UNIVERSITY OF ILLINOIS LIBRARY .T '.^ANA-CHAMPAIGN AGHICULTUP.r WON .CIRCULATING CHECK FOR UNBOUND CIRCULATING COPY UNIVERSITY OF ILLINOIS Agricultural Experiment Station BULLETIN NO. 145 QUANTITATIVE RELATIONSHIPS OF CARBON, PHOSPHORUS, AND NITROGEN IN SOILS (SECOND EDITION, MAY, 1912) URBANA, ILLINOIS, APRIL, 1910 CONTENTS OF BULLETIN No. 145 PACE A. Historical Resume 1. Carbon in Soils gi 2. Nitrogen in Soils 94 3. Phosphorus in Soils 96 4. Carbon and Nitrogen in Fundamental Rocks 103 B. Experimental Part 1. Mathematical (a) Influence of Age upon the Carbon-Nitrogen Ratio 105 (b) Carbon, Nitrogen and Phosphorus in Illinois Soils 108 (c) Factors for Calculating Organic Phosphorus no 2. Chemical (a) Analytic Results of Soil from Illinois South Experiment Farm.ni (b) Phosphorus Associated with the Matiere Noire 112 (c) Organic Phosphorus by Schmoeger's Method 119 C. Conclusions 121 D. Bibliography 123 QUANTITATIVE RELATIONSHIPS OF CARBON, PHOSPHORUS AND NITROGEN IN SOILS* BY ROBERT STEWART (A) HISTORICAL The literature on carbon, nitrogen and phosphorus in soils is voluminous. The resume given herewith by no means attempts to include all that pertains to these elements in the soil, but it is con- fidently believed that it fairly represents the literature pertaining to this particular phase of the subject. i. CARBON IN SOILS Carbon may exist in soils as inorganic and as organic carbon. The agricultural value of organic carbon, or organic matter, of soils has long been recognized by the practical husbandman, and the scientific man early recognized its value when the applications of science were made to agricultural problems. Mulder (i), in 1844, made an elaborate study of the organic matter of the soil, and seems to have been the first one to suggest that it consisted of other elements than carbon, hydrogen and oxygen. He separated the organic matter into various supposedly pure organic compounds of an acid nature, which were analyzed and studied by the usual organic method. Wolff (2) determined the organic matter of the soil by calcu- lation, by use of the factors 1.724 or 0.471 ; he multiplied the or- ganic carbon by the former factor, or the total organic carbon dioxid by the latter. The factors were derived from the concep- tion that "humus" contained 58 percent carbon. Detmer (9) attempted to isolate "pure" humic acid from the soil and to study its properties. He obtained a fairly pure product which he studied and submitted to analysis. A little later Grandeau (10) developed his well known method for determining the matierc noire of soils, which he regarded as of great importance. He stated that soils owed their color and *Submitted to the Faculty of the Graduate School of the University of Illinois in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1909. 91 92 BULLETIN No. 145 [April, probably their fertility to its presence, since it held in combination phosphorus, nitrogen, and certain mineral elements. Deherain (22) determined the carbon content of soils from plots which had received different treatment. He found that the soils from the plots which had not been manured had lost over 50 percent of their carbon. Kostytscbiff (24) studied the humus obtained from substances of known origin and which were converted into humus under con- trolled conditions. He learned that even with the albuminous sub- stances the carbon was lost more rapidly than the nitrogen, hence the ratio of carbon to nitrogen would be narrower in the resulting humus than in the original material. Berthelot and Andre (31) found that 67.1 percent of the total carbon in soils was soluble in dilute alkalis but that over one-half of this soluble carbon, or 40 percent of the total carbon, was not precipitated from the alkalin solution by the addition of an acid. Snyder (37) reported the results obtained by a study of the production and analysis of the humus obtained from such sub- stances as cow manure, clover, meat scrap, etc., etc., which were converted into humus under known conditions. The carbon con- tent of the humus varied from 41.95 percent in case of the hu- mus produced from cow manure to 57.84 percent in case of the humus produced from cane sugar. Hess (45) studied the effect of different systems of treatment on the humus of the soil. He found that the ratios of carbon to nitrogen and nitrogen to humus were not materially affected by the treatment applied. Andre (47) studied the action of potassium hydroxid on the carbon compounds of the soil, mould, compost and peat. He de- termined the insoluble and soluble carbon; the latter he separated into two classes ; the portion precipitated from alkaline solution by the addition ef an acid, and the portion remaining in solution. The results obtained are expressed in Table I. TABLE 1 PERCENTAGE OK SOLUBLE AND INSOLUBLE CARBON AND RATIO OF CARBON TO NITROGEN Peat Compost Soil Mould Ratio of C/N Percent carbon of total Ratio of C/N Percent carbon of total Ratio of C/N Percent carbon of total Ratio of C/N Percent carbon of total Insoluble portion 147.6 24.5 84.4 40.0 82.4 35.4 40.4 41.8 Sol- uble por- tion a. Precipitated by acids 26.9 44.8 16.6 23.0 27.1 36.6 10.8 18.7 b. Not precipi- tated by acids 17.7 30.7 9.8 37.0 16.7 28.0 10.1 39.5 jp/o] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 93 The ratio of carbon to nitrogen in the original material was : peat 22.7; compost 15.0; soil 24.7; and mould 12.8. He concluded that the more insoluble the compound the wider was the carbon- nitrogen ratio. The potassium hydroxid showed a tendency to dis- solve the compounds rich in nitrogen. Pagnoul (51) found no fixed relation between the carbon and nitrogen of the soil, but apparently the carbon, nitrogen, and hu- mus varied in the same direction, altho irregularly. Rimbach (53) concluded that since the matiere noire was readily nitrified it was the direct source of the nitrates of the soil, and thus the insoluble carbon was of insignificant value. Frear and Hess (54) found that lime caused a more rapid loss of carbon than of nitrogen in manured land, but the reverse in un- manured land. Dyer (55) studied the carbon and nitrogen content and the re- lationship between carbon and nitrogen in the soil taken from 22 different plots of the Rothamsted experiment fields. These data are furnished for each individual 9-inch section to a depth of 90 inches. The carbon and nitrogen contents of the higher depths were higher than those of the lower depths and the ratio of carbon to nitrogen is wider in the former. At the fifth to sixth depth the carbon and nitrogen contents seem to become fixed quantities and are apparently those derived from the original matter out of which the soil was formed. A study of various clays and other material taken from great depths seemed to indicate that a nitrogen content of .04 percent was indigenous to the subsoil of the Rothamsted station. Cameron and Breazeale (61) investigated the three general methods for determining the carbon content of the soil : namely, the "loss on ignition" method, the humus method, and two forms- of a combustion method. They concluded that the first two meth- ods were unreliable : the first, because there was no apparent re- lationship existing between the results obtained and the true carbon content; the second, since it made no pretense of giving the total carbon in the soil. It is interesting to note that they reported that the ammoniacal extract contained so much suspended material that it was found undesirable to work with until it was passed thru a Chamber- land-Pasteur filter, when a perfectly clear solution was obtained. Konig (67) recently studied the influence of hydrogen peroxid on the organic matter of the soil. He found that it consisted of two parts, one easily oxidized by hydrogen peroxid, the other not oxidized by this reagent. 94 BULLETIN No. 145 , [-4pril, Hopkins and Pettit (68) reported the total carbon, nitrogen and phosphorus contents of a great number of samples of the soils of Illinois. This work is thus made the basis of calculating the relationship of carbon, nitrogen and phosphorus reported in part (B) of this thesis. 2. NITROGEN IN SOILS The nitrogen in soils exists chiefly as organic nitrogen wtih a very small amount of inorganic nitrogen. The organic nitrogen may exist in some known and probably some unknown forms. Mulder (i) believed the nitrogen found in the humus to be associated with the organic matter in the form of the ammoniacal salts of the various organic acids obtained by him. Miiller (4) thought he detected a tendency for the nitrogen to vary inversely as the carbon. Detmer (9) believed that the nitrogen formed a definite com- pound with the organic carbon of the soil since the nitrogen could be liberated only with great difficulty and by the use of the most drastic chemical agents. Simon ( 1 1 ) believed that the organic matter of the soil pos- sessed the property of absorbing the free nitrogen of the atmos- phere and of converting it into ammonia which in turn united with the organic acids in the form of their ammoniacal salts. Sos- tegni (19) a little later discussed the work of Simon and reported a series of experiments to prove that Simon's assumption was . untenable. Berthelot (19), in 1886, reported the carbon and nitrogen con- tents of calcareous clayey soil, originally very deficient in organic carbon and nitrogen but which was gradually increasing in carbon and nitrogen content owing to the action of diatoms. Berthelot and Andre (20, 74, 75, 76) later carried on a series of experiments for the purpose of separating the organic nitro- genous rnaterial into its various compounds. They reported the amount of total, nitric, amido and ammoniacal nitrogen present in the soil. Eggertz (21) differed very materially from Mulder. He con- cluded that Mulder's contention, that the nitrogen associated with the organic matter of the soil existed only as the ammoniacal salts of the various organic acids, was untenable. If the nitrogen ex- isted simply as the ammoniacal salt of the humic acid, treatment with hydrochloric acid should liberate all the nitrogen as ammonia, which, experimental evidence showed, was not the case. Furthermore, artificial humic acid, treated with ammonia, did form ammonium humate, which, however, was readily decomposed /pzo] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 95 by treatment with a mineral acid ; yet, if this artificial humic acid be heated in a current of ammonia gas, combination took place, and the resulting compound could not be decomposed by treatment with mineral acids. He therefore concluded that the nitrogen formed an integral part of the humic acid radical. Berthelot and Andre (26) studied artificial humic acid pre- pared out of sugar. This acid formed salts with various bases, which were easily decomposed again by treatment with an acid, except in the case of the ammonium salt, the nitrogen of which could not be entirely liberated by this treatment. They concluded that the nitrogen, in part at least, formed an integral part of the humic acid radical. Hilgard and Jaffa (30), in 1892, propounded their well known view regarding the importance of the nitrogen associated with the extracted matiere noire. Berthelot and Andre (31) regarded the organic matter of the soil as of great importance since it prevented the loss of nitrogen thru drainage and since the nitrogen was held in insoluble combi- nation in the organic matter. Fulmer (38) determined the humic nitrogen in 53 samples of Washington soil and attempted to work out the relationship be- tween carbon and nitrogen by means of the formula c = ~ , where c = the percentage of nitrogen in the matiere noire; b = the percentage of the total soil nitrogen; a= the percentage of humus. By means of this formula the 53 samples of soil were separated into three classes ; the first class contained 19 samples in which the variation in the humic nitrogen calculated by means of the formula was within one percent of the analytical result; the second class contained 10 samples and the variation was from one to two percent ; the third class contained 24 samples and the varia- tion was anywhere over two percent. These results furnished good evidence that no one given relation would hold for all soils. Wheeler (48) found that both lime and gypsum caused a de- crease in the amount of humus but that the percentage of humic nitrogen was increased. Similar results were obtained by Frear and Hess (54) on manured land. Dojarenko (56) recently studied the "humic" nitrogen of soils. He determined the total, humic, amid, ammoniacal and amido ni- trogen in seven samples of black Russian soils. The results are reported in Table 2. 96 BULLETIN No. 145 [April, TABI,E 2. PERCENTAGE off TOTAI, HUMIC, AMIDO, AMID AND AMMONIACAL NITROGEN IN HUMUS Percent in dry substance Percent of total quantity of nitrogen No. Total humic nitrogen Amido nitrogen Amid nitrogen Ammo- niacal nitrogen Amido nitrogen Amid nitrogen Ammo- niacal nitrogen 1 2 '3 4 5 6 7 2.735 3.38 2.64 3.33 4.58 3.65 4.02 1.34 1.81 1.30 2.34 1.01 1.26 1.96 0.31 0.41 0.29 0.32 0.48 0.27 0.22 0.04 0.08 0.02 0.03 0.06 0.07 0.03 49.09 53.55 49.20 70.27 22.01 34.52 48.75 11.38 12.13 10.99 9.61 10.46 7.40 5.47 1.46 2.36 0.80 0.90 1.31 1.90 78 This did not account for all of the nitrogen present and so the question arises, In what form does the remainder exist? D'Utra (70) found that the humic nitrogen showed wide vari- ations. Hilgard (71) reported the average humic nitrogen of 466 samples of soil from the humid regions as 5.45 percent, while the average of 313 samples of soil from the arid section was 15.87 percent. Later (73) he found that the average humic nitrogen for 696 samples of humid soil was 5.00 percent, while that of 573 samples of arid soil was 15.23 percent. It must be remembered, however, that the total quantity of nitrogen of the t\vo regions is in the inverse order. The total nitrogen of -the uplands and low- lands of California, for example, is o.ioi percent and o.ioi per- cent respectively, w'hile the total nitrogen of the ordinary brown silt loam soils of the corn belt in Illinois varies from 0.218 percent to 0.337 percent 3. PHOSPHORUS IN SOILS The phosphorus of the soil may exist in the inorganic and in the organic condition. The greater part is in the inorganic form, with an unknown amount in the organic state. The form and amount of the organic phosphorus is uncertain, and indeed it has been questioned, especially during recent years, whether or not organic phosphorus occurred in the soil to any appreciable extent. Mulder (i), as early as 1844, noted that the organic material was not readily freed from phosphorus. The work of Thenard, and of Schutzenber (5, 6, 7) showed that union may take place between various forms of artificial humus and phosphates under certain conditions and indicated that combi- nation may possibly take place in the soil between organic carbon and inorganic phosphorus. Detmer (9) in the preparation of his "pure humic" acid, noted /p/o] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 97 that the material could be freed from phosphorus only with great difficulty. Grandeau (10) regarded the phosphorus associated with the extracted matiere noire as being of the greatest importance and probably in special combination with the organic matter. He re- garded it as an index of the fertility of the soil. Simon ( 1 1 ) believed that he had demonstrated that union took place between organic matter and phosphates. When freshly pre- cipitated humic acid was suspended in water and digested with calcium phosphate and then filtered, the filtrate showed an excess of phosphoric acid : this excess, he concluded, must be in union with the organic matter in solution. He thought that a double compound of ammonia and phosphorus existed in the soil. Schultz (12) showed that the addition of humus to "Basalt- boden" increased the absorption ability of the soil for phosphates. Eichhorn (13) repeated some of Simon's work and concluded that organic combination did not take place as indicated by Simon but that the humus had decomposed the tri-calcium phosphate with the formation of acid phosphate. Pitsch (14) determined the solubility of the various mineral phosphates, including iron and aluminium phosphates, in a solu- tion of ammonium humate itself. He concluded that since this solution exerted a solvent action on mineral phosphates, the am- monia extract of the soil contained phosphorus other than that originally associated with the organic matter in the soil and prob- ably part, at least, of the ammonia-soluble phosphorus was de- rived from the iron and aluminium phosphates. M. P. DeGasparin (15) found in calcareous clay soil 5 per- cent of the total phosphorus in organic combination. He noted, furthermore, that the mosses and lichens contained from 5 to 6 times as much phosphorus as the rocks on which they grew; the soil formed, therefore, from the debris of these plants should be relatively richer in phosphorus and should have a part of its phos- phorus in combination with carbon in the organic material. Eggertz (21) found that the ammoniacal extract of the soil., when treated with ?n acid, formed a precipitate of organic matter which always contained phosphorus. He concluded, therefore, that part of the phosphorus of the soil was united to the carbon in or- ganic combination. Later Eggertz and Nilson demonstrated that the amount of phosphorus soluble in dilute mineral acids showed a marked in- crease after ignition of the soil. Ignition rendered 10 times as much phosphorus soluble in 2 percent hydrochloric acid. They attributed this to the destruction of the organic matter which had held the phosphorus in combination and which would not yield up its phosphorus to acids. 98 BULLETIN No. 145 [-4f>ril, Van Bemmelen (23) believed that the iron, calcium, silica, phosphoric acid, etc., found in the ash of the matiere noire by Eg- gertz, were not originally chemically combined to carbon in the organic matter of the soil but were absorbed by the precipitated gelatinous matiere noire. According to Van Bemmelen the phos- phorus existed in the soil principally as calcium phosphate with a very small quantity occurring in the absorbed state in the form of a colloidalcn Humate-Silicat-Komplex. Two questions seemed to be of paramount importance to Wik- lund (25) regarding the work of Eggertz : First, was the amount of the ammonia-soluble phosphorus obtained from different soils constant? Second, did the phosphorus exist in the mullkorpers (matiere noire of Grandeau) in chemical combination with carbon, or simply as absorbed phosphorus? He concluded that there was a tendency for the ammonia-soluble phosphorus to be constant in different soils. He showed, further, that one digestion with 12 percent hydrochloric acid did not completely remove all of the acid-soluble phosphorus, but a second and even a third digestion still removed some phosphorus. Now, he reasoned, if the phos- phorus removed by the second and third digestion was simply ex- tracted from the absorbed phosphorus, extraction of the soil with ammonia after the first digestion with hydrochloric acid should yield a solution of matiere noire containing a higher phosphorus content than when the soil was completely extracted with the hy- drochloric acid. Such, however, was not the case; therefore, the phosphorus did not exist as absorbed phosphorus and must be in combination with carbon in the organic matter. Snyder (34) noted that some phosphorus, iron, etc., were ex- tracted with the matiere noire but he did not seem to think at this time that there was any evidence of combination with carbon. About the same time he observed the rapid loss of phosphorus as- sociated with the humus in continuously cultivated soil. According to Berthelot and Andre (27) phosphorus may be found in the soil (a) in inorganic or mineral phosphates, (b) in organic ethers and (c) in organic or mineral compounds not read- ily decomposed. Schmoeger (29) reviewed the rival claims of Eggertz and Nil- son, and Wiklund, on the one hand, and Van Bemmelen, on the other, regarding the phenomenon of ignition rendering the -phos- phorus of peaty soil more readily soluble in acids. It seemed possible to Schmoeger that the soil might possess such a tenacious absorbent power for phosphorus that it would not yield up its phosphorus to acid treatment before ignition. But he deduced experimental evidence to show that such was not the case. ip/o] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 99 Digestion of the soil itself and also the extracted matiere noire with a solution of potassium hydrogen phosphate failed to add any phosphorus which was not again recovered by treatment with hydrochloric acid. This was conclusive evidence to Schmoeger that the phosphorus did not exist as absorbed phosphorus and must, therefore, exist in organic combination. Two possibilities suggested themselves to Schmoeger: first, the phosphorus existed in the form of lecithin ; second, it existed as nuclein. Lecithin was found to be present only in traces. The characteristic property of nuclein to "split-off" its phosphorus in the form of phosphoric acid, when heated under pressure in the presence of water to a temperature of I5o-i6o, was utilized by Schmoeger. The soil under examination, treated in this way, yielded as much soluble phosphorus as did the ignited soil. This experimental evidence led him to conclude that nuclein or some closely allied bodies were present in the soil.- Later Schmoeger (39) confirmed his previous work and pro- duced additional evidence in favor of his view that nuclein or simi- lar bodies existed in the soil. Table 3 shows some of the re- sults obtained. TABI,E 3 PERCENTAGE OF SU^FURIC ACID AND PHOSPHORUS SOLUBLE IN DILUTE ACID Percent Sulfuricacid Phosphorus 0.122 0.290 0.939 0.043 0.083 0.095 Since sulfur is regarded by many authors as being a constitu- ent of plant nuclein, the increased solubility of this substance, to- gether with the phosphorus when the soil was treated as indicated above, was regarded as evidence in favor of his assumption. In a later article (40) he showed, by similar treatment, that analogous bodies existed in the moor grass out of which the moor soil was formed. This was regarded as additional evidence in favor of his view. Tacke (33) observed that the drying out of soil rendered the phosphorus available. There w r ere three possible explanations sug- gested to him : first, the phosphorus existed in the soil in organic combination which was destroyed by the process of drying; sec- ond, it existed in the soil in the colloidal form as suggested by Van Bemmelen ; third, the drying out of the soil gave rise to sub- stances of a strong acid nature w r hich acted upon the insoluble phosphorus compounds rendering' the phosphorus soluble. 100 BULLETIN No. 145 {April, In a later article (42) he showed that very little water-soluble phosphorus existed in the soil under consideration, but that dry- ing at 7O-8o rendered over 50 percent of the total phosphorus soluble in water. Snyder (36) reported results of a confirmative nature regard- ing the phosphorus associated with the humus in virgin and cul- tivated soils. Later (37, 41) he studied the product obtained by the conver- sion of known substances, under known conditions, into humus. - The ash of the matiere noire obtained from this material contained phosphorus, among other substances, and according to Snyder, "There is every indication that these elements are in organic com- bination with the carbon, hydrogen angl oxygen of the humus." As regards the question whether or not the humus united with the inorganic phosphorus of the soil, he concluded that his experi- mental evidence showed that such union did take place. Nannes (49) found that a well decomposed peat soil con- tained o. 166 percent phosphorus. He found that 0.057 percent of phosphorus was extracted with the matiere noire. When the am- moniacal solution of the matiere noire was treated with hydro- chloric acid, 0.039 percent of the phosphorus was found in the organic precipitate. He also attempted to isolate a definite organic phosphorus compound and he believed that he detected the pres- ence of lecithin and chlorophyllan. Ladd (43) found in a study of eight samples of different soil that an average of 41 percent of the phosphorus was associated with the extracted matiere noire; the variation, however, w r as from 10 percent to 90 percent. In a later article (44) he showed that as the humus of the soil increased the phosphorus associated with the extracted matiere noire also increased. From the fact that the organic precipitate, formed by neutralizing the ammoniacal extract, contained the phosphorus, he concluded that it existed in the soil in organic com- bination, but just what \vas the relationship was not clear. Emmerling (52) believed that there were four forms of "phos- phorus in the soil, one of \vhich was phosphorus in organic com- bination. Rimbach (53) found 6.15 percent P2O5 in the ash of the matiere noire which was precipitated from the ammoniacal solu- tion by the addition of gypsum and magnesium sulfate. Nagaoka (57) found that ignition of the soil for fifteen min- utes at a faint red heat materially increased the availability of the phosphorus. He attributed this action to the destruction of the humophosphates. /p/0] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 101 AsG (58) confirmed, in a general way, the results obtained by Schmoeger. He also found 0.049 percent of lecithin in the soil. He drew the following conclusions : 1. Phosphorus existed in the soil as inorganic and as organic compounds. 2. The organic phosphorus material was principally nuclein with a very small part cf lecithin. 3. Ignition rendered the phosphorus in organic combination available. Hartwell and Kellogg (60) found that an average of one-half of the phosphorus was associated with the organic matter in the soil taken from four plots which had received different treatment. Dumont (62) studied a complete manure, the composition of which was as follows: soluble matter (in dilute alkali) 50.4 per- cent; insoluble matter 40.6 percent; total nitrogen 1.6 percent; total phosphorus 1.27 percent. The soluble portion contained 35 percent of the nitrogen and 46 percent of the phosphorus. In order to obtain data upon the state of combination of the phosphorus, the ammoniacal solution of matiere noire was treated with various reagents with the result (recalculated to the element basis) shown in Table 4. 4. DISTRIBUTION OK PHOSPHORUS WHEN Matiere Noire Is PRECIPITATED Precipitating agent Phosphorus In precipitate In filtrate Citric acid 0.383 386 0.532 O.S66 0.584 0.203 0.199 0.053 0.019 0.0009 Hydrochloric 3>cid Ferric chlorid Aluminium sulfate These results furnished conclusive proof to Dumont that a part of the phosphorus of the soil was in organic combination. Later (64) he obtained better cultural results from application of humic phosphatic manures than from mineral phosphatic ma- nures and better even than from barnyard manure, which he at- tributed to the phosphorus in organic combination. In a still later article (65) he said that the organic phosphorus was derived from two sources : first, from the nuclein and lecithin of the decaying vegetable and animal debris ; second, from the union of the humus with the water-soluble phosphates of the soil. Evidence of the latter contention was obtained by precipitating the matiere noire in the presence of potassium hydrogen phosphate by different reagents as indicated in Table 5. 102 BULLETIN No. 145 [April, TABLE 5. AMOUNT OF PHOSPHORUS IN PRECIPITATED HUMUS AND FILTRATE Precipitating agent Series A Phosphorus in- troduced = .087 Series B Phosphorus in- troduced=.218 Series C Phosphorus in- troduced=.437 Phos- phorus in humus Phos- phorus in filtrate Phos- phorus in humus Phos- phorus in filtrate Phos- phorus in humus Phos- phorus in filtrate Acetic acid 0.056 0.054 0.057 0.057 0.031 0.033 0.030 0.030 0.057 0.055 0.059 0.061 0.160 0.163 0.158 0.156 0.058 0.054 0.059 0.062 0.374 0.381 0.378 0.372 Citric acid Hydrochloric acid Sulfuric acid Altho the amount of dipotassium phosphate added to the solu- tion had increased, the amount of phosphorus absorbed by the humus was practically constant, due to the formation of definite "composes phospho-humique." Konig (67) found that hydrogen peroxid oxidized from 40 percent to 70 percent of the humus present in the soil and that much more of the phosphorus was soluble in pure and carbonated water after oxidation than before, due, he believed, to the destruc- tion of the organic phosphorus compounds. Fraps (69), quite recently, made a study of the phosphorus extracted from the soil by 4 percent ammonia in the usual deter- mination of humus. He confirmed Pitsch's results regarding the possibility of some of the ammonia-soluble phosphorus being of inorganic matter. He separates the ammonia-soluble phosphorus into three classes as follows : 1. The phosphorus associated with the clay held in suspension in the liquid. 2. The phosphorus precipitated with the organic matter when the solution was neutralized with an acid. 3. The phosphorus which remained in solution after the pre- cipitation of the organic matter. With the soils under consideration he found that 1/9 of the ammonia-soluble phosphorus was in the first class, 1/3 was in the second class and 5/9 was in the third class. The phosphorus found in the first class was assumed to be as- sociated with the clay particles as iron and aluminium phosphates. He concluded that the phosphorus precipitated with the organic matter from the ammoniacal solution by the addition of acids was in organic combination. The phosphorus remaining in the mother liquor was assumed to be derived from the iron and aluminium phosphates of the soil. Mooers and Hampton (77) recently proposed a method for obviating the error introduced in the humus determinations by the /p/o] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 103 suspended clay. They claimed that filtration thru the Cham- berlain-Pasteur filter, as suggested by Cameron, introduced a seri- ous error inasmuch as the filter absorbed some organic matter. They proposed an evaporation method : by evaporation of the ammoniacal extract to dryness and re-dissolving in ammonia and filtration several times, a perfectly clear solution was obtained. Determination of the humus in this filtrate gave very concordant results. Hopkins and Pettit (68) found that in certain soils the min- eral composition had a tendency to be constant in the surface, sub- surface, and subsoil. This was indicated by the uniform potassium content of the surface, subsurface, and subsoil and by the fact that different samples of surface soil of the same type showed a wide variation in the phosphorus content but that this variation largely disappeared in the subsoil. The potassium exists in the soil in the inorganic form, the nitrogen exists chiefly in the organic form, while the phosphorus may exist in the inorganic and in the organic state. They suggested, therefore, a method for calculating the phosphorus in the organic state in the surface soil. The difference in amount of nitrogen in the surface soil and subsoil, and the dif- ference in the amount of phosphorus in the surface soil and subsoil gave apparently the amount of nitrogen and phosphorus associated together in organic combination. By means of this ratio and the total amount, of nitrogen in the surface soil the total amount of organic phosphorus present in the surface soil could be calculated. 4. CARBON AND NITROGEN CONTENT OF FUNDAMENTAL ROCKS The fundamental rocks out of which soils have been formed contain an appreciable amount of carbon and nitrogen which is in- digenous to them. Dellese (3) discovered that mineral matter (crystalline, sedi- mentary and eruptive) contained carbon associated with nitrogen. This mineral matter, which was formed under similar conditions of temperature, pressure, etc., had a tendency to contain a constant amount of carbon and nitrogen. The work of Lawes and Gilbert (16), Dyer (55) and Hall and Miller (66) on the clays and other fundamental rock material taken from various great depths indicated that an appreciable amount of carbon and nitrogen w f as indigenous to the underlying soil material. /9/o] CARBON, PHOSPHORUS AND NITROGEN IN SOILS 105 (B) EXPERIMENTAL PART The starting point of such an. investigation consists of a con- sideration from a mathematical point of view of the existing data regarding the relationships of carbon, phosphorus and nitrogen in the soil. i. MATHEMATICAL (a) INFLUENCE OF AGE UPON THE NITROGEN-CARBON RATIO From the data available in the literature it is possible to deter- mine within certain limits the influence of age upon the nitrogen- carbon ratio in soils. From the average results of a number of nitrogen determinations (68) and the carbon content obtained by calculation from the proximate analysis, it is possible to determine the approximate nitrogen-carbon ratio in the more common humus producing materials. The results obtained in this way will be found in Table 6. The materials naturally fall into two groups : in the first group the ratio varies from i :^2.2 for corn stover to i 184. i for wheat straw; in the second group the variation is from 1 116.7 for alfalfa hay to i :35-4 for timothy hay. TABLE 6. APPROXIMATE NITROGEN-CARBON RATIO IN THE MORE COMMON HUMUS PRODUCING MATERIALS Kind of material Carbon to 1 of nitrogen Corn stover 52.2 67.8 \Vheat straw 84.1 Timothy hay 35.4 Clover hay 21 3 Cowpea hay 19.5 Alfalfa hay 16.7 Albumin 3 2 Zein 3 4 Nucleiii 2 8 In Table 6 will also be found the nitrogen-carbon ratio in some of the compounds which might be expected to be found in humus. The ratio is very narrow and does not vary much from i 13. The next step in the study of the influence of age upon the nitrogen-carbon ratio would be to determine the ratio in as fresh humus as possible from known materials. Snyder (35), in his study of the production of humus from known materials, placed a weighed quantity of the material, together with a weighed quantity, 106 BULLETIN No. 145 [April, of soil having a low humus content, in a box and set it aside for one year. At the end of the experiment the humus was extracted and the carbon and nitrogen determined in the matiere noire. At first thought this would appear to furnish the desired information, but unfortunately no check was run with the untreated soil; so no cor- rection can be made for the carbon and nitrogen which may have been converted into humus from the unhumified material of the soil. This is evidently not a quantity which can be ignored, since the humus content of the original soil is .06 percent while the total nitrogen of the soil is .02 percent, showing that considerable unhumified organic matter was present ; otherwise the humic nitro- gen would be 33.33 percent, while it has been shown (71) that the humic nitrogen would more probably be nearer 5 percent. In ad- dition, the fact that the humus obtained from sugar contains some nitrogen is evidence that some of the unhumified organic matter of the soil has been converted into humus, since sugar does not con- tain nitrogen. The results, however, will be found in Table 7. In TABLE 7. MINNESOTA SOIL STUDIES: HUMUS PRODUCTION FROM KNOWN MATERIALS Percent Ratio of Material used Humus Carbon in humus Nitrogen in humus carbon to 1 of nitrogen Original soil 0.06 ? ? ? Cow manure 0.58 41.95 6 16 6 8 Clover 0.37 54.22 8.24 6 6 Meat scraps 0.31 48.77 10 96 4 5 Oat straw 0.46 54.30 2 50 21 7 Flour 0.47 51.02 5.02 10.2 Sawdust 0.59 49.28 0.32 153 8 Sugar 0.32 57.84 0.08 741.0 the first five substances the variation is from i 121.7 for oat straw to i 14.5 for meat scraps. The large number of carbon and nitrogen determinations made of the soils of Illinois (68) rendered it possible to determine the nitrogen-carbon ratio not only for the surface soil but also for the subsurface and subsoil. The average of 19 determinations for the soil type gray silt loam on tight clay, gave the ratios i :io.4, i :8.8, and i 17.6 for the surface, subsurface, and subsoil respec- tively. The ordinary brown silt loam soils, as an average of 68 determinations, gave a nitrogen-carbon ratio of 1:12.1, 1:11.5 and i :8.9 for the surface, subsurface and subsoil respectively. The black clay loam soils, as an average of 25 determinations, gave i :ii-7, 1:11.9 and 1.9 respectively in the surface, subsurface, and subsoil. The peat soil, as the average result of 5 determinations, gave i :n.8 and i :i2.9 for the surface and subsoil respectively. CARBON, PHOSPHORUS AND NITROGEN IN SOILS 107 TABLE 8. RATIOS OF CARBON AND NITROGEN IN ILLINOIS SOILS Soil type No. Soil types Mo. of analyses on which calculations are based Carbon to 1 of nitrogen Surface Sub- surface Subsoil 330 426 526 626 726 1126 1026 Gray silt loam on tight clay 19 11 8 6 4 30 9 10.4 12.5 13.2 11.4 11.9 11.9 12.0 8.8 11.7 12.9 10.5 11.1 11.5 11.5 7.6 9.6 8.7 8.7 8.8 8.6 9.3 Brown silt loam Brown silt loam Brown silt loam Brown silt loam Brown silt loam Brown silt loam Averages 68 12.1 11.5 8.9 420 520 1120 1220 Black clay loam .... Black clay loam .... Black clay loam .... Black clay loam ... 7 5 11 2 12.2 12.4 11.1 11.2 12.2 12.2 11.1 12.2 8.9 11.4 8.3 7.4 Averages ' 25 11.7 11.9 9.0 1401 Deep peat . , 5 11.8 12.9 12.9 The Rothamsted work furnished information regarding the nitrogen-carbon ratio of the soil in g-inch sections to a depth of 90 inches. These results will be found in Tables 9 and 10. The ratio for the Broadbalk wheat fields varies from i 19.5 to I 14.8 for the surface and ninth 9 inches respectively. After the fifth 9 inches there is very little change in the ratio. In the Hoosfield barley soils the ratio varies from I :io.6 to i :8.8 for the surface and sub- soil respectively. TABLE 9. BROADBALK WHEAT SOILS: RATIO OK CARBON TO NITROGEN Per( :ent Carbon to 1 Depth Carbon Total nitrogen of nitrogen First 9 inches (all plats) 1.155 .1222 9 5 Second 9 inches .640 .0784 8 2 Third 9 inches .492 .0666 7 4 fourth 9 inches .339 .0511 6 6 Fifth 9 inches .279 .0472 5 9 .256 .0430 5 9 Seventh 9 inches .248 .0420 5 9 Eighth 9 inches .215 -.0396 5.4 Ninth 9 inches .. .189 .0391 4 8 Tenth 9 inches .188 .0375 5.0 TABLE 10. HOOSFIELD BARLEY SOILS: RATIO OF CARBON TO NITROGEN Depth Carbon to 1 of nitrogen First 9 inches 10.6 Second 9 inches 8 8 Third 9 inches 8.8 108 BULLETIN No. 145 [April, Hall and Miller (66) reported the carbon and nitrogen con- tent, and the ratio of carbon to nitrogen, in samples of various ma- terials taken from such great depths as to preclude all possibility of weathering. Since the nitrogen was always found to be associ- ated with carbon it was regarded as being of organic origin and as being derived in part from the organic matter present in the clay at the time of its deposit. These results are shown in Table n. TABI