THE UNIVERSITY OF ILLINOIS LIBRARY 630.7 II6b co A6RICULTURAL UBBARY UNIVERSITY OF ILLINOIS Agricultural Experiment Station BULLETIN No. 233 SWEET CLOVER FOR NITRATE PRODUCTION BY ALBEET LEMUEL WHITING AND THOMAS E. RICHMOND URBANA, MAY, 1921 CONTENTS OF BULLETIN No. 233 PAGE INTRODUCTION 255 Characteristics that Make Sweet Clover a Valuable Green-Manure Crop . . 255 Studies by Other Investigators on Sweet Clover as a Green Manure .... 256 PLAN OF INVESTIGATION 258 . SWEET CLOVER FOR NITRATE PRODUCTION ON BROWN SILT LOAM 259 Results Obtained at Urbana, University North Farm, 1918 259 Results Obtained at Minonk Field in 1919 -. . .261 SWEET CLOVER FOR NITRATE PRODUCTION ON GRAY SILT LOAM ON TIGHT CLAY '... .263 Results Obtained at Toledo Field in 1919 263 Results Obtained at Newton Field in 1919 264 Results Obtained at Oblong Field in 1919 264 WEIGHTS AND NITROGEN CONTENT OF SWEET CLOVER . . . '. 266 CONCLUSIONS . ..267 SWEET CLOVER FOR NITRATE PRODUCTION BY ALBERT LEMUEL WHITING, CHIEF IN SOIL BIOLOGY, AND THOMAS E. EICHMOND, ASSOCIATE IN SOIL BIOLOGY INTRODUCTION Sweet clover offers the most promising indications that the problem of supplying a cheap source of nitrate nitrogen for farm crops can be solved. With the exception of manure, the animal sources of nitrogen, such as tankage, dried blood, fish meal, guano, and others, are limited in quantity, and their price is such that to use any of them except manure, as a source of nitrogen for crop growth, is out of the question. The chemicals, Chili saltpeter or sodium nitrate, calcium nitrate, ammonium nitrate, ammonium sulfate, ammonium phosphate, calcium cyanamid, and others are likewise too expensive to apply to soils on which staple crops are to be grown. The need for special studies to determine the value of sweet clover (Melilotus alba, sometimes called bee clover) for nitrate production, when employed as a green manure, was made apparent by the pre- liminary studies made in 1917 on the University North Farm, at Urbana. Consequently extended investigations were conducted dur- ing the season of 1919 on certain of the outlying experiment fields in addition to continuing those already initiated at Urbana. CHARACTERISTICS THAT MAKE SWEET CLOVER A VALUABLE GREEN-MANURE CROP Sweet clover possesses certain distinct characteristics which give it a marked superiority over other crops as a green manure for nitrate production. The most important of these characteristics are the fol- lowing : 1. Adaptability to a wide variation of climatic and soil condi- tions, provided the soil is not acid and inoculation is assured. 2. Hardiness to cold and drouth, and resistance to diseases and weeds. 3. Production of a large tonnage per acre at a time when it can best be utilized for soil enrichment; for example, its rapid growth in the spring of the second year and during the first year. 255 256 BULLETIN No. 233 [May, 4. Rapid decomposition in the green condition. 5. Its deep-rooting habit which enables it to assist in rendering impervious subsoils more porous and to feed at great depths. 6. Ability to obtain plant food from insoluble minerals more readily than other crops. A crop that possesses the combined ability to grow rapidly in early spring and to decompose rapidly, makes an ideal green manure for feeding such a crop as corn. By virtue of its rapid rate of growth and large dry-matter production, it conserves large amounts of soluble plant food, especially at times when the soil would otherwise suffer heavy losses. It possesses in common with other legumes, when prop- erly inoculated, the property of utilizing atmospheric nitrogen. This plant stores in its roots large amounts of reserve food (especially nitrogen), which becomes available upon growth in the spring of the second year. The leaves are very tender and decay rapidly when the crop is turned under green. The roots and stems decompose at slower rates than the tops. Thus the three different parts represent three sources of nitrogen that furnish three rates of nitrate production. Once a weed by law, sweet clover is now coming to be regarded by soil scientists as the greatest crop for soil improvement. Few soils are able to meet large crop demands for nitrate nitrogen. Since sweet clover will supply nitrogen at an insignificant cost, it could well be utilized as a green manure by most farmers. Wide use of it for soil improvement is sure to be made in many countries. STUDIES BY OTHER INVESTIGATORS ON SWEET CLOVER AS A GREEN MANURE The literature concerning sweet clover as a green manure and as a nitrate producer is indeed very meager. Having long been classed as a weed, it has received but very little consideration by agricultural investigators. Orth, in 1890, was the first scientist to test green sweet clover in comparison with manure. Eight tons of stable manure were found to be decidedly inferior to green sweet clover for the production of pota- toes in his experiment. The green sweet clover also gave large in- creases over no treatment in the yields of oats and corn. The late Doctor C. G. Hopkins early recognized the promise offered by sweet clover as a green manure. In his book on ' ' Soil Fertility and Permanent Agriculture ' ' he says of it : but it does give promise of great value as a green-manure crop, and it seems appropriate to emphasize the fact that 6.4 tons of dry matter furnish as much humus-forming material and as much nitrogen as would be furnished by 25 tons of average farm manure." SWEET CLOVER FOB NITRATE PRODUCTION 257 The reader is requested to view the results herein reported in the light of the prediction made by our former colleague. So firm was the belief of Doctor Hopkins in the importance of sweet clover that he introduced it into the rotations on most of the forty experiment fields thruout the state. It is interesting to note that he selected sweet clover as the crop upon which to base his foundation work for restoring the soils of Greece. Maynard, in 1917, studied the decomposition of sweet clover for nitrate production under greenhouse conditions and found it to be very rapid. The following quotation from his review of the literature is of interest in connection with the studies reported herein: "No record has been found of any study of the rate of decay of sweet clover as a green manure." The work of Mertz deserves special consideration as it represents an experiment especially designed to study the value of green manures, both legumes and non-legumes, the latter being reinforced with com- mercial nitrogen. The experiment was conducted in southern 'Cali- fornia. The green manures were grown in the winter and were fol- lowed by corn, potatoes, cabbage, beets, sorghum, and Sudan hay dur- ing the six years of the experiment. The legumes proved much superior to the non-legumes as green manures. Of the legumes, bitter clover (Melilotus indica}, an annual variety of sweet clover, was the most promising, both from the standpoint of vigor and of available supplies of seed. Its value as a green manure is brought out in the following statements by Mertz in Bulletin 292 of the California Ex- periment Station: "The use of Melilotus indica as a green-manure crop resulted in a gain over adjacent non-legume plots of nearly twenty bushels of shelled corn per acre. This is certainly indicative of the manurial value of this plant." Another statement which serves well as a direct answer to those who desire data on comparative tests of commercial nitrogen and "home-grown" nitrogen is as follows: "It is significant that the non-legume plot receiving nitrogen at the rate of 163 pounds per acre, gave for the six years an average increase of 51 percent, while the plot where Melilotus indica was used as a green manure showed an increase of 57 percent without the addition of any nitrogenous fertilizer. ' ' The forms of nitrogen used in the above experiment were sodium nitrate during the years 1910-1913, and dried blood during 1914-1915. The annual application of the sodium nitrate was 1,080 pounds per acre and of the dried blood, 1,200 pounds. The annual expense of the nitrogen in the sodium nitrate was from $30 to $35 per acre. In addition, it might be said that the legume plots had gained 400 pounds of total nitrogen per acre over the non-legume plots, to a depth of two feet, according to the analysis reported. No nitrate determina- tions were reported in this work. 258 BULLETIN No. 233 PLAN OF INVESTIGATION The investigation reported herein consisted of a study of the nitrate content of the surface soil, during the important periods of the season, of plots where sweet clover was used, where stable manure was applied, and where there was no treatment. The sweet-clover and manure plots receive limestone at the rate of two tons, and phosphorus at the rate of one ton, per acre, once in four years. One untreated plot at each end of the series was selected for a study of the nitrate content of the natural soil subjected to crop rotation only. The plots which are designated as sweet-clover plots receive wheat straw in accordance with the practice of returning to the soil the crop residues in the grain system of farming. It is found in actual practice, however, that manure carries large amounts of straw, perhaps as much as is applied in the grain system of farming. Since straw is recognized as a rela- tively unimportant factor in nitrate production, this variation in the treatment of the plots does not interfere with their value as a basis for comparing the two sources of nitrogen. In order that the study might include the conditions and soils of northern, central, and southern Illinois, representative fields in these sections of the state were selected for study. These fields are located at Urbana, in Champaign county; at Minonk, in Woodford county; at Oblong, in Crawford county; at Toledo, in Cumberland county; and at Newton, in Jasper county. The soil of the first two is a brown silt loam, the 'common soil of the corn belt, and that of the last three is gray silt loam. Samples of soil were collected as described in Bulle- tin 225 of this station and analyzed by a recognized method. Corn occupied all the plots from which these results are reported. No better crop could have been selected to have growing on the soil for the purpose of this study than corn because of its large demand for nitrate nitrogen during a limited period of time. This demand most frequently occurs from June 25 to July 15. If the nitrogen is present, a very large percentage of the amount required to produce the crop will be taken up in a few days. Thus it will be seen that these investi- gations were conducted under conditions typical of those that will be met in a practical solution of the nitrate problem. In most cases, the dry and the green weights of the sweet clover and the field weights of the manure are reported. The analyses of the clover tops are also reported. In considering the comparisons between the sweet-clover plots and the manure plots, it should be borne in mind that the manure used varies considerably in quality, and that it is not applied under ideal conditions to conserve its greatest value. SWEET CLOVER FOR NITRATE PRODUCTION 259 SWEET CLOVER FOR NITRATE PRODUCTION ON BROWN SILT LOAM The layman often thinks of brown silt loam as black, rich, corn- belt soil, that has no need for additional organic matter or for increased nitrate production. This is not a safe assumption, as much of the best corn-belt soil responds readily to additions of active organic matter. The total nitrogen content of a soil has been erroneously regarded as indicative of its ability to supply nitrate nitrogen to crops. If one soil has 5,000 pounds of nitrogen in the surface 6% inches and another has 2,000 pounds, there has been little discussion as to which would supply the most nitrate to a growing crop. The total nitrogen is not, however, a criterion as to the availability of the nitrogen if the nitrogen happens to be in a form that is resistant to decay. Many peat soils are poor nitrate producers, yet they contain as high as 30,000 pounds of nitrogen per acre. A small amount of active nitrogen in a poor soil may produce larger crop yields than a much larger amount of inactive nitrogen in a rich soil. It is most important to keep as much nitrogen as possible in the active state of decomposition at the time when it is needed by crops. If this is accomplished by inoculated legumes, then the total nitrogen will at the same time be taken care of. RESULTS OBTAINED AT URBANA, UNIVERSITY NORTH FARM, 1918 -The results of the first special study in which sweet clover was plowed under as a green manure are presented in Table 1. This work was conducted at Urbana on Series 300 of the University North Farm, in 1918. TABLE 1. NITRATE NITROGEN IN SOIL GROWING CORN IN 1918: SERIES 300, UNIVERSITY NORTH FARM, URBANA Pounds per acre in 2 million pounds of surface soil (about to 6^ inches), water-free basis Plot Treatment Nitrate nitrogen in soil on date .of sampling Apparent utilization of nitrogen in 7 days, June 24 to July 2 June 10 June 17 June 24 July 2 July 12 Aug. 2 301 302 303 304 305 306 307 308 309 310 None Ibs. 22.9 22.7 26.6 62.0 28.9 63.6 19.0 55.8 28.8 90.7 Ibs. 34.8 52.6 32.6 66.3 33.0 97.0 34.4 63.5 29.4 96.6 Ibs. 30.2 47.8 38.0 48.9 33.0 61.5 33.9 49.1 43.6 81.5 /6s. 28.2 26.0 22.2 16.8 15.9 21.4 23.0 21.5 27.2 42.9 Ibs. 30.3 40.0 29.9 20.7 15.6 16.0 23.5 16.5 20.4 30.2 Ibs. 26.8 29.3 21.1 19.3 27.9 22.7 22.0 20.0 16.2 20.8 Ibs. 2.0 21.8 15.8 32.1 17.1 40.1 10.9 27.6 16.4 38.6 R,SwCl M RL, Sw Cl ML RLP,SwCl MLP RLPK, Sw Cl. . MLPK MxLPx R= Residues Sw Cl = Sweet clover green manure M= Manure L= Limestone P Rock phosphate or bone meal K Potassium X=5 times usual application 260 BULLETIN No. 233 [May, The figures given represent pounds of nitrate nitrogen per acre in the surface 6% inches. In the last column are shown the apparent amounts of nitrogen that were utilized by the corn crop in seven days. These are introduced to emphasize the fact that large demands are made upon the soil nitrate by the corn crop. These figures make no allowance for the nitrate that may have been produced by the soil dur- ing this period and assimilated by the crop. They may be considered as representative of the minimum amounts taken up by the crop, since the rainfall was not sufficient to reduce the nitrate content, and the utilization of nitrate by the bacterial activities is rapidly decreasing at this time of the year in normal Illinois seasons. No dry-matter nor nitrogen determinations of the sweet clover were made the first year of this study. The manure was applied on January 7 and 8, 1918, in the following amounts, and remained on the surface until it and the sweet clover were plowed under on May 15. Tons of manure applied, field weight Plot 303 13.41 Plot 305 15.32 Plot 307 18.10 Plot 309 18.12 Plot 310 80.00 While it may be convenient farm practice to apply manure in this way, it must be admitted that it is poor economy because of the losses of plant food from leaching. An idea of the serious losses that may result are given on page 44 of Bulletin 225. The point is clear, however, that under the systems practiced, green sweet clover was decidedly superior to farm manure for nitrate production, even when the applications of manure were as high as eighteen tons. Plots 306 and 307 are comparable except for the fact that 307 has received large applications of manure for a number of years while sweet clover was on this series for the first time this year. Since no such increases in nitrate production as are recorded here are obtained from phos- phorus or limestone applications on this soil, the large increases must be attributed to the green sweet clover. The results obtained in 1919 on Plots 406, 407, 408, and 409 of the Davenport series on the University North Farm at Urbana are presented in Table 2. It is interesting to note that altho the stands of sweet clover on Plots 406 and 408 were not high when they were plowed under for corn on May 2 (see Table 7), noticeable increases in nitrate had neverthe- less occurred by May 26. The manure was applied January 3, 1919, and the land was plowed for corn on May 2. The weights of manure reported are for manure of ordinary quality and condition as hauled out from a city stable and weighed as applied to the soil. 1921] SWEET CLOVER FOB NITRATE PRODUCTION 261 TABLE 2. NITRATE NITROGEN IN SOIL GROWING CORN IN 1919: SERIES 400, UNIVERSITY NORTH FARM, URBANA Pounds per acre in 2 million pounds of surface soil (about to 6 2 /> inches), water-free basis Field Weight of sweet clover (tops only) Nitrate nitrogen in soil on date of sampling weight April Plot Treatment of man- ure Green Water- free 26 (before May 26 June 7 June 12 June 27 July 3 July 10 July 18 plow- ing) tons tons tons Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. 406 RLP, Sw Cl 5 85 1 06 9 6 25 5 16.4 23.0 39.5 24.2 29.2 12.5 407 MLP. 19.79 25.0 14.6 15.5 28.9 30.2 8.3 13.5 10.7 408 RLPK,SwCl 6 82 1 14 26 4 48 5 11 5 46 1 66 9 26 4 30.9 15.9 409 MLPK .... 19.81 23.0 12.5 21.8 20.7 28.4 12.7 16.2 11.5 When the land was plowed, striking differences were seen in the moisture content of the sweet-clover plots and the adjacent manure plots. The sweet clover had reduced the moisture on Plot 408, 28 tons per acre, as shown by a comparison of that plot and Plot 409, where manure had been applied. The lower moisture content is usually an advantage in this climate at this time of year, because of the possi- bility it offers for earlier plowing. If the soil were dry it would be wise to plow under the green sweet clover earlier in order not to rob the soil of undue amounts of moisture. The heavy rainfall occurring between May 26 and June 7 accounts for the low nitrate content found on June 7 (Table 2). The water-free weights of the sweet clover include only the spring growth of tops. The roots, the fall growth, and the residues left on the ground from the fall growth are not reported. The nitrogen con- tent of the spring growth and other data on sweet-clover yields will be found in Table 7. It is interesting to note the sudden drop in the nitrate figures, which is always coincident with the rapid growth of the corn crop. This year it occurred between June 27 and July 3. From this year's results on the brown silt loam it is evident that nitrate production on this type of soil is greatly enhanced by the plowing under of green sweet clover. Another important fact is that approximately one ton (water-free basis) of spring growth of sweet- clover tops, together with the roots and fall residues, furnished as much nitrate as 19.8 tons of average farm manure. RESULTS OBTAINED AT MINONK FIELD IN 1919 The results reported in Table 3 were obtained at the Minonk field, in Woodford county in 1919. The soil on this field is somewhat uneven, being practically a heavy clay on Plots 101 to 106 and a brown silt loam on Plots 107 to 110. 262 BULLETIN No. 233 [May, The manure had been applied on September 13, 1918. On April 30, 1919, an excellent stand of sweet clover on Plot 108 and the manure on Plot 104 were plowed under. The first samples of soil were taken April 26. The relatively high nitrate content of Plot 108 is of sig- nificance. Either the protection afforded the soil by the presence of the fall and spring growths of the sweet clover, or the decomposition of TABLE 3. NITRATE NITROGEN IN SOIL GROWING CORN IN 1919: MINONK FIELD Pounds per acre in 2 million pounds of surface soil (about to 6% inches), water-free basis Plot Treatment Field weight of manure Weight of sweet clover (tops only) Nitrate nitrogen in soil on date of sampling Green Water- free Apr. 26 (before plowing) May 30 July 1 July 14 Aug. 12 104 105 108 110 Manure, LP None tons 20.89 tons tons Ibs. 21.0 24.5 38.7 10.1 Ibs. 15.4 9.3 76.8 8.1 Ibs. 7.9 10.8 67.2 11.8 Ibs. 7.5 14.5 51.3 18.6 Ibs. 7.6 12.4 143.6 11.8 SwCl,LP . None 9.17 1.54 the fall growth, accounts for the 38.7 pounds of nitrate nitrogen found before plowing. By comparing the manure plot and the sweet-clover plot, it will be seen that the manure was greatly inferior to the sweet clover for nitrate production. It was only a little better than were the check plots, which from all appearances were suffering from nitrogen starvation during the critical period of crop growth. The amount of manure applied was 20.89 tons per acre. The water-free weight of the spring growth of sweet clover tops plowed under was 1.54 tons. The large nitrate production in August on the sweet-clover plot is worthy of note and suggests the advisability of plowing under the sweet clover at an earlier date, which would be of advantage in effect- ing a somewhat earlier decomposition and thereby avoiding the detri- mental effect of the first stages of decomposition upon the germinating crop, in conserving soil moisture, and frequently in promoting farm work in the rush season of spring. Another suggestion would be the utilization of the surplus nitrate by seeding a catch crop, such as rye or wheat, in the corn. This shows what is possible in the way of nitrate production where a large fall growth is allowed to remain on the soil and, together with the spring growth, is plowed under for corn. There was 185.5 pounds of nitrogen turned under in the spring and fall tops alone, in addition to the 38.7 pounds of nitrate nitrogen present in the soil. The value of green sweet clover on this field is apparent and needs no further explanation. SWEET CLOVER FOR NITRATE PRODUCTION 263 SWEET CLOVER FOR NITRATE PRODUCTION ON GRAY SILT LOAM ON TIGHT CLAY The need for increasing the active organic-matter content of the gray silt loam on tight clay is very apparent and quite generally recognized. However, the very low nitrate efficiency of this type of soil is not so well understood. RESULTS OBTAINED AT TOLEDO FIELD IN 1919 The Toledo field is located in Cumberland county on the soil type designated as gray silt loam on tight clay. The manure was applied in the fall of 1918 and the sweet clover and the manure were plowed under on May 13, 1919. Corn was planted on May 30. The applica- tion of manure was 6.25 tons, while the water-free weight of green sweet clover of spring growth was 2.43 tons. The sweet clover grew taller than was desired because rains prevented earlier plowing. From Table 7, it will be seen that this was the tallest crop that was sampled on the nine fields studied. The nitrate results are given in Table 4. TABLE 4. NITRATE NITROGEN IN SOIL GROWING CORN IN 1919: TOLEDO FIELD Pounds per acre in 2 million pounds of surface soil (about to 6?^ inches), water-free basis Plot Treatment Field weight of manure Weight of sweet clover (tops only) Nitrate nitrogen in soil on date of sampling Green Water- free May 12 (before plowing) June 18 July 4 July 17 Aug. 18 104 105 108 110 Manure ,LP None tons 6.25 tons tons Ibs. 13.0 6.0 7.0 8.1 Ibs. 15.5 13.6 31.5 8.0 Ibs. 11.8 12.1 30.3 7.3 Ibs. 11.7 9.1 15.9 8.9 Ibs. 19.0 27.0 34.2 24.1 Sw Cl, LP . None 12.87 2.43 Nitrate production on the sweet-clover plot was considerably superior to that on the manure plot. The nitrate content of the check plots increased from July 17 to August 18, and it should be said that on these plots there was practically no crop to use the nitrate. The accumulation of nitrate on the checks between May 12 and August 18 was 16 and 21 pounds. The manure plot supported a good crop and increased 6 pounds, while the sweet-clover plot supported a good crop and increased 27 pounds. The sweet clover, even in the dry months of July and August, has demonstrated that its decomposition will supply nitrate in ample quantities for the corn crop on one of the poorest types of soil recognized. 264 BULLETIN No. 233 RESULTS OBTAINED AT NEWTON FIELD IN 1919 [May, At Newton the difficulties encountered were greater than at Toledo, and yet the value of sweet clover on this exceptionally poor soil was out- standing, as may be seen by reference to Table 5. The plowing under of the manure and sweet clover was delayed by rain, and planting was not possible until very late. But 2.18 tons of manure was applied, and it was put on in the winter. The water-free weight of green sweet clover of spring growth was 2.29 tons on May 10, but the land was not plowed until the first week in June, and the corn was planted June 13. The sweet clover was 22 inches high on May 10, and much taller on June 1. This more advanced development undoubtedly greatly reduced the immediate yields of nitrate. On the two check plots the amounts of nitrogen reported as pres- ent on August 18 represent nearly the total seasonal production, for only a very small amount was used by the crop. The amount of manure applied to the manure plot was too small to permit of a legitimate comparison of- that plot with the sweet-clover plot, but both these treated plots supported larger crops than the checks, and the sweet- clover plot at the same time accumulated striking amounts of nitrate nitrogen. TABLE 5. NITRATE NITROGEN IN SOIL GROWING CORN IN 1919: NEWTON FIELD Pounds per acre in 2 million pounds of surface soil (about to 6% inches), water-free basis Plot 404 405 408 410 Treatment Field weight of manure Weight of sweet clover (tops only) Nitrate nitrogen in soil on date of sampling Green Water- free May 12 (before plowing) June 18 July 4 July 17 Aug. 18 Manure, LP None tons 2.18 tons tons Ibs. 13.2 4.7 18.6 14.0 Ibs. 17 A 18.3 36.8 22.6 Ibs. 7.0 6.6 30.6 9.2 Ibs. 11.7 8.9 13.9 9.7 /6s. 16.0 21.7 78.2 25.9 Sw Cl, LP None 12.6 2.29 RESULTS OBTAINED AT OBLONG FIELD IN 1919 The Oblong field represents better soil than that found at Newton or Toledo. The weight of manure applied was 6.42 tons per acre, and the ap- plication was made late in the fall. The land was plowed May 17 and the corn planted May 31. Altho no sweet-clover samples were obtained from this field, it is believed that as much sweet clover was plowed under as at Newton. The crop of corn developed normally on the manure and the sweet-clover plots. On June 18 the manure plot was the highest in nitrate content, and apparently met the demands of the SWEET CLOVER FOB NITRATE PRODUCTION 265 crop satisfactorily. The nitrate produced from the sweet clover was ample, but was undoubtedly less than it would have been if the clover had been plowed under before it had become woody. Apparently little loss occurred in the manure applied on this field this year. The figures in Table 6 indicate the same condition of decomposition as that found on the Toledo and Newton fields. Earlier plowing would have furnished more nitrate and at an earlier period. Delayed plowing gives the plant time to render its nitrogen compounds more stable, and this slows down the rate of nitrate production, but the disadvantage in the slower nitrate production which results from allowing the clover to become more mature is in part offset by the greater amount of total nitrogen which the more mature clover contains and which will ulti- mately become available for plant use in the form of nitrate nitrogen. Because of losses which frequently occur one must calculate on a nitrate accumulation that will somewhat more than meet crop needs. Since the greatest losses of nitrogen from the soil generally occur in the spring, it is of particular advantage to have the nitrogen at this time of year in a form which will not leach readily. This means that it should be either in active organic form or in a growing crop. In the plowing under of sweet clover, little of the nitrogen so added to the soil is lost by leaching, even after plowing if it is done about May 1, TABLE 6. NITRATE NITROGEN IN SOIL GROWING CCRN IN 1919: OBLONG FIELD Pounds per acre in 2 million pounds of surface soil (about to 6J3 inches), water-free basis Plot 104 105 108 110 Treatment Nitrate nitrogen in soil on date of sampling June 18 July 4 July 17 August 18 Manure, LP Ibs. 53.2 16.8 39.1 11.2 Ibs. 22.1 27.6 19.2 16.1 Ibs. 20.0 14.2 16.3 15.7 Ibs. 21.0 27.5 58.1 29:0 None Sweet clover, LP . . . None . since the process of nitrification will not ordinarily produce large amounts of nitrate before 21 to 28 days have elapsed. As earlier stated, this first rapid production is mostly from the leaves. Later the stems and roots produce nitrate. They act as a reserve supply, or as a second application of nitrate, and on account of their composition are likely to suffer little loss from leaching, while the nitrogen of nitrate of soda, ammonium sulfate, and other soluble forms of nitrogen is readily lost under similar conditions. 266 BULLETIN No. 233 [May, WEIGHTS AND NITROGEN CONTENT OF SWEET CLOVER In Table 7 are found the results of studies made on samples of sweet-clover tops of spring growth of the second year, cut from one square yard. Every precaution was exercised to cause the results to represent the minimum. The Urbana, Minonk, and Joliet fields represent central and north- ern Illinois conditions, while the others represent southern Illinois conditions. The stands on all fields except at Urbana were excellent. Plot 406 at Urbana had the poorest stand. The earlier season in southern Illinois gives the sweet clover an opportunity to grow to a greater height in the same time than in central or northern Illinois. The southern fields would have been plowed earlier if rain had not inter- fered. The weights of material produced were much greater on the south- ern fields than on the central and northern fields, altho the percentages of nitrogen ran consistently lower. This is accounted for by the heavier, more woody growth, as indicated by the height. The smaller weights of material on the central and northern fields, as compared with those of the southern fields, are partially offset by the richer crop growth. The most important consideration is the weight of nitrogen in the crop on the acre basis. As shown in Table 7, the amount of nitrogen in the tops at sampling time varied from 98 to 196 pounds. The sweet-clover plot at Toledo produced 80.6 pounds of available nitrogen, including the nitrate nitrogen found on August 18 and the amount taken from the soil by a crop of 30.9 bushels of corn. At the time the TABLE 7. WEIGHTS AND NITROGEN CONTENT OF SWEET-CLOVER TOPS OF SPRING GROWTH: 1919 Acre basis Field Plot Date sample was taken Height Green weight Water content Water-free material Weight of nitrogen per acre Percent- age of water Weight of water Weight Percent- age of nitrogen Urbana. Urbana Urbana Minonk .... Joliet 408 408 406 108 508 108 408 108 108 April 24 May 2 May 2 April 26 April 29 May 9 May 10 May 1 May 1 inches 10 14 13 12 9.5 26 22 19 18 tons 5.07 6.82 5.85 9.17 7.96 12.87 12.60 12.70 8.71 perct. 82.2 83.5 81.8 83.1 81.9 81.0 81.0 86.1 82.2 tons 4.16 5.70 4.78 7.63 6.52 10.38 9.80 10.98 7.65 tons .95 1.12 1.06 1.54 1.44 2.43 2.29 1.76 1.66 perct. 4.60 4.50 4.62 5.35 4.60 4.03 4.10 3.67 4.00 Ibs. 83.3 100.9 98.1 164.8 133.3 196.4 188.5 129.6 124.8 Toledo Newton .... Raleigh. . . . Enfield 1921} SWEET CLOVER FOE NITRATE PRODUCTION 267 sweet clover on this same plot was sampled (May 9), it had accumu- lated in the tops of the second-spring growth 196.4 pounds of nitrogen, or nearly two and one-half times as much as was produced in the soil while the corn crop was growing, even with sweet clover to assist in the production. Further the untreated soil produced only 27 pounds of nitrate nitrogen up to August 18, and limestone and phosphorus together, while increasing nitrate production above that of the un- treated soil, were not able to cause increases sufficient to account for the large amount of nitrogen in the tops alone. This evidence indi- cates that a very large fixation of nitrogen by sweet clover takes place on the gray silt loam types. Probably the fixation is smaller on the - brown silt loam, but even there it is large under favorable conditions. In considering the merits of sweet clover, its ability to conserve nitrate is also a large factor that should not be overlooked. In some cases the conservation of nitrogen may be as important as its addition. Sweet clover, when seeded in small grains and allowed to grow until time to plow for corn, supplies large amounts of total nitrogen to the soil. It decomposes rapidly and furnishes a sufficient amount of nitrate nitrogen for a large corn crop. In these studies, the tops, on a water-free basis, averaged 4.38 percent of nitrogen for all the fields. This means 87.6 pounds of nitrogen per ton of dry weight. That the roots weigh as much or more than the tops at this time of year, is known, and the nitrogen content is not much lower than that of the tops. This would indicate that there is nearly twice as much nitrogen in the entire plant as these figures given for the tops show. At the Toledo field, where the sweet clover grew to a height of 26 inches before being plowed under, 2.43 tons of tops (water-free basis) con- tained as much nitrogen as 20 tons of average farm manure. CONCLUSIONS 1. Sweet clover, when plowed under as a green manure for corn, furnishes large amounts of nitrate nitrogen for that crop. The data on which this statement is based were obtained from experiment fields in northern, central, and southern Illinois, located on brown silt loam and on gray silt loam on tight clay. 2. The large nitrogen content of sweet clover indicates that a large fixation of nitrogen occurs on these soils. In the present study the sweet-clover tops averaged 87.6 pounds of nitrogen per ton of dry matter at sampling time, which, in some cases, was about the time of plowing for corn while in others it was somewhat earlier. 3. The data reported herein furnish positive information con- cerning the value of sweet clover, when used as a green manure, for solving the problem of adding to, conserving, and making available for crop purposes the nitrogen of the soil. UNIVERSITY OF ILLINOIS-URBAN*