ONE oj the most important things in the soil is humus, the product of decayed organic matter. This article discusses soil humus Jrotn the broadest stand- point, telling what it is; what its Junctions are; what factors determine ivhether the supply is large or small and whether it is slowly or quickly used up; how different types oJ soil differ in their humus content; how humus is related to the productivity of the soil. Soil Organic Matter and Soil Humus THE fundamental c4iaracteristic.of soil is its productivity; that is, its capacity to produce green plants. The plants/during a period of their growth, absorb and accumulate a certain amount of the radiant energ}^ of sunlight and convert it into a form available to other living organisms, which are incapable of utilizing the energy of the sunlight directly. Livnig matter, including, as it does, the sum total of all living plants, animals, bacteria, etc., is unicßie among the formations of the earth. It possesses a number of peculiar features not found in nonliving matter, such as a dissemination of its mass in a nudtitude of individual organisms, a capacity for endless self-renewal by means of reproduction, and a marlœd adaptability to environment. It has been assumed that the amount of Uving matter on the earth remains more or less constant, and it can further be assumed that this amoimt is limited by natural conditions at the earth's surface, such, as temperature, air pressure, and amount of available water. Within the limits fixed by these factors the earth's surface may be said to be saturated by Uving matter. The distribution of the latter over the surface area, however, is very uneven, depending upon local conditions. The regions where natural conditions are favorable for life harbor conspicuous congestions of living matter, whereas regions of the opposite type appear as virtual biological vacuums. Every natural province or landsca.i)e ^ is characterized by its O\\TI concentration of living matter, or so-called biological pressure. Bio- logical pressure represents the relationship between the amount of '- ('oustantin C. NikiforofT is Soil Sciontiiit, Soil Siirvov Division. Bureau of CJioniistrv nnd Roils 2/nio word ' latidscapc" as used in soil geosraphy moans tho sum tottil of the charactofisTics that distin- iîiiLMi n cvvUnn aroa on the. earth's surface from other areas. Tliesc cluiraetoristies are tho n^sult not onlv otnatural forces but of human occupancy aîid uso of the land. Included arnons them arc siich featun^s a^ soil typt^s, vegetation, rock formations hills, valleys, streams, ciiltivated iu^lds, roads, buiidiniïs. AÍl of these features toíieíher ^>,i\e I he area its distinguishing pattern, which is the landscape. ni)is:r^—ns on 929 930 ^ Yearbook, 1938 living matter and the surface area; its intensity can be expressed as the total weight of living matter per unit area or by the variety and number of organisms per unit area,. The biological pressure of every natural landscape (note the word "naturaF') is in equilibrium witli the potential capacity of the region for the reproduction and support of life. The exact composition of living matter is not known. It is kno^\TL, liowever, that living matter is made up of many elements, the bulk of which consists of carbon, oxygen, hydrogen, and nitrogen. Water is one of the most important constituents of living matter, both as to function and as to bulk. The development, the functions, and the very existence of organisms depend largely upon a sufficient supply of moisture. Relative intensities of biological pressure correspond very closely to the amounts of available moisture, provided other conditions, such as temperature and supply of mineral nutrients, remain relatively constant. Irrigation of many arid regions, for example, has demonstrated that the actual biological pressure of such areas is considerably lower than the potential pressure because water is deficient. Because of the rather narrow limitation of the total amount of living matter on the earth and because of its repeated turn-over during long geological eras, the elements that comprise it must pass through constantly recurring cycles. A certain amount of living matter, whether whole organisms or only certain parts of them, is continually dying and decomposing into its elemental constituents. At the same time, equal amounts of living matter are being synthesized anew in the growth of new generations. Endless successions of gener- ations have deposited their residues in the soil, from which soil organic matter and sou humus have been formed. In spite of the contiiuial depositing of residues, neither the soil organic matter as a whole nor the humus portion of it accumulates endlessly. They are both unstable and gradually decompose into the simple elemental constituents. COMPOSITION OF SOIL ORGANIC MATTER AND SOIL HUMUS The bodies of dead organisms and the residues of living matter depos- ited on and within the soil form the material known as soil organic matter. Soil organic matter theoretically comprises only the dead residues of organisms and the various products of their decomposition. It is practically impossible, however, to separate this material from the living micro-organisms that inhabit the soil and perform the taslv of decomposing the residues. The bulk of the bodies of micro-organ- isms, together with their own residues, is therefore commonly regarded as a part of soil organic matter. This matter includes the dead roots, leaves, fruits, and stems of pla.nts; carcasses of insects, worms, and animals; live and dead bacteria, fungi, and protozoa; various products of decomposition of the dead residues; and the newly synthesized substance of active micro-organisms. Chemically, soil organic matter represents a niixture of a great many different substances w^hich can be classified into three groups: (1 ) Carbohydrates, (2) protein, and (3) fats, resins, waxes, and similar compoimds. The gradual decomposition of these organic snbstances into the most simple mineral compounds is spoken of as mineraliza- Soil Organic Matter and Soil Humus -^ 931 tion. The ultimate end products of mineralization are principally water and carbon dioxide and smaller amonnts of free nitrogen, ammonia, methane, etc., and a few simple mineral salts. The process of mineralization, or reduction of the fresh, residues to the simple end products, proceeds gradually with the formation of a number of intermediate substances. Some of these are products of the decom- position of the original material, and others are the result of resyn- thesis. Sooner or later the entire amount of organic residues turned over to the soil in any given time undergoes complete mineralization. Soil humus represents a stage in the decomposition of soil organic matter. Organic residues as a whole do not decompose uniformly. Some substances decompose rapidly and are reduced to their ele- mental constituents in a short time; other substances are more resist- ant to decomposition, and the process of their mineralization may extend over a period of many years. The resulting mixture of many different compounds formed during the decomposition pi'ocesses is humus. It has no definite chemical composition, but pln^sically it is a homogeneous, amorphous, dark-colored, and practically odorless material. Fresh organic residues, as well as those in an advanced stage of decomposition which still preserve a specific structure of the organized tissues, are not considered a part of soil humus, although they are sometimes referred to as raw humus. ACCUMULATION OF HUMUS IN THE SOIL Soil humus is not a stal)le material. As the organic matter decom- poses, new humus is continually being formed, and part of the old is being completely mineralized. The equilibrium between the two proc- esses determines the amount of humus present in a soil at a given time. During the development of a young and immature soil the amount of new humus annually added is greater than the amount undergoing mineralization, and a gradual accumulation occurs. As the soil develops and approaches maturity, the absohite amounts of humus undergoing mineralization gradually increase until they equal the amounts of newly formed humus. Fi'om that time on, the two proc- esses—formation and mineralization—proceed at an equal rate, and the soil may be said to have reached a state of maturity or one of equilibrium with its natural environment. The average content of humus in the mature soil remains relatively constant as long as no change in natural conditions occiu^s. Any change in the natural conditions that upsets the ecpiilibrium will be followed by a corre- sponding change in the humus content of the soil. A great many profoimd changes of natiu'al environment are caused by man. To mention only a few of the more spectacular meddlings with nature, irrigation of arid huid, drainage of wet land, deforestation, annual removal of crops, and the breaking of sod all lead to a rapid and marked change of the humus balance of the affected soil. The stability of the new soil conditions depends entirely on the stability of the changes effected in the environment. As a general rule, nature sooner or later tends to obliterate all these changes and to reestablish her own ways. Reestablishment of the old balance, however, usually takes a much longer time than it did to accomplisli the striking changes produced l)y artificial processes. 932 4- Yearbook, 1938 FUNCTIONS OF HUMUS Tl^c function of Imnms in any soil system and its significance for i]iiiintenance of the productivity of the soil is determined by its dy- namic character. Hiiiniis is not a mechanical accumulation of static or iîiort material. It represents a certain stage of an endless turn-OYer or oxcJiange of certain elements between liviTig matter and the mineral kingdom.