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  By 'soil' we understand (Vil'yams, 1931) a loose surface layer of earth capable of yielding plant crops. In the physical sense the soil represents a complex disperse system consisting of three phases: solid, liquid, and gaseous.

  The solid phase of the soil consists of individual diverse particles, according to their chemical composition and dimensions. The size of soil particles fluctuates within a wide range.

  The different properties of the soil and its fertility depend, to a considerable extent, on the composition and size of the particles. The soils are subdivided according to the size of particles as follows:

   Every soil contains particles of all the above-mentioned dimensions but in a different quantitative relationship. The soils are subdivided according to their mechanical composition an follows (according to Kachinskii, 1956:

  The quality of the soil as an environment for microorganisms is determined not only by the size of the particles but also by the character of their distribution, in other words, by the structure of the soil.

  The soil may exist In two states: in a dusty state or in a form of aggregates. In the dusty form all properties of the soil are distributed in all directions. Such soil reacts in a uniform manner to external factors. For example, it swells uniformly in all directions upon wetting, thus, becoming inaccessible to air and water. Such soils are called structureless. As a result of their purely physical and mechanical properties they are unable to retain water for long periods, they dry up rapidly, and are poorly aerated. The conditions for plants and microorganisms are not favorable.

  The structured soil consists of aggregates of different sizes, from 1 to 20 mm and more in diameter. The aggregates in the soil are more or less loose, the intervals between them are of different sizes and forms. These intervals or pores ensure the presence of water, air permeability and various foodstuffs for plants and microorganisms. In such a soil the evaporation of moisture is diminished.

  The structure of soils is of the utmost importance. With each increase in the degree of orderliness of the structure, the physical properties of the soil aeration, water permeability, etc are improved. When comparing these properties of the structured and structureless soil, Remezov (1952) of the Dolgoprudnoya Experimental Station gave the following figures:

  As can be seen from the above figures, the capillary porosity in the structured soil increases sharply and many other positive properties improve, thus securing high fertility and abundant proliferation of microorganisms.

  The structure which contains aggregates 1-10 mm in diameter, which are water-resistant, is of the greatest importance.

  In each soil aggregate the individual particles are in close contact with each other. The attraction bonds between them are quite different from those acting between the aggregates. The distribution of air, water and different elements of the soil within the aggregates is of different nature unlike those between the aggregates.

  Due to the ability of the structured soil to absorb and hold water, as well as to its other properties, the structured soils are better able to provide conditions indispensable for the growth and development of plants.

  The basic conditions for the formation of the structured soils is the presence of a sufficient quantity of silt-colloid particles, organic material, and other factors essential to the growth and life of microorganisms. The soil particles are glued together with the help of colloids, the products of microbial metabolism. Without these colloids the soil particles would not aggregate and would remain in an isolated state.

  According to Godroits, the binding of the smallest elementary particles into microaggregates precedes the formation of aggregates; these can be destroyed only by chemical means.

  In the presence of colloids small soil particles and microaggrogates combine into simple aggregates and these, in turn, into complex ones. These aggregates are of different size and form. Between the simple aggregates there are intervals or pores which, as we shall see later, are of the utmost importance in the life of microorganisms. The glueing of soil particles may be stable or unstable. The strongly glued particles determine to a considerable extent the fertility of the soil.

  There are various conditions and determining factors in the formation of soil structure. Humus and organic substances are of the greatest importance. The influence of manure, composts, and other organic fertilizers on structure formation is well known. P.A. Kostychev has empirically shown the influence of organic substances on the processes of soil-structure formation and noted the supreme importance of microorganisms.

  The formation of soil structure is closely linked with the plant cover. It was thought until recently that only grass considerably improves the structure of the soil while woods destroy it. Recent investigations have shown that forests, too, favor the formation of soil structure.

  Zonn (1954) gives the following data on the per-cent content of waterproof aggregates of soils having different plant covers. In the solonets, steppe grasses, waterproof aggregates comprise 17%; under oak forests, 58%; on chernozems plowed for 50 years, 17%; and under a forest belt, 47%.

  The formation of soil structures is a subject of the investigations of many scientists. The first attempts to explain the cause of aggregates was made by biologists.

  The scientific elucidation of the above process is due to the Russian scientist P. A, Kostychev. Kostychev mastered the methods of microbiological and physicochemical analysis of soils. He proved experimentally that there are the closest links between the microbiological processes of organic substance decomposition in the soil and the formation of structure. He showed that certain substances which serve as a cement are required for the soil particles to combine into aggregates. Such substances are the products of microbial metabolism and of the disintegration of animal and plant residues. According to his experimental data, the process of structure formation takes place only under conditions optimal for microbial proliferation, when the biological transformation of the organic mass, with the consequent formation of cementing substances, is possible.

  When studying the decomposition of plant residues, Kostychev (1889) noticed an abundant growth of fungi and bacteria which developed until the whole mass of the decomposing substance was covered with these lower plants. He also showed that in different cases of decomposition of plant residues the course of the process varies and is caused by different microbes. Sometimes bacteria are the first to develop on decomposing material and the plant residues become covered with slime; sometimes fungi develop first and then the surface of the substance never becomes slimy.

  When studying the growth of microorganisms on decomposing organic compounds, Kostychev noticed that when only bacteria develop the substrate never darkens. The substrate, according to his data, becomes colored only in the presence of fungi. On the basis of these findings, the author concluded that fungi play the most important role in the darkening of the substrate and the bacteria have no part in it. At the same time, Kostychev did not preclude the role of bacteria in this process. He pointed out that when the access of oxygen to the substrate is arrested, fungi stop their activity and their growth ceases, but the process of decomposition continues, though at a lower rate. (Kostychev--The formation of chernozem. The soils of the chernozem belt of Russia, their origin, composition and properties, 1889).

  It should be pointed out that Kostychev (1892) not only determined the importance of microbes in the decomposition of organic compounds but was the first to notice their synthetic role in the soil. He wrote that humus does not represent a dead mass but in every way is full of life in all its diversity. Not only are processes of decomposition of complex organic compounds taking place in it, but also the synthesis of complex compounds from simple ones.

  Under certain conditions the humus is destroyed with the liberation of mineral compounds utilized as nutrients of plants. The structure of the soil is then destroyed. If the soil in maintained under such conditions, an irreversible process of humus decomposition takes place, then, parallel to a certain accumulation of plant nutrients, the structure of the soil will be destroyed and, as a result, its fertility will decrease. Kostychev recommended that the structure of the soil should be continuously regenerated by growing perennial plants, pointing out that this process of regeneration takes place in the deposit (Kostychev, 1882).

  The ideas of Kostychev were developed and furthered by Vil'yams. Vil'yams paid special attention to humus in the structural formation of soils, pointing out that not every type of humus is capable of forming a stable soil structure; only freshly precipitated humus of given properties may contribute to the stable soil aggregate (Vil'yams-Lectures on the soil science given at the Moscow Agricultural Insitute in 1895-1898, 1897). Vil'yams (1897) thought that physicochemical agents and rain water play a considerable part in the destruction of soil structure. He noticed that rain water containing ammonia destroys soil aggregates liberating calcium, and, as a result, humus loses its cementing properties. According to him, the main factor in the regeneration of soil structure is the sowing of grasses. Later. Vil'yams suggested that biological processes, caused by the metabolism of microorganisms, are the most important factor in the formation of structure. The process of organic residue decomposition varies according to the degree of aeration and is caused by different types of microorganisms. Vil'yams distinguishes two entirely different processes in the decomposition of plant residues which are of great importance in the structural formation of soils and these are: aerobic (caused by fungi and bacteria) and unaerobic processes.

  The aerobic process takes place in the surface layers of the soil, where plant residues are decomposed by microbes, bacteria, and fungi. The bacteria decompose the residues with the formation of humic acid, which, under conditions of full aeration, is fully mineralized. Therefore, Vil'yams does not think that humic acid is essential for the glueing together of soil particles. Only when this acid penetrates the lower layers of the soils, where unaerobic conditions prevail, it undergoes denaturation and is transformed into more stable compounds capable of coalescing the soil particles into water-resistant aggregates.

  When the decomposition of plant residues is carried out by fungi, crenic and apocrenic acids are formed. These acids are highly soluble in water and are leached from the soil. They are, therefore, of no great importance in the formation of structure.

  According to Vil'yams, the anaerobes are of the greatest importance in the formation of soil structure. According to him. these microbes synthesize ulmic acid through the decomposition of root residues, especially of grassy vegetation, This acid upon denaturation is transformed into a colloidal state, thus obtaining cementing properties. Upon dehydration it becomes insoluble in water and, conesquently, the particles which it glues together become water-resistant.

  The ideas of Kostychev and Vil'yams, concerning the role of microorganisms in the formation and destruction of structures, were developed in the works of Soviet and some foreign investigators (Mishustin et al; 1945a, 1951, 1936; Gel'tser, 1940; McCalla, 1943, 1945; McHenry and Russel, 1943, 1944, and others).

  The various investigators of the role of microbes in the process of structural formation had different approaches to the problem of the mechanism of their action. Some of them assume that microorganisms, by decomposing plant residues, form intermediate decomposition products which are responsible for glueing together the soil particles. Others assume that soil particles are coalesced by the products of microbial metabolism, while plant residues serve as a substrate for their nutrition (Gorshkov, 1940; Gusev. 1940; Rubashov, 1949, and others).

  Some scientists (Kanivets, 1951, and others) suggest that fungi such as Trichoderma lignorum, Mucor intermedius, and Martierella isabellina are the most important factor in the process of structure formation. Foreign investigators tested Trichoderma köningii, Aspergillus niger and other fungi with positive results (Martin and Andersen, 1924; Martin and Waksman, 1940; Martin, 1945, 1946; Peels, 1940; Peels and Beale, 1944). Gel'tser (1940) suggests that lysates of fungal cultures formed by bacterial action and also colloidal protein compounds synthesized by bacteria represent the cementing substance.

  Kononova (1951) assumes that the cementing substance consists of a mixture of cellulose decomposition products and protoplasm of the decomposed cellulose bacteria.

  All these studies clearly demonstrate the essential, if not the exclusive, role of microorganisms in the formation of soil structure. However, the essence of this action remains unclear. The published data are not convincing and require verification.

  Considerable work was carried out by Mishustin et al. He studied the action of fungi, actinomycetes, and bacteria on the decomposition of organic compounds (peptone, albumin, saccharose, starch, malic acid) artificially introduced into the soil on the formation of soil structure. The author concludes that the aggregation of soil particles is accomplished most vigorously by fungi and actinomycetes of mycelial structure. Thirty-two to fifty-one per cent of aggregates of 0.25 mm and more in size is obtained in experiments with fungi. Experiments with actinomycetes yield 25-30%. This value never exceeded 2.8% in the control soil. The various kinds of fungi have unequal powers of causing aggregation. Various sporogenous and asporogenous bacteria were studied including representatives of the genera Azotobacter, Pseudomonas Rhizobium, Bacterium, Bacillus, and others. Their cementing ability was very weak. The number of aggregates did not exceed 2-3%. more often 1-1.5%, and only in sporadic cases were there more than 10%. The cementing of soil particles proceeds considerably weaker under the influence of the simultaneous development of fungi and bacteria than in experiments with pure cultures of fungi. The opinion of the author is that bacteria in mixed cultures lower the aggregating action of fungi and actinomycetes. This is apparently caused by the action of bacteria in supressing the development of fungi or by their destroying the cementing compound formed by the fungi. Mishustin distinguishes two types of structure formations: biological, caused directly by microorganisms, and the so-called type of "humus structure", formed by humus compounds. The first type yields an unstable structure and is considered as an original and indispensable stage in the natural circumstances of structure formation (Mishustin and Pushkinakaya, 1942; Mishustin, 1945a). Rudakov (1951) assumes that the cementing of soil particles into aggregates in carried out by active humus, which comprises a complex of compounds of uronic acids and bacterial proteins, or products of their lysis. The uronic acids are formed, in his opinion, by pectin-destroying or protopectinase bacteria, which are found in greater or lesser quantities in the soil and on decomposing plant residues. Bacteria possessing protopectinase grow on plant roots, penetrate intracellular spaces, destroy intermediate compounds such as protopectin, and in so doing, form sugars and uronic acids. The latter, by combining with bacterial proteins, form uroprotein complexes capable of cementing soil particles. The galacturonic and other uronic acids are formed at the expense of living roots of vegetating plants as well as at the expense of dead root residues.

  Not all bacteria, according to the author, possess protopectinase activity. The most intensive production of protopectinase is observed in the sporogenous bacilli, Bac. polymyxa, Bac. macerans (Clostridium), Ps. radiobacter, Bac. mycoides, Bac. asterosporus, and others possess weak protopectinase activity while Bact. coli is completely devoid of this enzyme,

  Employing these bacteria together with vegetative residues of clover as a fertilizer, Rudakov obtained the following results expressed as the percentage of water resistant aggregates of 0.5-3 mm:

  According to Lazarev (1941-1945), the formation of soil-particle structure is done essentally by the ß-humatic, partially by the a -humatic, and partially by the complex humus formed from the products of bacterial autolysis.

  Tyulin (1954) distinguishes two types of structural aggregates in forest soils of the podzol belt: unstable calcium-humate aggregates and stable iron-humate aggregates. These forms of aggregates and the differences between them can be detected by special methods devised by the author.

  The nature and properties of the soil aggregates are determined by the quality of the humus compounds and by the nature of their interaction with mineral particles. The glueing ability depends on the elements contained in the humus, The presence of a sufficient quantity of calcium confers upon it certain properties; the presence of iron or aluminum confers different properties (Antipov-Karataev, Kellerman and Khan, 1948; Ponomareva, 1951, and others).

  The glueing properties of humus are heavily dependent on the preponderance of humic and ulmic acids on one side, or crenic and apocrenic acids (fulvo acids) on the other. It is well known that the composition of humus varies in different soils. The humic acids of podsol soils differ from those of the chernozem or serozem soils, The fulvo acids of krasnozems and podsol soils also have a different composition (Tyurin, 1949; Kononova, 1953-1956, Ponomareva, 1956, and others).

  All these variations in the humus of various soils have a definite effect on the nature of the structure of the soil particles. Ponomareva (1951) studied the structure formation in soils in relation to the development and metabolism of worms. She observed that the excrements of worms in the upper plow layer comprises, on the average, about 52 tons per one hectare which is about 1,700,000 individuals. The soil near plant roots is infested with worms to a greater degree than the soil outside the zone of roots (approximately 50%).

  The excrements of worms are glued more compactly than the aggregates of ordinary soil. Their water resistance is conditioned by the cementing organic compound of worm intestines. The author showed that the degree of the soil structure due to worms varies in relationship to the vegetation. In oak forests the quantity of worms is larger than that in fields under oats and grasses, or in fir forests.

  Under two-year-old grasses, 1,790,000 worms were found per hectare, in fields under oats, 560,000; in oak forests, 2,940,000, and in fir forests, 610,000,

  The above data show how closely the mechanism of soil structure formation is connected with the biological and biochemical processes which take place in its depth. The plants, their composition on one hand, and microbial biocoenoses of the soil on the other, determine, under certain soil and climatic conditions, the direction and degree of the formation process of soil-particle structure.

  Owing to the structural forms and aggregates, interspatial spaces or pores of different sizes and configurations are formed in the soil (Figure 49 A and B). The biological activity of the entire soil population is concentrated in these pores.

Figure 49. Soil porosity:

A) porosity of cultivated structured soil (schematic): 1--thin, predominantly capillary pores in aggregates, which fill with water on wetting; 2--medium-sized pores (cells, channels), upon wetting they will fill with water for a short period and subsequently, after the resorption of water, with air; 3--capillary pores; 4--large pores, between aggregates almost always filled with air (according to Kschinakii, 1956); B), visible porosity of soil aggregate (reproduction from a microsection). Thin chernozem (southern): 1--micro-aggregates; 2--visible pores (according to Kachinskii et al., 1950).

  The total volume of all the pores in 1 cm³ of soil taken in its natural surroundings is designated as soil porosity. The porosity of soils is one of the most important factors determining their fertility. The porosity volume varies in different soils and depends on the type of soil, its state, external, seasonal, climatic influences and on the vegetation. However, the most important factors in determining soil porosity is the degree of soil structure, and the size of soil aggregates. The smaller the aggregates the less the spaces between them and the greater the total porosity. The size of the spaces between soil aggregates and particles varies from several microns to 2-3 mm and more (Kachinskii, Vadyunina and Korchagina, 1950).

  The total porosity is determined according to its volume and specific gravity. It is determined by the following formula:

where P--porosity, sp--the specific gravity of the solid phase, vol--weight by volume. The specific gravity of the solid phase (ratio of the weight of the solid phase to the weight of water) for different soils fluctuates from 2.4 (chernozem) to 2.7 (krasnozem).

  The total porosity of different soils varies (Table 7). The chernozems have the highest porosity, 63-58%, and the highest total aggregation, more than 50% in the upper horizon A and somewhat less, 46%, in horizon B. Solonets has a lower porosity: the total, 50%, aggregate, 29.55% in horizon A₁. In the sod-podsolic soil the porosity is lower than that in the aforementioned ones: the total, 47-49%, aggregate, 32%, and the interaggregate, 15-16%.

Table 7
Soil porosity
(According to Kachinskii et al., 1950)

Soils	Depth of the layer, cm	Total porosity, %	Intra-aggregate total porosity, %	inter-aggregate porosity, %
Lixiviated chernozem with slght clay admixture, Kurak Oblast', steppe	.
	A--0-4	63.86	40.54	23.32
	A1--10-14	61.17	39.30	21.87
	A2--40-44	58.75	--	--
	B1--55-59	58.93	36.43	21.82
	B2--80-84	57.85	36.03	21.82
Nutty-lumpy solonets with slight clay admixtures, Sverdlovak Oblast', virgin soil
	A1--10-14	50.00	29.55	20.45
	B1--15.19	50.18	20.27	29.91
	C--60-64	44.40	--	--
Medium podsolic loam, Moscow Oblast'
	A1--0-12	49.05	31.61	16.43
	A2--20-32	47.55	32.27	15.28
	B1--32-35	41.70	--	--
	B2--55-85	36.76	--	--
	B2--85-110	34.10	23.78	10.32

  Soil porosity is subdivided into intra-aggregate and interaggregate porosity. The first determines the free space between the soil particles inside the aggregate. The total size (volume) of this porosity varies considerably. The individual pores and slits inside the aggregates are, as a rule, smaller than the interaggregate pores.

  The interaggregate porosity consists of the intervals between the individual aggregates. As can be seen from the data in the table, it occupies considerably smaller volume than the total intra-aggregate porosity.

  Besides this porosity, there are capillary and ultracapillary porosites. The size of pores in these cases in very small. They possess peculiar physical and mechanical properties for retention of water and air.

  Soil water as a physical body may exist in three states: solid, liquid, and gaseous. The water is subdivided according to its mode of binding with soil particles into categories, forms, and types (Rode, 1952, Dolgov, 1946). There are several classifications of soil water. The most widely used one in that of Lebodev (1930). The following forms of water are recognized (see Figure 50).

Figure 50. Schematic representation of various states of water in sand (small circles denote molecules of water in the form of vapor):

1--sand grains with incomplete hygroscopicity, 2--sand grains with maximal hygroscopicity, 3-4--sand grains with film water, water moves from the grain with a thick film (4) toward the sand grains with a thin film (3) until the thickness of the films equalizes (dotted); 5--sand grain with capillary water (according to Lebedev, 1936).

  Water vapor. Water vapor saturates the soil air spaces. In the soil it moves from places of highest saturation to places of lower vapor pressure. When the soil is cooled or heated unevenly, the vapor pressure changes correspondingly. At night, for example, the upper layers of the soil cool more rapidly than the lower. Due to the difference of the vapor pressure thus formed, the latter moves from the lower layers upward where it condenses on the surface of cooled soil aggregates. During the day the reverse process is observed, owing to the warming of the upper layers of the soil.

  The soil can be enriched with water at the expense of the water vapor of the air, subject to the same law. Upon cooling of the upper layers of the soil, the water vapor of the air condenses upon them. This is the diurnal distribution of water vapor in the soil.

  Deeper movements of water vapor take place in accordance with seasonal temperature changes.

  The water vapor is consequently of great importance in the enrichment of the soil with moisture. It should be assumed to play an especially important role in southern and regions with a distinct continental climate, where sharp differences in day and night temperatures cause considerable vapor condensation in the soil,

  Hygroscopic water is physically closely bound to soil particles. It covers soil particles and is maintained there by the forces of molecular cohesion. Its density is more than 1; its specific heat is about 0. 9; it does not freeze, The forces of molecular cohesion are stronger than the earth's gravity; consequently, there in no hydrostatic pressure. When the soil is moistened to such an extent that its particles are covered with a continuous layer of water molecules, we talk of a state of maximal hygroscopicity. Similar conditions exist upon saturation of a space with water vapor.

  The hygroscopic water is so strongly bound to soil particles that it can move only after it has been transformed to vapor. The removal of this water from the soil can be accomplished only by heating to 105° C.

  The capacity of the soil to absorb or strongly bind water is called hygroscopicity. The latter depends upon many factors: mechanical composition of the soil. the content of organic compounds or humus, different organomineral compounds and metabolites. It increases with the content of humus and products of the metabolism of microorganisms in general.

  The smaller the soil particles, the higher their adsorption capacity. The process of water adsorption by the soil particles is accompanied by the liberation of heat of wetting. Its value in various soils varies from 3 to 10 cal. For the gray forestbelt soil, solonets, and chernozem it is 5. 8, 5.92, and 6.06 cal, respectively, and for peat soil, 14.80 cal,

  The film water to also physically bound to the soil, However, the bonds of its molecules to the soil particles are less strong than those of the hygroscopic water. It does not obey the law of gravitation upon movement, and there is no hydrostatic pressure either.

  The molecules of film water (according to Lebodev's scheme) cover soil particles with a continuous thin layer or with a film. Its force of cohesion to the soil particles is weaker than that of hygroscopic water. The film water is bound loosely with the soil, it moves as a liquid, but differs from liquid water in certain physical properties; it has a higher viscosity and a lowered freezing point. According to some data, part of this water does not freeze at -78° C and complete freezing is noted only at -150° C (according to Vilonskii, 1954).

  The amount of hygroscopic and film water in the soil changes in relation to various factors: osmotic pressure of the soil solution, soil composition, and others, In soils poor in soluble compounds such as some of the sod-podsols, the film water may exceed the maximal hygroscopicity. and in soils rich in dissolved substances it may equal or be even lower than the maximal hygroscopicity. In saline soils, loosely bound water may be completely lacking (Dolgov, 1940).

  The physically bound water is assumed to be inaccessible to plants, The suction power of roots cannot overcome the force of the attachment of its molecules to the soil. It is not known if it is accessible to microbial cells, This water comprises about 1% of the soil's dry weight in sandy soils poor in humus, in loams, 3-5%, in soils rich in humus, 10%, and in peat soils, above 10%.

  Apart from the aforementioned two categories of water, chemically bound water is also present in the soil. It enters into the composition of minerals and represents an integral part of compounds appearing as water of crystallization and hydration. This water can be removed only upon prolonged heating, whereby essential changes of the properties of the heated compounds take place. This water is unavailable to organisms.

  Gravitational or filtration water-- free water. It moves in the soil as a liquid under the pull of gravitational and capillary forces. It filters downward through the depths of the soil, obeying the law of gravity.

  Upon filtering downward it reaches subsoil water or is converted in higher levels into mobile capillary water. Its greatest quantity held by adsorption and capillary forces corresponds to the total moisture capacity of the soil. The ability of the soil to let through this water is called soil permeability. The gravitational water is accessible to plants and other organioms living in the soil. It can be converted into capillary water and, as a result, so it is assumed, it is rendered unavailable.

  Capillary water-- the free part of water which fills the pores (the capillary space between soil particles) and moves in them by means of capillary forces caused by surface tension and the wetting of the surface of soil particles.

  The water in capillaries rises higher, the smaller their diameter. At a pore diameter of 1µ the height of a water column is 15 m. Under natural conditions the water in soils and subsoils usually does not rise that high.

  Analyses of the soil pores show that they represent a complex system. They spread out in different directions and the capillaries merge with pores of large or small dimensions and different forms.

  The velocity with which the water rises in the capillaries is called the water-rising capacity. It to qualified by various conditions and also by the properties of the soil. The structure of the soil particles is of great importance. The larger the aggregates, the easier the penetration of the water through the soil. In soils without aggregates the pores are small, the water filters through these pores slowly, but it can rise quite high. It evaporates on reaching the soil surface. The less structured the soil, the more rapid is the desiccation of the surface layer. The higher the temperature of the air, the quicker the ascent of water in the capillaries and the desiccation of the soil.

  Swelling of the soil strongly affects the flow and ascent of water. When mechanical composition of the soil becomes heavier, the ascent of water is slowed down and can be completely arrested, as it is in the solonets soils or illuvial horizons of the sod-podsolic soils (Kachinskii, 1956). Capillary water may flow in any direction.

  Capillary water exists in three states:

  1. The capillary immobile water consists of individual small droplets separated from each other. In this case, stasis of water takes place, and the conditions for the development of microorganisms are different from those in continuously flowing water. The separated water droplets in soil capillaries resemble, to some extent, the nutrient solution in glass capillaries obtained artificially in the laboratory.

  2. The capillary mobile water is characterized by its discontinuous location in capillaries. It is available to the plants.

  3. The easily mobile capillary water fills the pores of the soil and the subsoil over the ground water. It is easily available to plants. The capillary water represents the most important part of the soil moisture being available to all forms of life in the soil.

  The classification of soil water given here is of a relative character. The quantitative content of free and bound water changes in relation to the properties of the solid and gaseous phases. The conversion of one form of water into another is also conditioned by changing external factors: temperature, pressure, various admixtures of organic and inorganic compounds, etc.

  The physically bound water can, under certain conditions, be partially converted into free water and vice versa.

  The soil water is in constant motion and this is of particular importance to organisms, since, together with the water, nutrients and oxygen required for respiration are also continuously supplied. Due to the difference in vapor pressure in various sections of the soil the water flows in different directions, vertical and horizontal. Only on rare occasions, when air pockets are formed in thin pores or capillaries, is the water stream interrupted and becomes static. 'This stasis in temporary or relative, since water in such cases still moves through conversion into the gaseous state,

  It follows that the soil pores are filled with moisture in the form of vapor and with free and bound liquid water. Water is found in the intervals between the soil particles no matter how small they are, between the aggregates and within them. All forms of free and loosely bound water are available to microorganisms.

  The individual intervals between aggregates and particles represent sites of concentration of individual types and groups of microbes as shown in the schematic figure (Figure 51). The latter form more or less isolated colonies in each of the intervals of the soil space. In these micro- and macrocenters the microbes live, reproduce, and carry out various biochemical processes. Products of their metabolism diffuse into the surrounding medium.

 

Figure 51. Schematic representation of the structure of a soil aggregate and the distribution of microorganisms:

a) aggregates consisting of many microaggregates; b) pores between the microaggregates filled with bacteria (b_l); c) pores between the aggregates within which develop individual cells (d) and colonies (e) of bacteria, fungi, and actinomycetes.

  The pore system determines the centers of development and the type of propagation of microorganisms. As the separate large pores are usually interconnected through smaller channels of capillary structures, the colonies of microbes which develop in large pores or centers are not separated from each other. There is an exchange of metabolites between the colonies of the individual types and groups of microbes, and an interrelationship of an antagonistic or nonantagonistic character is thus established. Investigations show that microorganisms and especially bacteria live not only in the pores and spaces between soil aggregates but also on the surface of the latter. As will be shown later, bacteria are adsorbed by soil particles and can live and grow in such a state. Different organic and inorganic substances which can be utilized by the microbial cells are also adsorbed on the surface of soil particles.

  The adsorbed bacteria react with the adsorbed vitamins, antibiotics, toxins, and other compounds, according to their nature. The microbial cells grow well or badly according to the conditions created on the surface of the soil particles.

  The structure of the system of pores, or porosity in general, determines, to a certain degree, the manner of distribution of microorganisms in the soil. With changing porosity the distribution of microbial cells also changes.

  Diverse manifestations of life activity of soil microorganisms are also reflected in the character of the soil-porosity system.

  The soil solution represents a very dynamic and indeed the most active part of the soil. Different chemical and biological processes take place in it. The composition of the soil solution is an important factor in the nutrition, growth, and reproduction of organisms. To a great extent it also determines the total productivity of the soil, G. N. Vysotskii (1902) compared the soil solution to the blood of animals.

  Molecularly and colloidally dispersed compounds of mineral, organic, and organomineral composition are present in the soil solution. The following mineral compounds can be detected in the soil solution: ammonia salts, nitrites, nitrates, bicarbonates, carbonates, chlorides, sulfates, phosphates in the form of salts of calcium, magnesium, sodium and potassium; compounds of iron, manganese, aluminum, and silicon; microelements: zinc, copper, cobalt, vanadium, boron, molybdenum, radium, and others,

  The amount of these compounds in the soil solution fluctuates in relation to the peculiarities of the soil and climatic conditions and also depends on the solubility of the compound (Vilenskii, 1954). The data on the solubility of the individual mineral salts present in the soil are given in Table 8. It can be seen from the table that the solubility of the salts fluctuates within a wide range. It increases with the rise of temperature. The solubility curve is different for the various salts.

  The presence of gases in the solution strongly affects the solubility of salts. For example, carbonic acid increases the solubility of calcium carbonate, converting it into bicarbonate, the solubility of which exceeds many times that of carbonate. Sodium chloride in solution increases the solubility of gypsum; and sodium sulfate, on the contrary, lowers it. The concentration of salts in the soil solution changes relatively to the soil moisture.

  When the soil dries, the solution concentration of salts increases, then the salts crystallize and precipitate, First to precipitate are the carbonates of alkali metals, then gypsum, and, finally, the easily soluble compounds.

  With the increase in moisture the concentration of the solution decreases and the majority of salts redissolve.

  There are also gases in the soil solution which are either absorbed from the atmosphere or formed, in the soil. Especially large amounts of carbon dioxide and oxygen are found. Their solubility changes in relation to barometric pressure, temperature, and certain other factors. The higher the temperature, the lower the gas solubility, The solubility of gases is directly proportional to the partial pressure of the gas. Since there are relatively more gases in the soil and their pressure is relatively high, it follows that their concentration in the soil solution is higher than in water in an open space, The presence of electrolytes in the soil solution decreases the solubility of gases.

  Colloidal compounds in soil solutions comprise, according to Gedroits, 5-20% of the dry weight of the solution's residue. The majority are of organic origin. Silicic acid and iron and aluminum hydroxides may be found in the soil solution in the colloidal state.

  The soil solution contains all the soluble organic compounds formed and released by plants, animals, and microorganisms, and also many substances synthesized directly in the soil outside the organisms by free extra-cellular enzymes. One can also find in the soil solution humus compounds, humic acids and their salts, various acids, alcohols, esters, amino acids, antibiotics, and toxins.

  The soil solution represents a nutrient medium for the entire population of the soil and especially for the microorganisms. In all cases when the medium is favorable and there are no hindering factors, the amount of organisms is abundant. The more nutrients in the solution, the more intense the development and metabolism of soil microorganisms. Fertile soils and soils fertilized with large amounts of humus have a high concentration of nutrients in their soil solutions. Soils of low fertility, not containing humus, have small concentrations of nutrients in their soil solutions and growth of microbes will be slight.

  We have compared the nutritional values of soil solutions of four samples of soils. One sample was taken from the garden, well fertilized; the second from fertilized fields of the sod-podsolic belt (Chashnikovo, Moscow Oblast'); the last two samples from the chernozem of the Moldavian SSR and serozem of the Uzbek SSR. The soil solutions were obtained from wetted soil (60% of total moisture capacity) under high pressure (about 100 atm.) with the aid of a press. The solution obtained in one series of experiments was sterilized in an autoclave at 110° C for 40 minutes, in the second series it was filtered through Seitz filters; the third portion remained unsterilized. All three portions of the soil solution were Inoculated with six kinds of bacteria preliminarily tested and proven to be pure cultures.

  The results are given in Table 9.

  As can be seen from the data the garden soils and especially the chernozem soils contain more nutrients for the bacteria than soils from fields with low contents of humus. Serozems have more nutrients than sod-podsolic field soils.

  Sterilization by autoclaving improves the nutritional value of the soil. Apparently it in due to the hydrolysis of certain organic compounds which become more available for assimilation by the organisms. Upon filtration of the soil solutions, some substances are hold back by the filter, reducing the amount of nutrients.

  Not all bacterial genera react similarly to the nutrients of the soil solution. For example, Bacterium No 1 develops most abundantly in the soil solution of a garden soil of the podsol belt, and Bacterium No 5 in the solution of chernozem. The culture of Bacterium No 23 grows preferably in the solution of the serozem soil. The root-nodule bacteria of lucerne develop well in the solution of chernozem and serozem and almost do not grow at all in the solution of the field soil of the podsol belt,

  The reaction of the soil solution is of great importance for life processes in the soil as well as for many physicochemical and biochemical reactions. Too acid or too alkaline a solution is of little use, or of no use at all, for the growth and development of organisms.

  The reaction of the soil solution is conditioned by the dissolved salts. The acidity of the soil is caused In some cases by hydrogen ions present in the soil solution, in other cases by adsorbed ions. The first in called an active and the second , a potential, acidity. Besides these two, we also distinguish a total or titrable acidity or alkalinity which is determined by ordinary titration.

  The following soils are pH distinguished according to the active acidity: Strongly acid, 3-4; Acid 4-5; Weakly acid 5-6; Neutral 6-7; Alkaline 7-8; Strongly alkaline 8-9.

  Podsol soils, marshy soils, and gray forest soils have an acid reaction; serozems have a neutral or alkaline reaction; solonets soils have a strongly alkaline reaction (Vilenskii, 1954).

  The presence of carbonic acid, organic acids, carbonates, and other compounds effects the reaction of the soil solution. The presence of carbonates and especially of sodium and calcium bicarbonate causes an alkaline reaction.

  The oxidation-reduction potential of the soil solution. The concentration of hydrogen ions in the soil solution is of great importance to biological processes. The oxidation-reduction potential of the solution determines the direction and character of chemical and biochemical reactions and the solubility of biologically important components of the medium, as well an the products of microbial metabolism, The degree of dissociation of water ions (H and OH) has a great influence on the solubility of the different mineral salts: those of silicic acid, sesquioxides of iron, aluminum, and others. Bivalent iron (Fe++) dissolves in a weakly acid solution at pH = 4-6 and precipitates at pH = 7. Trivalent iron (Fe +++) dissolves in a strongly acid solution at a pH below 3, and at pH = 3 it precipitates. The same takes place with manganese and some other elements.

  In the surface layer of the soil where the amount of oxygen is sufficient, oxidation processes proceed vigorously under the influence of aerobic microbes. With increasing depth, the amount of oxygen in the soil solution diminishes. The solution loses its oxidizing properties on the so-called oxidation-reduction border. Below this border, reduction processes take place. The depth of location of the oxidation-reduction border varies in different soils. It may fluctuate in one and the same soil depending an moisture, temperature, and other external factors. This border should be considered as relative. Experiments show that in the upper layers both oxidation and reduction processes may take place and aerobes, as well an anaerobes, may develop. On the other hand, In the deep layers, oxidation processes may take place an well as reducing processes. However, the former are considerably weaker than the latter.

  V. R. Vil'yams assumes that aerobes in the upper layers grow in pores between the aggregates, where oxidation processes take place. Inside the aggregates, anaerobes with reducing function predominate. Anaerobic processes may be conditioned by an abundant growth of aerobes. The latter absorb oxygen and create anaerobic conditions in a closed system.

  The buffering capacity of the soil solution. The buffering capacity, or the ability of the solution to resist changes of the active reaction upon acidification or alkalization, is one of its characteristic properties, It is caused by the content and composition of soil colloids and their adsorptive capacity. The higher the adsorptive capacity of the colloidal particles, the greater the buffering capacity of the solution. The buffering capacity of the soil also depends on the adsorptive capacity at the solid particles of the soil. Soil is a strong buffer. Different chemically active compounds are neutralized or inactivated by the soil: e.g. , acids, toxins, antibiotics, vitamins, and other substances of microbial and other origin.

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