 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide at Different Soil Moistures and Soil Temperatures (as Determined by Season) on Wild Morning-Glory in Yolo Clay Loam Method of treatment Percentage kill Date of treatment, and soil moisture Amount per square rod Holes per square rod Dosage per hole Depth Anchor Brand Activated pounds number ml inches per cent per cent 20.0 90 80.0 6 97 20.0 120 60.0 6 92 91 Feb. 25, 1941, soil at field capacity. . < 20.0 150 48.0 6 94 25.0 90 100.0 6 98 25.0 120 75.0 6 87 91 k 25.0 150 60.0 6 95 95 ' 15.0 60 90.0 6 94 15.0 90 60.0 6 97 97 June 13, 1941, soil at field capacity.. . • 20.0 30 240.0 6 90 93 20.0 60 120.0 6 96 k 20.0 120 60.0 6 96 95 ' 10.0 30 120.0 6 50 25 10.0 120 30.0 6 75 40 12.5 30 150.0 6 50 85 Aug. 6 to Aug. 10, 1941, soil dry < 12.5 120 37.5 6 50 95 and cracked on top, moist below. . 15.0 30 180.0 6 60 15.0 120 45.0 6 40 10.0 30 120.0 9 25 30 , 10.0 120 30.0 9 25 30 ' 15.0 60 90.0 6 70 15.0 90 60.0 6 70 Aug. 20, 1941, soil at field capacity.. < 15.0 120 45.0 6 80 90 20.0 60 120.0 6 85 90 20.0 90 80.0 6 95 , 20.0 120 60.0 6 90 95 ' 20.0 30 240.0 6 75 20.0 120 60.0 6 98 . . Nov. 13, 1941, top 7 inches moist, 25.0 120 75.0 6 96 . , less moist below i 20.0 20.0 60 120 120.0 60.0 8 8 93 97 95 25.0 60 150.0 8 99 . 25.0 120 75.0 8 99 99 ' 20.0 90 80.0 6 98 20.0 120 60.0 6 90 90 Jan. 15 to 29, 1942, soil at field < 20.0 150 48.0 6 99 , . capacity 25.0 90 100.0 6 75 92 25.0 120 75.0 6 75 k 25.0 150 60.0 6 99 92 low compaction in the deeper cores indicates lack of structure of the type causing high permeability. Plot results on this soil (table 8) show excellent control in February, August, and November at all dosages of 12.5 pounds or more. Dosages of 7.5 and 10.0 pounds were definitely insufficient in August. Spacing seemed un- important, as did the brand of chemical used. Results on Yolo clay loam treated in February, June, August, November, Herbicidal Use of Carbon Disulfide 35 and January were less impressive than on the coarser-textured soils. Air-flow studies are illustrated in figure 13. Though somewhat denser and less pervious in the top 12 inches, this soil had no high resistance strata ; and it varied less throughout the profile than many. roe SO 60 70 SO £>e/>ffi //? ce/7///7?e/ers 90 /OO //o t20 Fig. 22. — Changes in resistance to air flow (R), in compaction (C), and in moisture content (Pw) with depth in Yolo silt loam. Average of two columns. Compaction is ex- pressed in units of density (grams per ml). TABLE 10 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide under Different Conditions on Wild Morning-Glory in Yolo Silt Loam at Paicines Method of treatment Date of treatment, and soil moisture Percentage Amount per Holes per Dosage per Dpnth kill, Anchor Brand square rod square rod hole pounds number ml inches per cent 10 30 120 6 60 10 . 60 60 6 35 10 90 40 6 20 Sept. 20, 1940, surface dry, moist 15 30 180 6 100 below, fallowed field ^ 15 15 60 90 90 60 6 6 100 100 20 120 60 6 100 10 60 60 10 80 , 15 60 90 10 100 r 15 60 90 6 50 15 90 60 6 95 Aug. 26, 1941, dry in top 12 inches, < 15 120 45 6 99 moist below 20 20 60 90 120 80 6 6 50 97 , 20 120 60 6 99 The plot results (table 9), though less consistent than in coarser-textured soils, show certain significant relations. The lowest effective dosage, 15.0 pounds, was adequate only in June, whereas 20.0 pounds gave some good results at all treatment dates. Spacing results were inconsistent. Control with the two brands was not consistently different. Treatments were not very successful in August, when the topsoil was dry and cracked. Yolo silt loam in Paicines, California, was studied. Figure 22 shows the air-flow results. This soil, loose and permeable in the top 2 inches (5 cm) , was 36 California Experiment Station Bulletin 693 denser and less permeable through the 2- to 10-inch (5- to 25-cm) cores, and then more permeable to a depth of about 24 inches (61 cm), where resistance rose through four fractions to drop again at about 3 feet (92 cm). The first dense region represents plow sole ; the second resulted from stratified material of variable textures. /oo do v 60 \ £0 N^ / V*** fi % & \ c^ 40 zo A l-/o /SO /so /40 /so /so B <£ < c k^# <* ffilV — TC 1 __ _ "~ ""' {Pk - - • /0 ZO SO 40 SO 60 70 o Depf/? //? cesf/znefers /o zo so *o eoo /so <3 140 > I /ZO /oo Fig. 25. — Changes in resistance to air flow (R), in compaction (C), and in moisture content (Pw) with depth in two California soils: A, Gridley loam; B, Greenfield coarse sandy loam. Resistance (R) and moisture content (Pw) for both graphs can be read from the percentage scale on the left. Note that the scales for apparent density, in grams per ml, are different for the two soils. through the cores to a depth of 32 inches (82 cm) . There was a hardpan layer at 3.5 feet (102 cm) . Density in the plow-sole layer was 1.67 grams per ml, one of the highest values recorded. On September 1, 1941, plots in a morning- glory infestation on this soil were treated at a depth of 6 inches (15 cm) with 15.0 and 20.0 pounds of carbon disulfide. Two months later the maximum con- trol was 25 per cent; and one year after treatment no effect was evident. This is an example of one type of soil that might require special methods to yield successful results with carbon disulfide. Greenfield coarse sandy loam is a dense, gritty soil from granitic alluvium. The plots were located south of Salinas in a Sudan-grass pasture. As figure 25B shows, this soil had an apparent density of 1.69 grams per ml in the top 38 California Experiment Station Bulletin 693 core, and the lowest density was 1.46 grams per ml. Resistance, although fairry high in the top layer, dropped throughout the top 14 inches (36 cm) to 40 per cent. As table 12 indicates, a 15.0-pound dosage was fairly effective at 120 holes per square rod, while 20.0 pounds proved successful at all three spacings. TABLE 12 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Wild Morning-Glory in Moist Greenfield Coarse Sandy Loam (Treatment on August 25, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 6 inches : 15 number 30 60 120 30 60 120 ml 180 90 45 240 120 60 per cent 80 90 95 100 99 100 per cent 15 15 80 20 99 20 20 100 TABLE 13 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Johnson Grass in Gridley Loam ; Top Soil Dry, Water Table at 30 Inches (Treatment on August 13, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 4 inches: 15.0 number 120 120 120 30 120 120 ml 45.0 60.0 37.5 180.0 45.0 60.0 per cent 75 90 50 87 50 93 per cent 20.0 96 4 inches: 12.5 15.0 15.0 93 20.0 93 The Gridley loam was taken from an undisturbed area near the town of Gridley. This soil (fig. 25 A) was firm but permeable in the top core, very light and loosely bound in the second and third cores — a condition caused by the presence of relatively large amounts of plant residues. Resistance and com- paction both increased immensely in the 8-inch core because of an old plow sole, and then decreased gradually to intermediate values. Plots in Johnson grass were treated on this soil at the time indicated in table 13. The topsoil was dry and hard; there was 12 per cent moisture at 12 inches (30 cm), and the water table stood at 30 inches. Results were rather erratic, as shown by Herbicidal Use of Carbon Disulfide 39 readings taken one year after treatment; but the surviving rhizomes were confined to the top 4 inches (10 cm) of soil, from which they sent down roots and resprouted. Because of the plow sole, few roots had penetrated deeply, and these were killed. The live rhizomes could have been killed by shallow plowing, to make all treatments 100 per cent. The best results were obtained /o 20 SO 5t? /40 \ /■zo > //o 90 /oo //o tfO /ex? 40 SO 60 70 SO £>e/>//? //7 ce/?h/7}efers Fig. 26. — Changes in resistance to air flow (-R), in compaction (C), and in moisture content (Pw) with depth in Hanford fine sandy loam. Average of two columns. TABLE 14 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Wild Morning-Glory in Dry Hanford Fine Sandy Loam (Treatment on August 12, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 6 inches: 10.0 10.0 12.5 number 30 120 30 120 30 120 30 ml 120.0 30.0 150.0 37.5 180.0 45.0 120.0 per cent 40 80 40 25 25 25 60 per cent 85 85 12.5 15.0 15.0 10 inches: 10.0 with 20 pounds; there was no significant difference between Anchor Brand and "Activated" carbon disulfide. Hanford fine sandy loam was studied at a location near Elk Grove. Figure 26 shows the air-flow results. Resistance, fairly high in the top core, dropped and rose again in the plow sole, lowered in the subsoil to a minimum at about 20 inches (50 cm), and rose again to a value as high as that in the top core. In the upper fractions, compaction followed resistance. In the lower ones, however, it fluctuated because of stratification with finely divided sediments that had little structure. Plot results (table 14) were poor in this soil. Morning-glory infestations on a farm near Gridley were used in studies on 40 California Experiment Station Bulletin 693 Honcut loam. Air-flow results (fig. 27) show that this soil was rather dense on top with a permeable layer beneath, then a dense impervious plow sole having an apparent density of 1.66 grams per ml, and below this a lighter, highly permeable subsoil. Treatments at 6 inches (15 cm) with 15.0, 20.0, and 25.0 pounds of carbon disulfide met with little success, presumably because of the highly compacted plow-sole layer. Had the plow sole been broken up /OO so v. 60 \ 20 1 I~c V* J ' \ / — jPw ^_ ■ - — — — — — " /7S /6S /55 ^ I /35 /Z5 o /o eo JO 40 SO 60 70 so s>o /oo //o /so 0epf/? /'/? ce/7///ne/ers Fig. 27.- — Changes in resistance to air flow (H), in compaction (C), and in moisture content (Pw) with depth in Honcut loam. One column. /oo So v. 60 \ c^> 40 20 A 4* C >^ fPw r^^ r ' /./$ /OS 095 085 —I 1 1 1 1 IO 75 /O SO 30 40 SO 60 O /O 0ep//7 //7 cen///7?e/ers B ^P Purrs' _ / .. ""v^ - f^\ • ^Y\ ' c^ \ /oo OSS t 090 & OS5 \ I oso so 30 40 so 60 O/S Fig. 28. — Changes in resistance to air flow (R), in compaction (C), and in moisture content (Pw) with depth in two California soils: A, Lockwood gravelly loam; B, Sacra- mento clay. Resistance (B) and moisture content (Pw) for both graphs can be read from the percentage scale on the left. The scales for apparent density, in grams per ml, are dif- ferent for the two soils. by subsoiling and the chemical placed beneath, the results would undoubtedly have been better. Lockwood gravelly loam is an old alluvial soil lying west of the Salinas River, south of Salinas. As figure 2SA shows, this soil had a well-developed plow sole even though the apparent density was low. Below the 10-inch (25 cm) level the soil was loose and permeable. Treatments were put in at 6 inches (15 cm), with two plots at 10 inches (25 cm). Results (table 15) were not particularly successful, probably be- cause of the plow-sole layer. At close spacing, 20.0 pounds gave 90 per cent control of the morning-glory, but 10.0 and 15.0 pounds were unsatisfactory. Sacramento clay is a highly colloidal soil, rich in organic matter and having a large water-holding capacity. Figure 285 shows the air-flow results. Resist- Herbicidal Use of Carbon Disulfide 41 ance and compaction, starting at medium values in the top fraction, drop within the plowed layer, rise abruptly in the plow sole, which in this soil is between 10 and 14 inches (25 and 36 cm), and then descend, to rise again in a dense clay layer at about 20 inches (51 cm), and fall in a peaty layer at 24 TABLE 15 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Wild Morning-Glory in Lockwood Gravelly Loam j Soil Dry on Top, Moist Below (Treatment on September 12, 1940) Depth and amount per square rod Holes per square rod Dosage per hole Percentage kill, Anchor Brand pounds number ml per cent 6 inches: 10 30 60 90 120 60 40 40 10 60 10 60 15 30 180 70 15 60 90 70 15 90 60 75 20 120 60 90 10 inches: 10 60 60 00 15 60 90 70 TABLE 16 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Hoary Cress in Sacramento Clay; Soil Dry on Top, Subirrigated Below (Treatment on August 12, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 6 inches: 15 number 30 120 30 120 30 ml 180 45 240 60 75 per cent 90 93 80 98 99 per cent 15 85 20 20 95 25 98 inches (61 cm). The gradually increasing water content reflected soil mois- ture conditions above a standing water table at around 4 feet (122 cm). Plot results (table 16) show some increase in effectiveness with close spacing at the 15.0- and 20.0-pound dosages, success being achieved on most plots. Hoary cress was the weed treated. Samples of San Joaquin sandy loam were taken from a grainfield near Montpelier. As figure 29 shows, the topsoil was coarse textured and permeable. Both resistance and compaction increased rapidly to a level near 10 inches 42 California Experiment Station Bulletin 693 (25 cm), where a dense plow sole was present. Below this, resistance dropped for about 8 inches (20 cm) and then increased sharply, because of a very compact sandy clay overlying the hardpan. Apparent density, after remain- ing constant for 8 inches (20 cm) below the plow sole, increased sharply to 1.75 grams per ml, the highest value found for any soil in the studies. /oo 00 60 40 20 A ~y c P*2 - //S /■65 I /S5 Ai /4S X /O 20 SO 40 £0 60 70 00 Pep/6 //? ce/7//mefers 90 /oo //0 /20 /35 /20 Fig. 29. — Changes in resistance to air flow (-R), in compaction (C), and in moisture content (Pw) with depth in San Joaquin sandy loam. Average of two columns. TABLE 17 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Wild Morning-Glory in San Joaquin Sandy Loam ; Dry on Top, Moist Below (Treatment on August 31, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 6 inches: 12.5 number 30 120 30 120 30 120 ml 150.0 37.5 180.0 45.0 240.0 60.0 per cent 50 25 60 50 85 70 per cent 12.5 25 15.0 15.0 25 20.0 50 20.0 Plot results (table 17) were unsatisfactory on this soil, probably because of the high compaction and low permeability. Wide spacing seemed better than narrow, but this difference may not be significant. The highly compact nature of the subsoil would make questionable the value of carbon disulfide treatment in this soil. Shallow-rooted weeds such as Bermuda grass and John- son grass might be controlled. Cores of Sorrento fine sandy loam, taken from a grainfield near Westley, were studied for compaction and resistance. In this soil the two curves (fig. 30) are similar and indicate a close correlation between permeability and density. There was a definite plow sole and, at about 32 inches (82 cm) , a very dense layer with intermediate values between. Plot results in this soil were poor because the injections were made into loose, dry topsoil above a dense Herbicidal Use of Carbon Disulfide 43 /oo ac x to 4> «> 20 A >/i pvx ^ I /■/o /oo Fig. 30. — Changes in resistance to air flow (R), in compaction (C), and in moisture con- tent (Pw) with depth in Sorrento gravelly fine sandy loam. Average of two columns. 1 /oo SO €0 \ « 20 '/? s^ P*' /JS /6S /SS /■4S /O 20 JO 40 SO 60 70 SO Depf/i //? ce/)f//7?efers so /oo //o /2o /■SS /2S \ Fig. 31. — Changes in resistance to air flow (-K), in compaction (C), and in moisture content (Pw) with depth in Wyman sandy loam. Average of two columns. TABLE 18 Effect of Dosage, Spacing, and Depth of Injection of Carbon Disulfide on Wild Morning-Glory in Wyman Sandy Loam ; Dry on Top, Moist Below (Treatment on September 10, 1941) Depth and amount Holes per square rod Dosage per hole Percentage kill per square rod Anchor Brand Activated pounds 6 inches: 15 number 30 120 30 120 30 120 ml 180 45 240 60 300 75 per cent 50 75 75 98 85 95 per cent 15 50 20 20 90 25 25 99 plow sole. A 20.0-pound plot with 30 injection points gave 90 per cent control. At 120 points the control was only 50 per cent. The results of air-flow studies on Wyman sandy loam, a coarse-textured soil from basic igneous alluvium, are shown in figure 31. In this soil, resistance 44 California Experiment Station Bulletin 693 was high, but compaction was low in the top fractions. Apparently, there were slick or compacted layers, probably caused by improper tillage procedures. After a rise at the 6-inch (15 cm) core — the depth to which this soil is peri- odically disked — resistance dropped to a very low value ; but it rose again in the bottom 10 inches (25 cm) . The plot results (table 18) indicated a definite correlation with spacing, best control being found on the 20.0- and 25.0-pound plots with 120 injections. Under the conditions of treatment, 15.0 pounds was an insufficient dosage. DISCUSSION OF RESULTS Before the results of these experiments are reviewed, it should be pointed out that the object was to determine the effects of dosage, spacing, depth of injection, and the like, under varying soil conditions such as high and low soil moisture, presence and absence of plow sole, and variations in profile character. A high percentage of control was not necessarily sought. Whereas the laboratory studies indicated that soil moisture was the pre- dominating factor in controlling gaseous diffusion, air-flow and plot studies show that, in the field, soil structure is the major factor. Temperature deter- mines the rate and extent of diffusion and enters the problem through its rela- tion to spacing. There is some evidence that narrow spacings are required in cold soils, whereas wider spacing (with a consequent saving in application cost) may be used in summer when soils are warm. Texture may sometimes be important. Very fine-textured colloidal soils apparently adsorb the chemical and require larger dosages. Field experience corroborates this conclusion, for 25 and even 30 pounds per square rod have been found necessary in treating some such soils. Where a permeable structure is established, soil moisture seems to have little influence upon gaseous diffusion. However, because a dense plow sole may be rendered practically impervious by moisture approaching field ca- pacity, soils having moist plow sole should not be treated unless the injections can be made through this dense layer. This requires deeper injection than is ordinarily recommended. Such deep treatment, however, must be followed by thorough compaction of the surface. Moisture has a definite relation to surface sealing, since a recently sprinkled soil or one upon which rain has fallen is impervious. Moist surface soil may be tightly sealed by rolling or tamping, and losses from the surface thus pre- vented. Judging from several of the plot tests, soils in which the surface layer is light and pervious, with increasing density in the subsoil, will present real difficulty; but a soil with a dense surface layer and decreasing density with depth may be treated with much more assurance of success. Where the struc- ture provides for ready diffusion of gas into the subsoil, 15 or even 10 pounds per square rod has proved satisfactory (see data in tables 6, 7, and 8). Such soils are usually coarse to intermediate in texture and capable of surface com- paction for sealing. Spacing in them may usually be widened, with consider- able saving in application cost. In all soils, localized regions of restricted permeability should be subsoiled or deep plowed and then thoroughly packed, particularly on the surface, to provide a proper seal. Herbicidal Use op Carbon Disulfide 45 In fine-textured clay and adobe soil types, use of carbon disulfide in weed control should be preceded by plot studies to determine the adaptability to treatment. Some such soils, if reduced in moisture content by growing a vigor- ous crop, and then plowed and disked thoroughly to produce a deep mulch, may be treated with success when dry. With others it has proved best to pro- ceed as in preparation for seeding in the spring and, maintaining the mulch, to treat with carbon disulfide as soon as the soil becomes warm, in early summer. Even fine-textured soils of the more fertile types have sufficient structure to be pervious to vapor while moist ; in these, the obtaining of proper conditions for surface sealing is more important than reducing the soil moisture content. If the drying of a fine-textured soil into clods makes it impracticable to prepare a dust mulch, then treatment of the moist soil is the most feasible procedure. Where a water table maintains a high content of soil moisture in the subsoil, one should try carbon disulfide with caution on a small scale before using it in quantity. Depth of injection is another problem that must be related to local condi- tions. In general, the deeper the treatment the less the surface loss. This rule, however, must be interpreted to meet each situation. If the weeds have shallow roots, and the topsoil is moist, and the injection is very deep, resprouting may be expected. Under these circumstances, treatment to a depth of only 4 inches has been suggested. Such treatment, however, is bound to suffer sur- face loss. Where possible, it seems better to inject 6 inches deep or deeper, and to destroy resprouting crowns by plowing, disking, cultivating, or knifing after the deeper roots have been killed ; in fact, such a practice is recommended in some regions. In dry topsoil the rule is to treat as deeply as convenient, not to exceed 16 inches. In such cases there should be a dust mulch to prevent surface loss. PRACTICAL USE OF CARBON DISULFIDE As was stated in the Introduction, carbon disulfide has long found practical use in controlling perennial weeds. This bulletin aims to describe clearly the various factors that determine the effectiveness of this chemical and to point out certain limitations. Physiological studies have been discussed, and soil investigations described in detail. Season of Application. — According to tests the optimum season for apply- ing carbon disulfide is summer, especially in regions of very cold winters and wet heavy soils. Admittedly, results with the standard dosage are satisfactory at any season (p. 5) in the more temperate climates, and successful control has followed injection even through a layer of frozen soil. Comparative tests on one soil throughout the year show, however, that less chemical is required in summer. Where the standard dosage has produced satisfactory results in cold soils, these experiments indicate that lower dosages could be used with equal success in warmer seasons. On the other hand, if labor shortage or ad- verse soil conditions prevent application in summer, then fall, spring, or even winter treatment may be used, the dosage being increased to meet the situation. Dosage in Relation to Soils and Soil Type. — Table 19 gives the relations between dosage in terms of milliliters per injection, spacing, and over-all dosage in pounds per acre. Calculations were based upon a density of 1.26 grams per ml for liquid carbon disulfide at 20° C. OS En "a 85 £ 1 CO CO o o ■f ^< a a 05 <*> s "5 '- OS CM o •o cm && CO 1 -8E 8.8. CO •*»< >o cm eo CM o t^ t^ o GO ■^< eo CO 1 11 -r CO CM o -r eo -^ CM o ' - oa *— i "5 && LO CO CO CS« Is CB __ SO CO J^- CM 3 -r c^ is. co co i—* 9 O O) a co CO 00 CN» — d O ,£S § c Re. h *™^ os co ■H c>- uo m CO 3 a o CO I- CO r; uO r>. ^»< o O as O o CI IC CM o 1 11 CC •fl- "O CO CM -»f Cl CC Cl CD — CO CO no I- r^ o — ' IS H OS ■«* 8.8. ■*)* CO CO CI ~~ •^ 00 £ a 1 2 « p^ a cc o 0B CO ■C OB — S Cl 00 • *— < TT o ■T c; r>. o CO co CM C) ^H VH oo *> a :- .O oo rc r- -r 3 5 >o Cl c= t^ uo •— < I— 1 T— » Aft o J» M9 B0 -t r» or »-> CM eo CM Q <• ^c — CO "5 CO 3 t- BO '"' t« CO ■^ ^f o u t~ ~. ■>*< cc Cl OS t-~ CO CD CT3 3} o 5C '-£ 05 a> o3 - •- OJ c ^ co « ° os a co'o ^^ 03-^ |c t- c d.2 •— CO co oo c a ■3* a^ C 'S c"3 .2 c '^3 o3 _cs .£ "2 cu • -X 03-^ -*3 O CO 4; fi 2 U CD °*is .B-S 2 c ft Q. Hi ® p >; «H ^ O c ost; * 2H •e 8 P CB Herbicidal Use of Carbon Disulfide 47 A gallon of carbon disulfide weighs approximately 10.5 pounds. The stand- ard dosage (p. 5) is equivalent to about 2 gallons per square rod, which is approximate^ 20 pounds (3,200 pounds per acre). Most fertile agricultural soils have good structural characteristics — that is, adequate pore space per- vious to gas. With them, accordingly, the effect of soil texture upon diffusion need not be considered. Texture, however, determines adsorption and hence has a direct bearing on dosage. According to the plot experiments previously described, carbon disulfide may give satisfactory results on coarse-textured, open soils with as little as 10 pounds per square rod. In fine-textured soils the standard rate of 20 pounds will be required, particularly if the subsoil tends to be dense or very moist. This rate will be necessary in coarse-textured soils treated in winter or in an excessively moist condition. Dosage in 'Relation to Soil Moisture. — As already indicated (pp. 14 and 21) , soil moisture may be important in carbon disulfide treatment. Moist soils can be successfully treated, but only if the operator carefully considers soil moisture conditions in determining dosage and depth. In general, warm, dry soils are most favorable for the use of carbon disul- fide. This statement presupposes, however, that surface sealing is adequate. When the topsoil is dry, injection should be as deep as practicable (about 12 inches or 30 cm), and there should always be a dust mulch. Treatment to any depth through open clods will fail. If the topsoil is moist, injection should be at such depth that the crowns are killed ; otherwise the crowns should be plowed, disked, or scalped before they can reinf est the area. Deep treatment, with subsequent killing of crowns, minimizes surface loss and reduces the dosage needed. A dense plow sole that turns up cloddy if plowed dry may have to be broken while the soil is still moist and then worked down with the disk, har- row, and packer. Treatment may well be made to a depth of 8 inches or more, for the plowing and disking usually kill crown growth of weeds. Dosage in Relation to Compaction. — As the experiments have shown, any compacted layer restricts carbon disulfide diffusion. This is the reason for tamping or rolling the surface to prevent excessive surface loss. Where dense plow or cultivator soles exist, the treatment should be made through this layer; or the layer should first be broken up. Injection through a dense sole is practically impossible with the hand applicator, but may be accomplished with the tractor-drawn subsoiler. Great care should be exercised ; the surface must be thoroughly sealed behind the applicator. When the subsoiler is drawn through tough, refractory sole that is fairly dry, the cloddy soil can be worked down only with difficulty. Hardpan layers are usually dense enough to restrict root growth as well as to hinder gas diffusion. For this reason they usually do not affect carbon disulfide treatment. When, however, they cause a perched water-table condi- tion in irrigated soil, they may prevent success. Effect of Wind on Carbon Disulfide Treatment. — As much field experience has proved, heavy wind tends to bring about excessive surface loss of carbon disulfide and hence to reduce materially the effectiveness of treatment. In moderate wind, deep injection with good surface sealing should be used. The work should not be attempted in very windy weather. 48 California Experiment Station Bulletin 693 Response of Different Perennial Weeds. — Various weed species show dif- fering susceptibilities to carbon disulfide treatment. According to careful analysis of field-plot results, the major differences in response may relate to variations in soil requirements and root distribution more directly than to inherent differences in cellular behavior. It is difficult to retain a high vapor density of carbon disulfide in the top 4 inches of soil. Accordingly, if the perennial portions of the underground structures of a weed are largely re- stricted to this surface layer, killing may be difficult even though the cells are just as subject to the toxicity of the vapor as the cells of more easily killed species. Bermuda grass is such a weed. Uniform distribution of the vapor throughout a mass of wet clay soil is, likewise, seldom achieved. For this reason, it may be almost impossible to destroy species that characteristically inhabit such soil — hence, perhaps, the seeming difficulty in killing hoary cress in fine-textured soils. Camel thorn (Alhagi camelorum Fisch.) also has proved resistant to carbon disulfide. This pest often inhabits dense clay soils of saline character and resprouts from laterals at depths of 4 to 5 feet. The problem may be one of vapor distribution, not of any specific ability to tolerate carbon disulfide. Bermuda grass, nutgrass (Cyperus spp.), and other relatively shallow- rooted weeds have been hard to control with carbon disulfide in lawns, in sandy soils, or wherever the root distribution requires shallow treatment. One way of handling this situation has been to inject the chemical to a depth of only 2 or 3 inches and immediately cover the area with wet canvas or vapor- proof building paper. This covering is sealed at the borders of the infestation by burying it in a trench and covering it with soil. To prevent escape of the vapor the canvas must be kept wet, or the building paper must be sealed at the edges or lapped. Covering quickly with 2 to 4 inches of soil has also proved effective. Although this expensive method is applicable only to small areas, it will control patches of noxious weeds in lawns and golf greens where the only other possibilities would be complete soil sterilization or removal of the infested soil to a depth of 12 inches or more. Problems of Eeinfestation. — One problem inherent in the use of most tem- porary soil sterilants is that of reinf estation by seedlings. Usually, sometime during the development of a serious infestation, a seed crop has been allowed to mature ; and often a succession of seed crops have been plowed under or worked into the soil, where most w r eed seeds will remain viable for several years. Such seeds, if returned to the surface by subsequent plowing, will germi- nate. After carbon disulfide is injected, therefore, every effort must be made to prevent reinf estation. A few rules should be followed : (1) keep such areas in intertilled crops as long as feasible after treatment ; (2) avoid deep plowing, but cultivate, and watch the area very closely, killing all seedlings of peren- nial weeds even if hand-hoeing or pulling within the crop rows is required ; (3) avoid such crops as small cereals, grain hay, and flax, which allow reinf es- tation in the spring and early summer while ripening for harvest ; (4) if the land is needed to produce feed, use alfalfa, Ladino clover, or some similar crop that tends to hold weeds in check through shading and competition. All uncropped areas (for example ditchbanks, roadsides, and fence lines) should be watched carefully to prevent seed growth. Often such areas should Herbicidal Use of Carbon Disulfide 49 be treated with a permanent soil sterilant if they cannot be mowed or periodi- cally sprayed. Only a complete and comprehensive program of control can eradicate deep-rooted perennial weeds, and only a program aimed at eradi- cation will justify the use of carbon disulfide. Economics. — The practicability of using a weed-control method that costs over $100 an acre has often been questioned. On the other hand, past and present use of hundreds of carlots of carbon disulfide proves that the method is feasible in many places. Obviously there are situations where the expense would not be justified. Just as obviously, there are many places where carbon disulfide is the best and most practical chemical to use. Large acreages of densely infested land will seldom bear the cost of such treatment unless they can be immediately cropped with beans, cotton, vege- tables, or seed crops that render a quick cash return. Cropping and tillage methods are available for reducing the density of large infestations to a point where chemicals can be used to clean up the areas. (For discussion of such methods see : Ball et al., 1940.) After cropping or tillage programs, small local infestations of deep-rooted perennials may survive unobserved in the field, along fences, ditches, headlands, and in other out-of-the-way places. These immediately act as sources of reinfestation ; if left uncared for, they may spread and return the area to its original w r eedy state. For such a situation carbon disulfide is an ideal herbicide : it quickly kills the weeds and permits almost immediate use of the land. Though carbon disulfide may be utilized in a program of large-scale weed control, probably its most practical use is to prevent the spread of deep-rooted perennials in the early stages of infestation. Many thousands of acres of highly productive land have only small scattered infestations of wild morning-glory, Russian knapweed, hoary cress, or other noxious perennials. Where beans, cotton, melons, corn, vegetable and seed crops are being produced, these infes- tations not only reduce yields, but act as sources of seeds and root pieces for constant increase. Under such conditions carbon disulfide is the most valuable herbicide available : it can be applied at almost any stage in the crop-produc- tion program, it leaves no poisonous residue on the crop or in the soil (except as mentioned in footnote 2 and explained on p. 51) ; and the land can be re- turned to use in a minimum of time. The expense should be levied, not against the actual areas of infestation but against the total acreage jeopardized. Used in this way, carbon disulfide is probably the most valuable herbicide for combating perennial weeds. Effects upon the Soil. — An application of 20 pounds of carbon disulfide per square rod is equivalent to 3,200 pounds per acre. Of this, 84 per cent, or about 2,700 pounds, is sulfur; and, though some of this is lost by diffusion, the bulk is retained and oxidized to sulfuric acid, w^hich dissolves bases and is con- verted to sulfates. For this reason carbon disulfide tends to correct alkalinity, and renders some soils more fertile than before treatment. Being poisonous, carbon disulfide has a distinctly toxic effect upon soil organisms such as nematodes, wireworms, and bacteria. This amounts to a temporary partial sterilization, which usually favors subsequent growth of higher plants. Furthermore, with the death and decay of the roots of deep- rooted perennial weeds the soil is left permeable to moisture and is well 50 California Experiment Station Bulletin 693 aerated. Crops on areas treated with carbon disulfide are distinctly better than those on adjacent untreated areas. Figure 32 illustrates this result with barley. Pure carbon disulfide has no known deleterious effect upon the soil. Although this bulletin considers carbon disulfide only in connection with Fig. 32. — Eight, barley from carbon disul- fide treated land; left, barley from a similar area that was not treated. (From Bobbins, Crafts, and Kaynor, Weed Control, by permis- sion of McGraw-Hill Book Company, copy- right owners.) weed control, the chemical is invaluable in treating phylloxera of vines, armil- laria root rot (oak root fungus) of trees and vines, and other soil pests. It is being applied before the replanting of orchards and vineyards where trees or vines have died. It is also widely used as a fumigant and poison in con- trolling insects and rodents. Herbicidal Use of Carbon Disulfide 51 Brands of Carbon Disulfide. — As already mentioned, 17 there are two brands of carbon disulfide. Anchor is a clear, colorless form, relatively pure, whereas "Activated" is a reddish liquid containing" a chemical amendment that re- duces its vapor pressure. Developed to minimize vapor losses from the soil surface, this latter form may be the more effective of the two in warm, dry soils ; and it may allow a somewhat shallower in- jection because it develops less pressure in the soil and consequently has less tendency to leak through the surface seal. On the other hand, it tends to stay in the soil longer than Anchor and may delay planting of crops after treatment. The chemical amendment used in "Activated" carbon disulfide is itself toxic and may temporarily sterilize the soil against the growth of annual weeds. Applied during fall, the "Activated" form has been known to render treated soils toxic to annual weeds throughout the following winter. Methods of Application. — During the time car- bon disulfide has been used in weed control, many methods of applying it by hand have been devised. Early workers made holes by driving iron rods into the soil ; they poured the liquid chemical into the holes through funnels. Improvements on this method have involved use of a prod with a stirrup to enable the operator to remain standing. Use of this instrument, however, is limited to moist soils. Another method involves an electric drill with a long shank. Funnels have been extended for use in a standing position; and one such funnel has been fitted with a small 2-fluid-ounce measuring cup, which, upon being filled, can be tipped di- rectly into the throat of the funnel. A long-spouted teakettle has proved as handy as any other con- tainer for handling the liquid. Another hand method, mentioned in the early literature and still utilized, especially in the sod of grass pastures, is to force a spade or shovel to full depth of the blade into the soil and then to push forward on the handle, form- ing a crack between the blade and the soil. The liquid carbon disulfide is poured down this crack, whereupon the blade is removed and the soil tamped firmly. The most popular hand method now practiced makes use of the Mack Anti- Weed gun. This applicator (figure 33) consists of a hollow prod, a gallon container, and a piston pump with adjustable stroke for measuring the charge. The end of the prod is hollow, with many small perforations just above the tapered point. First, the prod is forced into the soil by means of the foot stirrup. Next, the handle is pulled up, filling the pump with carbon disulfide. 17 See footnote 2. Fig. 33. — Hand prod for ap- plying carbon disulfide. By raising and then pushing down on the top handle, a measured charge of liquid is forced into the soil through holes at the tip of the prod. (Courtesy of Wheeler, Eeynolds & Stauf- fer.) 52 California Experiment Station Bulletin 693 Then the handle is pushed down, forcing the liquid out into the soil through the perforations near the point. The collar, with its set screw on the handle shank, can be set to deliver any amount of carbon disulfide up to somewhat Fig. 34. — A, Complete equipment for power application of carbon disul- fide in weed control. The drag behind the subsoiler fills the furrows left be- hind the standards, and the water-filled roller packs the soil. B, Detailed view of improved applicator. Liquid from the tanks is metered through three pumps and delivered by force to the tubes at the rear of the subsoiler stan- dards. The pumps of this applicator are made of stainless steel to resist corrosion. (Courtesy of Wheeler, Eeynolds & Staffer.) more than 2 fluid ounces. One important precaution is to wash the machine out thoroughly with kerosene, diesel fuel, or light lubricating oil after use each day. If it is left full of carbon disulfide or empty without washing, the pump and valves will corrode, and the apparatus will be ruined. Early machine application was made with a subsoiler to which was attached a tank with a small delivery tube running down behind the subsoiler standard. Herbicidal Use of Carbon Disulfide 53 Calibration was done by setting a valve in this outlet line. Such a machine had two faults : First, the dosage varied somewhat with the height of liquid in the tank. This variation was compensated for somewhat by altering the speed of the machine, but calibration could not be accurate. Second, application was continuous ; if a fire occurred, as occasionally happens when the subsoiler strikes a rock and makes a spark, the whole application was liable to be lost. Improvements on this machine have involved the use of positive-acting metering pumps, which, running off the wheels of the subsoiler, discharge measured volumes of carbon disulfide at the desired intervals. Dosage is not affected by the level of liquid in the tank, nor by the speed of travel of the machine. One such apparatus is illustrated in figure 34. This machine has three standards and will cover up to 4 acres per day. The surface is sealed by the drag and w 7 ater-filled roller shown in figure 34A. Proper spacing of injection points, where applications are made by hand, has been accomplished by various means. One method has been to use guide lines of wire, marked by placing drops of solder at the proper intervals ; or rope lines marked by binding with twine or dark thread. Four lines are used : two head lines to space the rows, and two w T ork lines to space the injection points in the rows. The two head lines are stretched parallel, at a distance equal to the length of the work lines. The first w r ork line is stretched between the head lines with the knots or markers adjacent to those at the ends of the head lines. The second work line is stretched between the second markers on the head lines, but with one head-line marker lying halfway between the first and second marker of the w T ork line. This gives the staggering required to place the injections at the vertices of a triangle rather than at the corners of a square. Such spacing has proved, from long practice, to be best. Usually, according to the plot tests reported in this bulletin, the small difference in spacing represented by this staggering of the injections is of little signifi- cance. It probably is needed, however, to take care of the occasional situation where the dosage might be just on the border line of sufficiency. Particularly is this true where wider spacings and lower dosages than standard are used. Other methods of spacing involve, for example, the use of markers (as in planting row crops) or the use of marked rods. Convenience is the main con- sideration, and no one method has proved best under all circumstances. After an application by any method, the soil should be compacted to form a seal. Where the w r ork is done by hand, the operator should close the hole by pressing the soil down with his heel. After treatment, a garden rake should be used to fill in the holes and cracks, and the area rolled as in sowing a law 7 n. Where large areas are being prepared for carbon disulfide application, plowing followed by thorough disking will provide the mulch needed to make possible a proper sealing by rolling. If the topsoil is dry, the disking should be at least 10 inches deep; and the soil should be worked sufficiently to provide a dense dust mulch. Such preparation should be done several weeks before application because time must be allowed for regrowth of the weeds. Plowing or disking immediately before treatment scatters root segments, many of which might escape notice if not allowed to sprout. Precaution. — Carbon disulfide vapor is inflammable and burns with a scarcely visible blue flame. Since a mixture of the vapor with air is explosive, 54 California Experiment Station Bulletin 693 the liquid must be handled carefully. Empty drums containing this explosive mixture are particularly dangerous. The combustible vapor of carbon disul- fide may be ignited by the discharge of static electricity generated by the motion of a rubber-tired vehicle. A man walking through stubble or across a dry, cloddy field may generate enough electricity to cause a spark as he touches the bung or spigot on a drum. The operator should ground each drum from which the chemical is to be withdrawn, by placing a chain across it with one end buried in the ground. Even the flow of the liquid from a spigot has caused a spark and ignited the vapors and liquid. Smoking must be prohibited while working with or near carbon disulfide. As is evidenced by its use in fumigation, the vapor is poisonous. It should not be inhaled, and all handling of the liquid for weed control should be done out of doors. SUMMARY Carbon disulfide is a volatile liquid widely used for controlling deep-rooted plants. When injected into the soil it forms a toxic gas, which permeates the pore spaces and renders the soil atmosphere poisonous to plant roots. The standard treatment, evolved over the years, is to inject 2 fluid ounces of liquid in holes 18 inches apart each way to a depth of 6 to 8 inches. Effectiveness of this herbicide is determined by many factors, mostly re- lated to the soil. Rapid diffusion of the gas is favored by high soil tempera- ture, low moisture content, and loose, open structure. Of these factors, structure (under most field conditions) predominates. To prevent losses of ..the vapor from the surface is just as essential as to obtain uniform distribution through the soil. For this reason, the surface soil is sealed after an injection. Compaction by rolling and tamping is part of the standard procedure. Treatment followed by sprinkling or made after early fall rains is also common. Application methods depend on the soil relations mentioned above. Depth of injection depends upon soil moisture, which affects sealing, and upon weed species. Shallow injection is used where the surface soil is moist and can be effectively sealed; deep injection where the topsoil is dry. Effective mulching is necessary in dry soil. Spacing of injection points and dosage are interdependent. Wide spacing is economical, but only under favorable conditions of soil temperature and structure can the distance be greater than the standard 18 inches. Where conditions are adverse, spacing may be as close as 12 inches. Dosage on an area basis is determined by soil type and structure, weed species, and the conditions that determine sealing. Warm, coarse-textured, pervious soils that can be effectively sealed may require only 10 pounds per square rod, or 1,600 pounds per acre — just half the standard amount. In moist, fine-textured soils and under adverse temperature conditions, dosage may be increased to 25 or even 30 pounds per square rod. Under such condi- tions the 2-ounce injection is usually retained, but the spacing is reduced. Though carbon disulfide has been used with success at all seasons, summer is the most favorable time from the standpoint of soil temperature and moisture. Plow sole is the commonest soil structure that interferes with the treat- Herbicidal Use of Carbon Disulfide 55 ment ; if dense and wet, it may be almost completely impervious to the vapor. As a rule, the plow sole should be broken up before application. Where this cannot be done, soil moisture should be reduced by growing a vigorous annual crop, and injection should place the liquid on top of the dry sole beneath at least 10 inches of well-pulverized dust mulch. Treatment through a mass of open clods is useless. Hardpan because of its position does not interfere with weed control by carbon disulfide, for hardpan that can be penetrated by roots is usually per- vious to the gas; one that cannot be penetrated prevents downward diffusion and intensifies the killing action. Carbon disulfide should not be applied during heavy wind. Light wind does not interfere if the surface sealing is adequate. Different weed species respond differently to carbon disulfide, primarily because of different soil requirements and root distribution of the weeds. Hoary cress in wet, fine-textured soils or in peat is hard to control. Camel thorn, in addition to habitating clay soils, is deep-rooted. Bermuda grass, nut- grass, oxalis, and other shallow-rooted weeds require special methods. After shallow injection or sprinkling on the surface, such weeds are covered with gastight paper or wet canvas that is covered with soil at the edges. Another method, often used with wild morning-glory in moist soils, is to inject to a depth of 4 inches, sealing the surface carefully. After the deep roots have been killed, any surviving crowns are destroyed by scalping, disking, or knifing, followed by thorough cultivation. Such a treatment may be neces- sary even after injection to 6 or 8 inches if the topsoil remains moist. Carbon disulfide may be applied by several hand methods, such as the use of the Mack Anti-Weed gun ; or by machine, using a modified subsoiler. Though seldom justified in very large-scale weed control, prompt use of carbon disulfide is recommended to prevent the spread of small infestations ; for cleanup after large-scale cultural or cropping programs ; and for general use around gardens, nurseries, landscaped areas, and the like. For such pur- poses it is the most effective herbicide for controlling deep-rooted perennial weeds. It has proved invaluable in large areas devoted to beans, and to vege- table and seed crops where weeds annually cause heavy losses if not controlled. 56 California Experiment Station Bulletin 693 LITERATURE CITED Ayres, J. C, II. W. Hulbert, and C. B. Ahlson. 1926. Idaho weeds. How to know and control them. Idaho Agr. Col. Ext. Bui. 65:1-74. Ball, W. S., A. S. Crafts, B. A. Madson, and W. W. Robbins. 1940. Weed control. California Agr. Ext. Cir. 97:1-111. (Revised by R. N. Raynor.) Barnum, C. C. 1923. The control of wild morning glory. California Agr. Exp. Sta. Cir. 256:1-22. (Out of print.) Beers, C. W. 1913. Weed exterminator. California Cultivator 41(22) :533. Buehrer, T. F. 1932. The movement of gases through the soil as a criterion of soil structure. Arizona Agr. Exp. Sta. Tech. Bui. 39:1-57. Bywaters, M. F., and A. G. Pollard. 1937. Studies in soil fumigation. I. Preliminary observations. Ann. Appl. Biol. 24: 883-94. Fleming, W. E. 1923. Fumigation of potting soil with carbon bisulfide for the control of the Japanese beetle. New Jersey Agr. Exp. Sta. Bui. 380:1-45. Fleming, W. E., and F. E. Baker. 1935. The use of carbon disulfide against the Japanese beetle. U. S. Dept. Agr. Tech. Bui. 478:1-91. Frost, K. R. 1932. Experiments in weed eradication by machine applied chemicals. Agr. Engin. 13: 79-80. Gastine, G., G. Couanon, and L. Gastine. 1884. Traitement des vignes phylloxerees. Emploi du sulfure de carbone contre le Phyl- loxera. [Treatment of phylloxera of vines. Use of carbon disulfide against phyl- loxera.] 276 p. G. Masson, Paris. Hagan, R. M. 1941. Movement of carbon disulfide vapor in soils. Hilgardia 14(2) : 83-118. Hannesson, H. A. 1945. Movement of carbon disulfide vapor in soils as affected by soil type, moisture con- tent, and compaction. Hilgardia 16(10) : 503-10. Hickman, J. B. 1922. Hoary cress. Weeds of California and methods of control. California State Dept. Agr. Mo. Bui. 11(2-3) : 286-87. 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Eapport sur les experiences et sur les applications en grande culture effectuees en 1877. [Treat- ment of grapes affected by phylloxera with carbon disulfide. Report on experiments and on field applications carried on in 1877.] 171 p. P. Dupont, Paris. O'Kane, W. C. 1922. Diffusion of carbon bisulfide in soil. New Hampshire Agr. Exp. Sta. Tech. Bui. 20:1-36. Penman, H. L. 1940. Gas and vapour movements in the soil. I. Diffusion of vapours through porous solids. Jour. Agr. Sci. [England] 30:437-62. Peterson, Wm., and D. C. Tingey. 1928. Weeds: losses, agencies in distribution, nature, and control. Utah Agr. Exp. Sta. Cir. 71:1-52. Rogers, C. F. 1928. Canada thistle and Russian knapweed and their control. Colorado Agr. Exp. Sta. Bui. 348:1^4. Rogers, C. F., and Ira Hatfield. 1929. Carbon disulfide for eradication of perennial weeds. Colorado Agr. Exp. Sta. Bui. 347:1-23. Shaw, C. S., and G. B. Bodman. 1928. The plowsole. 4 p. California Agr. Exp. Sta. Soils Division. (Mimeo.) Simmons, P., and G. W. Ellington. 1926. The discovery of the insecticidal property of carbon bisulfide. Science 64(1657) : 326-27. Spence, H. L. 1937. Eradicating perennial weeds with carbon bisulfide. Idaho Agr. Col. Ext. Bui. 106: 1-10. Spence, H. L., and H. W. Hulbert. 1935. Idaho perennial weeds. Their description and control. Idaho. Agr. Col. Ext. Bui. 98:1-20. Taylor, A. L. 1939. Efficient spacing of soil fumigants for field applications. Helminthol. Soc. Wash. Proc. 6(2): 62-66. Walker, Ernest. 1906. Suggestions upon care of apple orchards. Arkansas Agr. Exp. Sta. Bui. 91:141-210. Wilcox, E. V. 1909. Carbon bisulphid for killing weeds. Hawaii Agr. Exp. Sta. Press Bui. 25:1-4. 16m-12,'45(5035)