CHEMICAL NATURE.of ORGANIC MATTER or HUMUS in SOILS, PEAT BOGS, and COMPOSTS SELMAN A. WAKSMAN New Jersey Agricultural Experiment Station, New Brunswick, N. J. INTRODUCTORY
T
HE BEGINNINGS of the study of the chemical nature of soil organic matter date back to the latter part of the eighteenth and early nineteenth centuries, when organic chemistry was still in its infancy, and when organic compounds were considered to be simple in composition, similar to inorganic substances. As a result of this conception, humus was believed to comprise a few simple, dark-colored compounds, obtained by treatment of rotting wood, of soil, or of peat with alkali solutions. When a mineral acid was added to the& extracts, dark amorphous precipitates were produced, which were designated as "humic acids." Various hypothetical formulas were attached to these, based entirely upon their elementary compositions. These "humic acids" were mere preparations rather than chemical entities; they varied considerably in chemical composition, depending on the nature of the material from which they were ohtaiied and on the method of preparation. Not only different sources, such as coal, peat, or mineral soil, hut even the same general source, taken from different regions, yielded different preparations. The nature of the alkali and its concentration, the time and temperature of extraction, the nature and amount of acid used for neutralization-all modified the nature of the preparation obtained. Such eminent chemists as Vauquelia, Sprengel, Berzelius, Mulder, and Hermann (I) gave prestige to the numerous "humic acids" that were swn described. Various dark-colored preparations were also obtained by treatment of sugars and other carbohydrates with acids or alkalies; these were also designated as "humic adds" and were believed to be chemically the
same as the natural preparations. Some chemists went so far as to claim-that the natural humus compounds originate by processes similar to those employed in the formation of these artificial preparations. In spite of a century and a half of progress in the study of humus, considerable confusion still exists today in regard to this group of ill-defined complexes, which play such an important r61e in soil fertility by serving as a reservoir of plant nutrients, a n d which represent the greatest storehouse of avaikble energy on this planet. Humus formation in nature, whether in soil or in composts, in peat bogs or in water basins, must be considered as an accumulation of substances, of plant and animal origin, resistant to further' decomposition. However, although it is now generally agreed that humus is formed as a result of decomposition of plant and animal residues, primarily the former, by microorganisms, many chemists still persist in their belief that this process is a result of action of atmospheric agencies or of inliltration of inorganic compounds. The most important limitation in the majority of investigations on the origin and chemical nature of humus was the lack of recognition of the agencies concerned in its formation under natural conditions; further, humus is not simple but highly complex in composition and is formed not only by processes of decomposition but also by those of synthesis. Only when the nature of the microbial population of the soil became better understood, when the r61e of these microijrganisms in the decomposition of plant and animal residues hecame established, was there a definite attempt made to correlate the chemical cornposition of humus with that of the mother substances
512 from which it originated and with the activities of the microorganisms bringing about its formation. The latter were shown to be the primary agents in the decomposition of residues of organic life in field, pasture, and forest soils, in composts, and in peat bogs. Under certain conditions, as in well-aerated composts and in cultivated soils, the organic substances are decomposed rapidly and completely, with the result that hnmus does not accumulate; under other conditions, which are unfavorable for the activities of many microorganisms, as in peat bogs and in acid forest soils, decomposition is delayed and humus accumulation takes place. Our knowledge of humus is still incomplete. Although some of its physical and chemical properties are well established and the conditions of its formation and decomposition are better understood, the function of the various plant and animal constituents in its formation, and the chemical nature of the humus as a whole are still questions subject to considerable debate and speculation. Such arbitrary questions as the nomenclature of hnmus, the nature of the complexes which should be included under this term, the contribution of the major plant constituents, namely the cellulose, pentosans, proteins, and lignins, to its formation, still continue to arouse considerable interest. Some of the major reasons for the existing confusion in the literature on humus are: (1) the use of the term "humus" to designate diierent preparations; (2) the continued assumption that humus is made up of a few simple hypothetical "humic acids"; (3) the belief that it is formed by a specific "humification" process distinct from the normal processes of decomposition. The term "humus" has been applied to: (a) the soil organic matter as a whole, (b) the alkali-soluble portion of the organic matter, (6) the alkali-soluble and acid-precipitated part of the humus, (d) that fraction of the decomposing organic matter which is acted upon by weak oxidizing agents, (e) that part of the hnmus which is not acted upon by acetyl bromide ("pure humus").' It is now commonly agreed thatqhumus is not a single chemical compound, but is complex in composition, depending on the nature of the materials from which it has originated, the extent of their decomposition, the conditions under which decomposition has taken place, and the nature of microorganisms active in the decomposition processes. A number of humus types are found in nature, in a state far from homogeneous; they are distinct from one another in their physical and chemical properties; however, they still possess the characteristics of humus as a whole. Humus is thus found to possess, on the one hand, certain generic properties which distinguish it from other types of organic matter in nature, namely plant substance and animal substance; on the other hand, different forms of humus possess specific properties which can be used as a basis for its subdivision into a variety of forms. One returns, therefore, to the early conce&ion and klassification i f humus by ~inneaus,
with the only difference that this early botanist considered humus as a specific ecological body; however, the present basis for its separation is the accumulated knowledge of its chemical composition and processes of its formation. Since humus, in whatever condition it is, forms an excellent medium for the growth of numerous types of microorganisms, it is never constant in composition, in a natural state; the same type of humus may, therefore, vary considerably, since it is in a dynamic state and undergoes continuous change. CHARACTERISTICSOF HUMUS
Humus is a complex aggregate of brown to darkcolored amorphous substances, which have originated during the decomposition of plant and animal residues by microorganisms, under aErobic and ana*obic conditions, usually in soils, composts, peat bogs, and water basins. Chemically, humus consists of certain constituents of the original plant bodies resistant to further decomposition; of substances undergoing decomposition or resulting from decomposition, by processes of hydrolysis, oxidation and reduction, and of various compounds synthesized by microorganisms. Humus is a natural body; it is a composite entity, just as are plant, animal, and microbial substances; it is even much more complex chemically, since all these materials contribute to its formation. Humus possesses certain specific physical, chemical, and biological properties which distinguish it from other natural organic bodies. Humus itself, as well as the products of interaction of some of its constituents with certain inorganic compounds, forms in the soil complex colloidal systems, the different constituents of which are held together by surface forces; these systems are adaptable to changing conditions of reaction, moisture, and action of electrolytes. Humus is practically inso!uble in water, although a part of it may form a colloidal dispersion in pure water. It dissolves to a large extent in dilute alkali solution, especially on boiling, giving a dark colored extract; on neutralization with mineral acids, a dark flocculent precipitate is formed. Humus has a higher carbon content than the plant, animal, and microbial substances from which it originated, usually 52 to 55 per cent., and frequently as high as 58 per cent. Humus contains a considerable amount of nitrogen, namely 3 to 6 per cent., in lowmoor and sedimentary peats, and in mineral soils. Frequently, the nitrogen concentration is less; in the case of certain highmoor peats, for example, the nitrogen content may be only 0.W3.8 per cent.; however, it may also be higher, especially in subsoils, frequently reaching 8 to 10 per cent. Many forms of humus contain the elements carbon and nitrogen in a ratio close to 10: 1; this is true of the majority of field and garden soils and of the humus in sea bottoms. This ratio varies considerably with the nature of the humus, the stage of its decomposition, the nature and depth of soil from which it is obtained, and the climatic and other environmental conditions under which it is formed.
been kept under different conditions, to the formation of ammonia and nitrate from the manure, and to the influence of fresh or composted manure upon the physical and chemical conditions of the soil and upon plant growth; however, one is still largely in the dark concerning the major processes which take place during the decomposition of manure under aerobic and anaerobic conditions, and especially on the relation of manure to the formation and accumulation of humus (3). A knowledge of the various processes involved in the decomposition of fresh organic residues in compost heaps or in soils involves an understanding of: (a) the chemical composition of these residues; (b) the nature of the microorganisms active in the decomposition processes, especially under various conditions of temperature, aeration, and moisture; (c) the transformation of the specific organic and inorganic constituents brought about by the agency of microorganisms; (d) the influence exerted bv one mouo of constituents upon the decomposition of another, as in the influence of carbohydrates upon the transformation of nitrogenous complexes; (e) the mechanism of liberation of the nutrient elements, namely the carbon, nitrogen, phosphorus, and potassium, in forms available to plant growth; (f) the gradual formation and accumulation of organic complexes which are more resistant to decomposition, namely the humus substances. It is not at all necessary to make a detailed analysis of the numerous organic niaterials which find their way into the soil, or to account for all the chemical compounds which they contain. The chemical nature of some of the compounds is still imperfectly understood (e. g., the lignins and certain hemicelluloses). However, for most practical purposes, it is sufficient to make only a proximate analysis of the chemical composition of the different materials. Various systems of analysis have been proposed, in which the more abundant noups are determined, namely the cellulose, hemicelluloses, lignins, proteins, fats and waxes, water-soluble substances, and mineral constituents; these groups account for 85 to 96 per cent. of the total constituents in the mbjority of plant and animal residues; in some cases, as in sphagnum mosses and algze, the common method has to be modified so as to include the polyuronides and certain other compounds (4).
-
A
are high in lignin and in polysaccharides, but low in nitrogen; the cereal straw and the corn stalks are high in carbohydrates, they contain much less lignin, and are low in nitrogen; the leguminous plants are high in nitrogen and low in lignin. When plant residues undergo decomposition, the different organic compounds are not attacked at the same rate. This is brought out in the next three tables. In the destruction of wood by various fungi, there is a rapid loss in cellulose and in pentosan, and an increase in the lignin constituents, the latter being measured by the methoxyl content; however, the lignin does not remain as such but is modified considerably, as shown by its greater solubility in dilute alkali solution (Tahle 2). This type of wood decomposition is not universal, however, since other fungi can attack both the cellulose and the lignin in the nlant material. When rve straw was allowed to undergo aeobic decomposition for a period of two months, in the presence of added available nitrogen and inTABLE 2
CB*NOES I N TBB
co-mmw
CHB~CAL
CREXICAL. C o m s m r o w OR VA~UOVR PLANIMAIBR~ALS (Per cent. of dry material)
Chcmicol Cmsliluanll Ether- and alcoholsoluble fractions Cold-and hot-watersoluble fractions Hemieellulores Celluloee Lignior protein Ash Total aemunted fm
Corn Stalks
Rye Slmm
Ook Lcoocr
5.99
5.33
6.44
Old AIfolfn Pine Plnnls Nccdlcs 10.41
23.92
C ~ r n s Wwd 5.45
The proximate compositions of several typical plant materials are reported in Table 1. The wood products
WOODAS
* RE9m.T
OR ITS
A m LISSB)
(Per cent. df dry wood)
Chenricnl Carlilurnlr Fresh woad Partlydemmposedwood Fully deeompmed wood
Mclhoxyl AlkaliCelldole Penlolon Crowp soluble 58.96 41.66 8.47
7.16 6.79 2.96
3.94 5.16 7.80
10.61 38.10 65.31
Melbyl#r"loson 2.64 3.56 6.06
organic minerals, and at optimum temperature (25'C.) and moisture (7573, there was a rapid reduction of the cellulose and pentosan, a limited reduction in lignin content, and an increase in the concentration of protein; the latter was due to the synthesis of cell substance by microorganisms, .which use the energy made available in the decompbsition of the carhohydrates and assimilate the inorganic nitrogen added to the compost (Table 3). As a result of these changes, the resulting compost contained a lower concentration of carhohydrates and a greater concentration of lignin TABLE 3
Pmsh S1row
TABLE 1 PRO-TB
O.
DBcoxPoslnolr (Rose
Total organic matter ( i f f f f m f h and water-soluble constituents) Pentman Cellulose Lignins Protein
100. W 26.00 41.44 22.52 1.19
Dscom9orrd Organic Mollcr on Basis of RElnfioa Original Com*od1ion Molerid 100.00 17.71 31.54 34.43 5.92
58.03 10.28 18.30 19.98 3.41
and protein. Similar results have usually been reported for the decomposition of the various plant constituents during the cornposting of stable manure; this is brought out in Tahle 4, in which some of the earliest and most careful analyses on the decomposition of plant and animal residues are reported. Most of the plant residues which undergo decomposition in soils and in composts are very low in
TABLE 4
Condil~mts
high, itdecomposes very rapidly, as can be conveniently
RBSIDVBd r m s MOSHIOOXmeasured by the course of evolution of carbon dioxide; G m m (Heasar ~ A m , Hem) a large part of the nitrogen is rapidly liberated as (Per emt. of dry material) ammonia, which is then gradually oxidized, under spent Fresh Camgorlrd Mushroam favorable conditions of ahation and reaction, to Monwc Manure Monurc
CABXICALCOY~OSIT~ON os M m m s
AND
nitrate. When the nitrogen content of the plant is about 1.5 per cent. or above, there is just suBicieut nitrogen for the decomposition to proceed rapidly; after about one month, a t optimum moisture and temProtein perature, some of the nitrogen begins to become Ammonia nitrogen liberated. The higher the nitrogen content of the material undergoing decomposition, the greater the nitrogen. The rate of their decomposition under relative amount of nitrogen that is liberated in an aerobic conditions is usually limited by the supply of available form. When the nitrogen content of the available nitrogen, or by the rapidity with which this plant is less than 1.5 per cent., additional nitrogen is nitrogen becomes available. Because of this rela- required to bring about rapid decomposition of the tionship, carbohydrates and other nitrogen-free sources material; this added nitrogen is transformed, in the of. energy influence markedly the liberation of the process of decomposition, into an organic form. If nitrogen in an available form. I t has been known for a longer period of decomposition is allowed, the amount a long time that ammonia formation from proteins by of nitrogen necessary for optimum decomposition will microorganisms can be repressed by the addition of be less. carbohydrates. This influence was a t 6rst ascribed In the decomposition of plant and animal residues, to some injurious action of the carbohydrates upon the water-soluble carbohydrates, starches, and simple the microorganisms or to the production of some acid nitrogenous compounds are the first to disappear; in the process of the* decomposition. However, it these are followed by the free proteins, pentosans, and was later found that this effect was due to the pref- true cellulose. These complexes are not attacked by erential utilization of the carbohydrates as sources of the microijrganisms in steplike processes; the rapidity energy with consequent preservation of the proteins. of decomposition of each compound depends on its In the decomposition of carbohydrates and other non- specific chemical nature and upon the nature of the nitrogenous sources of energy, microorganisms utilize microorganisms. The lignins, certain hemicelluloses, a certain part of the carbon for the synthesis of their notably polyuronides, and the resinous substances cell substance. In order to bring this about, a source are the most resistant plant constituents; as a result, of available nitrogen is also required. Under aerobic they tend to accumulate in the process of decomposiconditions of nutrition and given a source of available tion (6). Of these resistant, complexes, the lignins energy, the organisms are able to synthesize a quan- play the most important r61e; however, they do not tity of cell substance proportional to the amount of remain in the original form in which they exist in the carbohydrate decomposed; in this process, nitrogen plant material, but their molecules are variously is converted from an inorganic into an organic form. modified, both by biological and chemical agencies The ratio between the carbohydrate decomposed and (7). This modification consists in: (a) a gradual the nitrogen required by the.microorganisms for cell chemical oxidation, especially .under slightly alkaline synthesis was found to vary with the organisms and conditions and in the presence b f oxygen, ( b ) gradual conditions of nutrition. I t has Been demonstrated darkening in color, ( 6 ) increased solubility in dilute (5), for example, that pure cultures of fungi, growing alkali solutions, and (d) reduction of the methoxyl on a simple mineral medium with cellulose as the only groups. In view of the last modification, the measuresource of energy, are able to consume one part of ment of the abundance of methoxyl in composts and nitrogen for every 30 parts of cellulose decomposed; in soils as an index of lignin transformation (8) is not in the case of mixed microbial populations, 40 to 50 always justified; but even such measurements were parts of the cellulose may be decomposed for every sufficient to establish the accumulation of the lignin unit of nitrogen assimilated. This relationship be- as a result of the decomposition of stable manures in tween the available energy and the amount of nitrogen composts and of plant residues in soil. Under certain required by the organisms for cell synthesis lies at the conditions, lignin can undergo active decomposition basis of our understanding of the liberation of nitrogen by specific groups of microorganisms, notably by in an available form when plant and animal residues various higher fungi, as in the case of certain forest are undergoing decomposition, especially when plant soils, as well as in the decomposition of manure commaterials low in nitrogen are plowed under or used posts by the edible mushroom Agaricus cnmpestris in the preparation of artificial manures; it also explains and in various tree diseases (9). the injurious influence of straw upon plant growth Side by side with the decomposition processes, and the influence of the age and nature of the plant brought about by microorganisms in soils and in upon the rapidity of its decomposition. composts, in water and in sewage, considerable synWhen the plant is young and its nitrogen content is thesis of microbial cell substance takes place. The
synthesized material consists largely of proteins and of certain hemicellnloses. A definite relation was found to exist between the processes of decomposition and synthesis; this relation is considerably modified by the nature of the organisms and by the conditions under which decomposition is takimg place. It has been shown (10) that no matter what orotein is added to the soil, gftk a certain period of'decomposition, it changes into a typical soil protein; this change is accompanied by the transformation of the larger part of the nitrogen of the protein into ammonia. The formation and accumulation of the latter is largely prevente* when available carbohydrates are also added to the soil (11); all the nitrogen is then utilized for microbial synthesis, because of the abundance of available energy. Through these processes of decomposition and synthesis, accompanied by various chemical reactions, notably omdation, plant and animal residnes are gradually transformed into humus. Geologists and chemists have devoted considerable attention, especially from the point of view of the origin of peat and coal, to the mother substances of humus, as if the latter were formed in toto from one specific group of plant constituents. Neither lignin, nor cellulose, nor hemicelldoses can be considered as the sole group of compounds contributing to humus formation. Under conditions of active decomposi. tion, carbohydrates are more or less completely destroyed, and are not transformed directly into "humus," if one conceives of the latter only as the dark-colored, alkali-soluble fraction of the humus complex; however, they contribute indirectly to humus formation, through microbial synthesis. Carbohydrates may be, therefore, considered as contributing materially to the hnmus complex. The fact that dark-colored substances can be produced from cellulose under high pressure and by the action of acids, alkalies, and heat has only a limited bearing upon the formation of humus under natural conditions. Varying amounts of cellulose and hemicellnlose$ can also be present as constituents of the hnmus complex, especially in the case of humns formations that hzve not undergone extensive decomposition. This is true of the rawhumus forest soil formations, the humus of heath soils, alpine humus formations, and highmoor peats. Certain polyuronides may prove to be more resistant to rapid decomposition than other carbohydrates and may, therefore, also accumulate in the humus (12). Some of the polynronides in the humns are produced as a result of microbial synthesis, although in the case of certain types of humus, as in marine bottoms and in highmoor peat, the plant constituents which are high in polyuronides may contribute directly to this group of complexes. The extensive synthesis of polyuronides by various bacteria and fungi has been definitely established; it is also known that some of these compounds are decomposed only by certain highly specific groups of organisms. Humns formation is thus shown to consist of a gronp of processes which can be briefly summanzed as
follows: (a) decomposition of the water-soluble complexes, of most of the carbohydrates, of certain fats, and of proteins; (b) synthesis of new proteins and of certain hemicellnloses; ( 6 ) accumulation of lignins, in a modified condition, whereby they are rendered more readily soluble in dilute alkalies and become darker in color; they may actually become so changed, either by polymerization or by other reactions, as to become distinctly different in nature from the original lignin; (d) interaction of the proteins with the lignins and their derivatives, to give rise to ligno-proteins, whereby the active peptide linkages of the protein are bound and made more resistant to bacterial attack; (e) the fixation of the ligno-protein complex in the soil by bases, notably calcium and magnesium, thus giving rise to humus-rich prairie soils (tchernozems), or its partial removal by leaching, in the absence of sufficient bases, and its subsequent deposition in the lower horizons, as in podsol formation. CHEMICAL NATURE OF HUMUS
Humus contains substances derived from the original plant and animal residues, either in an unmodified or in a chemically modified form, their transformation products, as well as substances which have been newly synthesized by the numerous fungi, bacteria, and lower animals inhabiting the soil or the compost. Humns itself is not absolutely resistant to further decomposition; it is gradually destroyed, under favorable conditions, by various specific microorganisms, but at a slower rate than the original plant and animal residues; as a result of its continuous decomposition, a t a gradually diminishing rate, humus tends to reach a certain chemical equilibrium, and in that state it is usually spoken of as "completely humified material." This equilibrium is in itself not stable; at this point, humus is found to consist primarily of two groups of chemical complexes: (a) the more stable group, consisting of lignins or lignin derivatives and of proteins, the first being largely of p l a m origin (residual) and the second largely of microbiat origin (synthesized); (b) the less stable group, consisting of carbohydrates (cellulose, hemicelluloses), a small amount of nitrogenous complexes, fatty and waxy substances. The more stable group of humus constituents consists of a varying proportion of lignin and lignin derivatives to protein, depe'ndig on the nature of the original materials, the extent of their decomposition and conditions under which decomposition took place. This humus fraction is commonly spoken of as "humic acid," "pure humns," "true humus," or a-humus; in order to avoid confusion between this complex and other humus constituents, the designation of it as the "humus-nucleus" has been suggested (13). The less stable group of humus is more variable in composition and is less resistant to decomposition, depending also on the nature of the original materials and on the extent and conditions of their decomposition; certain compounds which can be classified with this group, such as various polynronides and resinous substances,
may be as resistant to active decomposition by the majority of microorganisms as those included in the first group. For the sake of convenience, the second group of humus constituents may be designated as the group of "accompanying substances." On treatment with mineral acids, one can separate these two groups of complexes, not on a quantitative basis however; the "accompanying substances" are largely hydrolyzed by the acid treatment, while the "humus-nucleus" remains more or less unaffected. C e m o s m o ~on P L AMATBBI*W ~ m
OII PEA=HDYOS(W*RSU*N AND Srsveas) (Per fmt. on bnsir 01 dm materid) SphngClodium. num. HighScdiCrccw h m o o r Young man W&Y lncnlnry Nofur. of Molniol Plan1 Pcol Plants Peal Pad Peal Ph"1 Conrlilurnlr:
1.14
1.10
1.47
organic m a t t e r, (C X factor)* 96.17 78.19 74.20 18.46 4.33 8.36 5.94 Total nitrogen 0.84 1.75 1.56 0.42 0.15 0.26 0.15 Nitrogen in humus 0.87 2.24 2.10 2.28 3.46 3.11 2.53 C:N ratio 59.5 24.7 26.6 24.6 16.3 18.3 21.8 (Comp%aion of humus, per cent.) Ether- and alcoholsoluble fractions
TABLE 6
Ether-soluble substances Water- soluble substanm Hemieellvlores Cellulose Ligoinr and ligoin derivative3 Crude protein Aah
TABLE 6
3.96
3.22
0.07
...
0.70 14.93 0
Cold- and hot~watersoluble fraction^ Hemicelluloses Cellul~e Lignins and lignin derivatives Protein 'The factor used for this csleulatioo
was
(1.684
+ 0.004 X
C/N).
sharply the nitrogen content of the humus as a whole, with the result that its carbon-nitrogen ratio becomes narrower and tends to reach a certain equilibrium. 6.97 38.26 60.73 23.07 20.W 50.33 18.72 6.58 14.30 20.69 5.88 7.19 These facts are largely responsible for the high carbon 1.50 3.18 3.30 25.30 3.89 10.13 content of the humus in most mineral soils, which was The chemical composition of humus as compared found to approach gradually 58 per cent., and also with the plant materials from which it has onginated for the more or less uniform relationship of carbon to is brought out in Tables 5 and 6. In the case of low- nitrogen in mineral soils, which tends to approach moor peat, one finds that nearly 70 per cent. of the 10 to 1. If the ratio between the carbon and nitrogen total material is made up of the ligno-protein.groups, is much wider than 10:1, it indicates that either a as compared with 36 per cent. in the original plant considerable amount of carbohydrate is present in the material. In the case of the highmoor peat, however, humus and is subject to rapid decomposition, or that which has not undergone as extensive decomposition an insufficient amount of protein has as yet been formed, and in which the organic constituents vary consider- or that the process of decomposition is highly specific ably in nature from the corresponding constituents in in nature and is brought about by certain characterthe grasses, the ligno-proteins make up only 35 per istic organisms as in the destruction of wood by "white cent. of the material, as compared, however, with 13 rot" fungi. A ratio of carbon to nitrogen much per cent. in the fresh sphagnum. The woody peat narrower than 10: 1 indicates either insufficient lignin has undergone changes similar to the lowmoor peat concentration in the plant residues, which results in as shown by the high ligno-protein content (75 per an excess of protein being formed in the decomposition cent.); it is important to can attention to the high processes, or that considerable destruction of the lignin protein content of this peat, as compared with that has taken place due to the specific conditions, such of wood (Table 1). In the case of the forest humus, as arid soils, or to specific organisms, as in the case of one can see a gradual reduction of the fatty substances, mushroom fungi. The above relationships are modified by the condiwater-soluble substances, and carbohydrates, and an increase of the ligno-proteins, as one proceeds from the tions of decomposition, especially aaation, reaction, and presence of bases. In the case of mineral soils, fresh plant residues (litter) to the true humus. The two major mmtituents of the "humus-nucleus" these relationships are responsible for the formation form not one but a series of compounds; these are of: (a) tchernozems, in which the humus is fixed by capable of combining with bases to form soluble or the calcium and magnesium of the mineral part of the insoluble "salts" (humates). Due to the high carbon soil; (b) podsols, in which certain of the organic and content of the lignin (62-64%) and of the protein inorganic constituents of the soil are removed by (5&53%), and to the low carbon content of the con- drainage waters and deposited in another horizon; stituents found in the group of "accompanying sub- (G) arid soils, such as chestnut soils and serozems, stances" (38-44%), the gradual removal of the latter in which the humus is characterized by a high protein with the advance in decomposition of the plant residues content and a narrow carbon-nitrogen ratio (Table 7). tends to increase the carbon content of the residual In the case of forest soils, these relationships are rehumus. Further, due to the fact that the nitrogenous sponsible for the specific vegetation and the formation substances in the humus are largely present in the of: (a) raw humus soils and (b) mull soils. In the "humus-nucleus,' the removal of the readily de- case of peats, they lead to the formation of: (a) highcomposable constituents tends to increase even more moor, (b) lowmoor, and (c) sedimentary peats, each 6.87 21.45 28.31
1.24 8.95 0
3.86 30.82 21.13
4 19.91
5.44 2.68
nature of humus as a whole, the methods of its determination have also undergone numerous changes. These fall into two major groups. (1) Some of the methods were developed for the purpose of determining the specific complex in the humus which gives to i t its characteristic black color. (2) The other group of methods has dealt primarily with the determination of the total humus content of the soil. Among the first group of methods, one need only mention: (a) the colorimetric determination of the alkali extract of humus, (b) the evaporation of the a m moniacal extract of humus (matidre noire), (c) the precipitation of the alkali extract with an acid, (d) the oxidation of the alkali extract with an oxidizing agent, (e) the oxidation of the total humus with a dilute oxidizing agent (6% H2O1), (f) the treatment of the humus with acetyl bromide. The second group comprises methods ranging from the loss on ignition to the calculation of the humus content on the basis of the organic carbon, using a known factor (1.724). Most of these methods, especially those based upon the determina ion of a fraction of the humus, are open to considerable criticism. By far the most reliable is the last one based upon the fact that the carbon content of the humus approaches 58 per cent. In the case of peats or the surface layer of forest soils, the loss on ignition is quite sufficient.
The action of humus upon the soil can, therefore, be considered as physical, chemical, and biological. Physically, humus modifies the color of the soil, it improves its texture, rendering i t less impervious to water, it imparts to i t a higher water-holding capacity, and a greater capacity for absorption of heat, salts, and gases; humus also acts as a binder for the loose inorganic particles. Humus is present in the soil largely as a hydrophilic colloid, with water as the dispersing medium, which is itself adsorbed. It forms various compounds with different inorganic soil constituents, the combination with bases in the haseexchange complex being of special interest; it increases the buffering properties of the soil against rapid changes in reaction; it also modifies the chemical composition of the soil solution. Finally, humus offers a favorable medium and substrate for the growth of the numerous soil microijrganisms as well as for the root systems of higher plants. As to the specific d e c t s of humus upon plant growth, it is sufficient to call attention to the recent ideas concerning the formation in soils and in composts of plant-stimulating substances and to the emphasis laid upon the relation of these to plant hormones and to animal vitamins. It is claimed (17) that these substances ("auximones," "phytamins") are synthesized by microorganisms and are absorbed by the plant, giving rise to hormones and vitamins. It was suggested HUMUS AND PLANT NUTRITION that the beneficial effect of composts of stable manures Humus in mineral soils is not absolutely resistant and plant residues upon plant growth is due to this to decomposition, but under certain conditions, es- relationship. pecially with proper aeration (cultivation), favorable Humus can be looked upon, in its relation to higher reaction (liming), temperature (summer), and mois- plants, as a storehouse of nutrients, which nature ture, it can be gradually decomposed. As a result keeps in reserve for plant life,, The key to this storeof this, the elements carbon, nitrogen, phosphorus, house is found in the activities of the numerous microsulfur, and others, so highly essential for plant nutri- organisms which inhabit the soil and which act upon tion, are again liberated in forms available to the grow- the humus in hundreds of different ways. Without ing plant. Humus influences plant growth in various the formation of humus, the soil would never be what other ways, which are still imperfectly understood; i t is and plant life on this planet, apd probably animal this is largely due to the presence or possible formation life as well, would have had an aspect totally different in humus of certain specific substarRes which are from that which it has today. toxic or stimulating to plant growth. LITERATIJRE CITED
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