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Part II, continued: 

  The gaseous or air phase of the soil plays an essential role in life processed. The free space between the soil aggregates is filled with air, provided it is not occupied by soil solution.

  The air in the soil exists in three states: a) free, filling the free space between the soil particles and the aggregates, b) dissolved In the soil solution; c) adsorbed by the solid phase of the soil.

  All these states of the air are of importance in the life of the soil. The air is adsorbed on mineral and organic colloidal particles, the largest amounts being adsorbed by organic matter in the dry state. With the increase in moisture, the adsorption of air diminishes. When the moisture somewhat exceeds the maximal hygroscopicity, the adsorption of air is completely arrested. Water molecules are adsorbed by the soil particles more tenaciously than gas molecules.

  Different gases are adsorbed by soil particles with varying tenacity, according to the following series; NH₃ > CO₂ > 0₂ > N₂ > H₂S > CH₄. The capacity of soils to adsorb and retain gases varies, and depends upon the composition of the colloidal fraction. Humus and iron hydroxide adsorb most strongly.

  With increasing temperature, the capacity of the soil to retain gases decreases. The free air and the air dissolved in the soil solution in of the greatest importance for the biology of the soil. The total free air content of the soil depends on its porosity and moisture. Since water and air occupy the same sites in the soil, any increase in the volume of one of these components leads to a corresponding decrease in the volume of the other.

  As already mentioned, the porosity of the soil is not a constant value. It fluctuates, for different reasons, between 25-50% and, In rare instances. rises to 60%.

  Only in dry soils the soil space almost entirely filled with air. In rainy seasons the pores are filled with water which supplants the air.

  The upper layer of the soil (0-10 cm) of Northern Caucasus chernozem contains the following quantities of air per 1,000 CM³ of soil under different plants (A. A. Shmuk, 1924): Virgin soil 80 cm^3; Winter wheat 240cm³; Oats 200cm³; Sunflower 260cm³; Tobacco 280cm³; Fallow 320cm³.

  It can be seen from these data that the amount of air in one and the same soil changes in relation to Its state and degree of cultivation, the vegetative cover, and in relation to other factors. There is less air in virgin (8%) than in cultivated soil , and in soil under cereals there is less air than under intertilled crops. The largest amount of air (32%) is found in fallow soil. Similar data were obtained by studying sod-podsolic soils (experimental fields of the Agricultural Academy of Timiryazev). Their air content fluctuated from 15 to 30%; the bare fallow soil contained the greatest amount of air (Vilenskil, 1954).

  The air content in the soil changes but little during the whole growth period provided that soil moisture is kept constant. The amount of air diminishes correspondingly with increasing moisture.

  The composition of the soil air. The air in the soil never has the same composition as atmospheric air. It is more diverse in the qualitative and quantitative content of its constituent gases.

  As in well known, the atmospheric air consists of 79.01 % nitrogen, 20.96 % oxygen, and 0.03 % carbon dioxide. In addition, negligible quantities of other gases (neon, krypton, argon, xenon, helium and others) can be detected in the earth's atmosphere.

  Soil air differs from that of the atmosphere by its higher CO₂ content. The oxygen content varies within a smaller range, and that of nitrogen is almost constant. Besides atmospheric gases, there are other gases in the soil formed by the metabolism of organisms and the respiration of the soil. Various volatile organic and inorganic substances can be detected in the soil: ammonia. hydrogen sulfide, methane, organic acids, alcohols, esters, tars, and many other compounds, products of the metabolism of bacteria, plants, and animals.

  The composition of the soil air is not well known, The most important component of the soil atmosphere is carbon dioxide, the final decomposition product of organic matter. The intensity of the biochemical processes taking place in the soil can be judged by the amount of carbon dioxide released.

  The amount of carbonic acid in soil varies noticeably in relation to the composition and type of the soil, to the metabolic activities of the soil population, and to climatic conditions and other factors. The formation of CO₂ depends to a large degree on microbial metabolism. Everything that favors growth of microorganisms increases the generation of CO₂. Lundergardh (1924) assumes that 2/3 of the total amount of CO₂ in the soil atmosphere is formed as a result of bacterial metabolism, the remaining 1/3 being formed by plant roots.

  In soils rich in organic compounds of humus, the CO₂ content is, as a rule, greater than in soils poor in humus.

  Vilenskii (1954) gives the following figures on the formation of CO₂ in various soils (CO₂ in kg per hectare per hour): Nonfertilized clay soil, 1.25; Nonfertilized sandy soil, 2.0; Sandy soil, high in humus, 4.0; Sandy loam, not fertilized, 4.0.

  Beneath the canopy of forests the air in more saturated with CO₂ than in the field (Zonn. 1954); in autumn (10-17 September), the following amount of CO₂ was found to have been released: In an oat field between forest belts, 2.09 kg/hectare per hour; In a forest belt 60 m wide (forest 50 years of age) 17. 35 kg/hectare per hour; In a forest composed of oaks, acacias and ash trees (28 years) 10.50 kg/hectare per hour; The same, but composed of oaks and honeysuckle, 5.88 kg/hectare per hour.

  The amount of carbonic acid in the soil changes sharply in relation to the composition of plant residues. According to the data of Stoklasa (1906), one gram of dry root substance released the following quantities of CO₂ on decomposition: Barley, 70. 5 mg/24 hours; Potatoes, 82.3 mg/ 24 hours; Wheat, 74.6; Beet, 130.6; Rye, 110.9; Clover, 1.46; Oats, 118.9; Lucerne,160.5.

  According to the data of Makarov (1952, 1953), the liberation of this gas by the soil fluctuates in the range of 400 to 800 kg/hectare in 24 hours. In fields under crop rotation the following amount of CO₂ is released during one season (in tons/hectare): fallow, 35; under winter rye, 05; under oats, 79; under first-year grass, 98.

  The formation and release of CO₂ under different vegetation varies. For example, under clover 0.550 g of CO₂ is released in unit time; under serradella, (Ornithopus sativus), 0.305 g; under mustard, 0.218 g; under rice, 0.285 g. All data refer to release of CO₂ per 1 m² of soil (Reinan, 1927).

  The largest amount of CO₂ in released from soil under legumes, i.e. , clover, lucerne, etc. This can be explained by the activity of the Rhizobium bacteria. According to the data of Bond (1941), respiration of Rhizobium bacteria on soya roots was 3 times higher than the respiration of the roots per unit of dry weight. The total mass of root nodules released more CO₂ than the root mass of the entire plant.

  The CO₂ in the surface layer of the air may reach 10% as a result of its release from the soil. In the deep layers of the soil the air contains more CO₂ than in upper layers.

  The dynamics Of CO₂ liberation changes according to the phase of plant vegetation. The liberation of CO₂ from soil under wheat is greatest during flowering but under other grasses it is greatest before the harvest. Makarov connects the maximum CO₂ release with the greatest development of the root system in the given plant phase. According to our observations, the period of maximum CO₂ liberation coincides with the maximal growth of the microflora near the roots (Krasil'nikov, Rybalkina, and others, 1934; Krasil'nikov, Kriss, Litvinov, 1930 a).

  The great effect of temperature on microbiological activity, and, consequentl y, on CO₂ liberation, should be mentioned. Experiments show that a rise In temperature from 15 to 28° C increases the formation of CO₂ in the soil twofold (Bunt and Rovina, 1955).

  The dependence of soil respiration on some of these factors to shown in the following curves (Figure 52).

Figure 52. Dependence of soil respiration on its moisture content and the growth of microorganisms (according to Makarov, 1953)

Curve A--soil respiration (release Of CO₂); curve B--total amount of microorganisms; curve C--soil humidity as of dry weight.

  The quantitative fluctuations of oxygen in the soil is the reverse of that of the CO₂, In the upper layer, to a depth of 30 cm, oxygen comprises 15-20% of the total amount of gases. With increasing depth its quantity decreases sharply. In the spring the amount of oxygen, at a depth of 60-90 cm, comprises 0.3-0.8%. In the summer the amount of oxygen in deep layers of the soil rises and in July, at the same depth, reaches 15-19%; in August it comprises 11-13%, even at a depth of 180 cm. In October the amount of oxygen again diminishes. These fluctuations are caused by temperature and humidity.

  It is clear that, with the varying oxygen content, all biological processes will be modified. not only quantitatively, but also qualitatively. In the presence of a sufficient influx of oxygen, oxidation processes will take place predominantly; if the amount of oxygen is inadequate, reduction processes will predominate.

  The gases of the soil atmosphere can exist in a dissolved state. As already mentioned, the soil solution always contains a certain amount of air and gases present in the soil and in the atmosphere.

  The solubility of gases in the soil solution depends on their characteristics, partial pressure, temperature. and on the concentration of salts in the solution. According to Henry's law, the solubility of a gas in liquid is directly proportional to its pressure. If the liquid is in contact with a mixture of gases then each of the gases will dissolve, not under the influence of the total pressure, but according to its own partial pressure.

  Of all the gases in the soil the most soluble are CO₂, NH₃, H₂S, and some others. Oxygen is less soluble; nitrogen dissolves with difficulty. The dependence of the solubility of gases on temperature is shown in Table 10.

Table 10
The solubility of gases at different temperatures (1 cm³ / l)

Temperature °C	CO2	O2	N2
0	17.1	0.49	0.24
10	8.8	0.31	0.15
30	6.6	0.26	--

  There are always large amounts of electrolytes in the soil solution; consequently, the solubility of gases in it is lower than in pure water. The soil solution of saline soils contains less gasses than that of nonsaline soils. The adsorption of gases by soils rich in humus is higher than in soils poor in humus.

  As a result of microbiological activity, ammonia, hydrogen sulfide, hydrogen, methane, and other metabolites of aerobic and anaerobic microflora. as well as such organic volatile compounds as acetic acid, butyric acid, alcohols, esters, aromatic compounds and others, can be detected in the soil. The specific scent of the earth in caused by the volatile metabolites of microbes. especially actinomycetes, whose, nature is not clear. There are many other compounds which. in the soil air, are a source of direct and supplementary nutrition, and also certain volatile compounds which suppress the growth and development of specific microbes.

  By direct experimentation N.O. Cholodny (1944 a, b, c, 1951 a, b) discovered the existence of nutritional substances in the atmospheric and soil air. He showed that some bacteria and fungi, and also excised tips of plant roots, can grow satisfactorily in a drop of a medium in which soil vapors served as the only source of nutrition.

  The presence of foodstuffs in the soil atmosphere was also established in our experiments in the following way: a culture of an asporogenous bacillus; Bact. album, when isolated from soil, is incapable of growing in the synthetic medium of Chapek. However, this bacterium placed in a drop of this medium in a soil chamber begins to grow and yields many generations. It follows that volatile substances were released from the soil and found their way into the drop of the medium, thus securing normal growth of the culture.

  Meisel', et al., (1946, 1950) showed that separate components of vitamins--thiamine, nicotinic acid, para-aminobenzoic acid, present in the air, are used by microorganisms. Biotic compounds enter the air and soil atmosphere from the soil and plants. According to Cholodny (1944c), vitamins given off into the air by plants are utilized by soil bacteria and by the plants themselves. The air of forests and meadows is the richest in volatile vitamins (Grummer, 1955),

  Shavlovskii (1954) detected thiamine and nicotinic acid in the soil atmosphere of gray forest soil and podsol chernozem.

  In the soil atmosphere, volatile compounds, toxic to certain microbial types, can be detected, Our experiments have shown that the staphylococcus Staph. aureus, placed in a hanging drop in a soil chamber prepared from forest sod-podsolic soil, do not grow or grow very slowly, whereas the control cultures of Staph. aureus placed in a chamber with other soils (chernozem, garden soil) grow normally. The suppressing activity of soil vapors (from soil under flax) was observed, an well as the absence of such activity by the vapors of soil under clover.

  Radioactive substances can be detected in the soil atmosphere, usually in the form of disintegration products of radium or other substances.

  The composition of the soil air changes constantly. A continuous exchange between the atmospheric air and the soil air taken place, This exchange is of the utmost importance for the life of the soil. Without this exchange the CO₂, H₂S, methane and other gases formed would quickly fill up all the pores of the soil, the oxygen would be exhausted, and many biochemical processes would stop. The population of the soil--plants, animals, and microorganisms--would be poisoned. Without the influx of atmospheric air, without replenishment of the soil with oxygen, anaerobic conditions would be established.

  The replenishment of the soil air is accomplished under the influence of many factors. The main ones are.

  a) temperature fluctuations, diurnal and seasonal;

  b) changes in barometric pressure;

  c) diffusion of gases;

  d) dynamics of life processes; utilization and formation of certain gases by the living population of the soil.

  The first method of air exchange is accomplished due to the property of gases to expand upon heating and to shrink upon cooling, With the rise in soil temperature, the air in the soil increases in volume and leaks into the atmosphere. When the temperature drops the reverse process takes place; the soil air shrinks, its volume decreases, a vacuum is formed in the pores, and atmospheric (external) air is sucked in. Such fluctuations take place periodically (diurnal fluctuations). There are also seasonal fluctuations. These are less sharply pronounced and apparently are of less importance for the respiration of the soil. These periodic fluctuations of the soil temperature are responsible for a regular gas exchange between the soil and the atmosphere. It appears as if the soil respires. The characteristic feature of this respiration is, as of any other respiration, the uptake of oxygen and the release of carbon dioxide.

  The respiratory activity of the soil can be increased or decreased by various factors, such as humidity, wind, and others. The water and the air are antagonists, The humidity of the soil leads to a decrease in the amount of air in the pores of the soil. During the rainy seasons the soil gets so wet that the air in almost completely supplanted. With the drying of the soil the reverse process takes place.

  The CO₂ and oxygen content of the soil air varies seasonally. The largest amount of CO₂ in the upper layer of the soil is found in spring and summer. From April until September, in the temperate belt, it reaches 2-4% at the depth of 30-60 cm. In the autumn and winter the amount of CO₂ decreases considerably. Together with these findings there are observations which show that the activity of the microorganisms does not stop in winter, According to our data, in winter, at a soil temperatures of about 3-5° C above freezing point, certain forms of actinomycetes and bacteria multiply abundantly. In one gram of soil 10-15 thousand actinomycetes Act. globisporus were counted in the spring and summer; in winter their number reached 100-500 thousand and more. Sauerland and Groetner (1953) found that the release of CO₂ in the soil increases in winter.

  The barometric pressure strongly influences the respiration of the soil. Observations have shown that with the change in pressure, the gas content in deep layers, at a depth of 2 m and more, also changes. With decreasing pressure the gas volume increases and the gases are released into the atmosphere; with rising pressure the picture is reversed.

  Diffusion also plays an important role in the gas exchange of the soil.

  According to some authors, diffusion alone can secure the gas exchange of the soil and maintain the composition of the soil air at a level sufficient to maintain the life processes of the soil population.

  In the soil (except when frozen), biological processes of synthesis and decomposition take place continuously. In this process various organisms form CO₂, O₂, esters, acids, alcohols, ammonia, hydrogen sulfide. methane, etc. These compounds serve an nutrients for other organisms, especially microbes. The content of gases in the soil will change according to the prevalence of these or other microorganisms In a given soil and the direction of the biochemical processes.

  There are indications that plant roots, not only release, but also actively absorb CO₂. The amount of CO₂ taken up, from the soil may be of the same magnitude an that coming from the atmosphere or may even exceed it. The intensity of CO₂ absorption from the soil depends on its concentration. The higher the concentration of CO₂ in the soil, the quicker it finds its way into plants via the roots (Kursanov, 1954; Samokhvalov, 1952).

  The CO₂ of the soil in taken up by many microorganisms. The soil is known to be the habitat of many kinds of autotrophs which use CO₂ as a source of carbon for the synthesis of organic compounds. Besides, there are many kinds of heterotrophs in the soil which can also use CO₂.

  We have cited opinions of individual workers who maintained that the CO₂ released mainly represents the product of metabolism of the soil microflora. We agree with this opinion. Experiments show that as soon as the activity of microorganisms is hindered, the release of CO₂ decreases. The reverse picture can be observed when compounds which increase the vital processes of microorganisms are introduced into the soil. Vincent and Nissen (1954) introduced into the soil small doses of antibiotics and obtained a noticeable increase in CO₂ output. In the control series the release of CO₂ amounted to 51.2-56.9 mg/40 g of soil. Upon introduction of penicillin, the amount of CO₂ released reached 112.6 mg, with chloromycetin, 85.2 mg; and with terramycin, 148.7 mg.

  Numerous investigators consider the release of gases from the soil as directly linked to microbial activity, The biological processes taking place in the soil under the influence of microflora may be judged by the respiration of the soil. There can be no doubt that other organisms take part in this, too. Their role, however, is much smaller than that of microbes.

  The respiration of the soil, is an indication of the biological and biochemical processes taking place in it, it can also serve as an index of soil fertility as a whole, as was maintained by Stoklasa (1905) and later on by many other investigators, Lundergardh, 1924; Makarov, 1953; Lees, 1949, Jensen, 1934; Bunt and Rovina, 1955 and others.

  The thermal regime is of an especially great importance in the life of the soil.

  The main source of heat is the solar radiation. Other sources, such as the internal heat of the planet and the heat obtained from chemical and biochemical reactions, are negligible and are not taken into account. The heat effect of radioactive reactions has not yet been studied.

  As in well known, the surface of the earth absorbs heat from the sun. Air layers surrounding the earth prevent the earth from cooling and generally exert a great influence on its heat regime. The clearer the air and the less water vapors it contains, the less is the retention of the heat radiated from the earth surface.

  The surface of the earth is not heated uniformly by the sun; it is most strongly heated at the equator and most weakly heated at the poles. The heat absorption is conditioned, not only by the geographical location, but also by its qualitative content, particularly by the color of the soil. Darkly colored soils absorb more heat than the lightly colored ones. The chernozems, for example, absorb 86% of the radiant energy of the sun; gray soils, 80%; and white soils, only 20%.

  Soils also differ from each other in their heat capacity. This depends on various factors. Of greatest importance is humidity, since water possesses greater heat capacity than the solid particles of the soil. Dry soils warm up more rapidly than moist soils. Heat conductivity also depends on soil moisture. Dry soils conduct heat slower than moist ones.

  The surface of the soil becomes warm during the day and cools in the night. This creates a diurnal fluctuation of soil temperature. The greatest amplitude of these fluctuations can be observed in the summer, especially in places with a sharp continental climate.

  Heat waves are formed in the soil as a result of the alternation of warming and cooling. These waves are most sharply pronounced in the surface layers; they lessen with depth and disappear completely at one meter below the surface. In deeper layers, the temperature of the soil remains relatively constant.

  Besides diurnal fluctuations, there also exist seasonal (annual) thermic fluctuations. The depth to which the soil freezes depends on regional and climatic peculiarities of the locality. There are regions where the soil does not thaw in the summer or it does so only in the upper shallow layer. This is the region of eternal freeze. The snow cover strongly influences the thermal conditions in the soil. It protects the soil against the winter freeze. In forests the soil freezes to a lesser extent than in the fields. Vegetation slows the warming up of the soil in summer and lessens the degree of freezing in winter. In the same way, it eases the diurnal temperature fluctuations in summer.

  The freezing of the soil in winter exerts a definite effect on the biological processes. Microorganisms are known to be unharmed by low temperatures. Frosts of 20-30°C and more do not affect them. Many forms survive the temperature of liquid air, In our experiments Azotobacter, and root-nodule bacteria survived a month's storage at 180°C below the freezing point.

  These are data on the increase of the metabolism of microorganisms under the influence of frost. After a three-week storage at -15 to -20° C (in frozen state) Azotobacter, for example, grows and multiplies more rapidly, root-nodule bacteria become more active and virulant, and yeasts are more active at fermentation of sugars, etc. This apparently is the explanation of the vigorous outburst of metabolism in the soil in spring.

  The spring outburst of microbial activity is sometimes observed in laboratory conditions in bacteria grown in pure cultures. The periodicity of the change of summer and winter temperature apparently manifests itself in hereditary characteristics, which become fixed to a certain degree and are transmitted from generation to generation for some time. We have observed such an outburst of metabolic activity in some cultures of azotobacter isolated from the soil near Moscow. The influence of seasonal and meteorological conditions on the metabolic activity of bacteria was noted by some other investigators (Bortels, 1942).

  Under the influence of frost a noticeable change in the chemical and physicochemical properties of the soil takes place. The concentration of the soil solution varies; a number of compounds precipitate, for example, ulminic acid into ulmin. According to our observations, the toxic substances of the soil are decomposed and inactivated. Soils exhausted by clover become less toxic after frosts. Inactivation of antibiotics produced by the soil microbes is noticed after the soil has been frozen for a prolonged period. It is assumed that many other organic and inorganic compounds in the soil are subjected to sharp changes under the influence of frost, and the soil as a whole becomes more fertile.

  The insolation of the soil has as yet been little studied. The soil is irradiated by the sun's rays only on the surface. The thicker the vegetation the less is the solar radiation reaching the surface of the soil.

  Most rays of the spectra do not penetrate into the deep layers of the soil. There are data on penetration of infrared rays to a depth of one meter. Algologists assume that the algae found at this depth grow only because of the presence of these rays.

  The importance of sun rays in the life of the soil is not clear. Undoubtedly, the effect of sun rays on the growth of microorganisms in the soil and, in particular, in the surface layer is very great. The study of microorganic metabolism on mountain summits has shown that biological processes are more vigorous there. According to our observations, the nitrogen-fixing activity of microbes on mountain summits is more vigorous than in the valleys. Some nitrogen-fixing bacteria are powerful accumulators of molecular nitrogen, in high places (Krasil'nikov, 1956b). On comparing the biochemical activity of soil bacteria in mountain soils with that of bacteria from valley soils we were able to establish an essential difference between them. The first, as a rule, possess more powerful proteolytic, amylolytic, and lipolytic activity.

  Similar data were obtained by Mishustin (1947). The mountainous climate and especially insolation affect the natural characteristics of bacteria. These characteristics are not lost for some time after their transfer to valleys. These as yet isolated observations give grounds to assume that insolation strongly affects the life of soil microbes.

  Summarizing, it can be said that the sites occupied by microorganisms in the soil are all the spaces between the soil particles and the aggregates. Microbes flourish in large and small pores. They inhabit microscopically small pores and capillaries. The soil solution is their nutrient medium. Its nutritional value varies, depending on the concentration of the nutrients, the presence or absence of toxic and biotic substances, the gaseous phase, intensity of air exchange in the pores, the income of atmospheric oxygen, and on the elimination Of CO₂ from the soil,

  The soil solution with its nutrient properties is to a certain degree an index of the fertility of the soil and the capability of microorganisms to grow in it. It determines, not only the composition of the microbial population, but also the qualitative distribution of the individual genera and groups.

  Organic matter is one of the main components of the soil and conditions its fertility. According to their composition, the organic compounds of the soil are unique and complex. They are formed from plant and animal residues as a result of microbiological metabolism.

  All organisms living above the earth's surface and in the soil (animals, plants, and microorganisms) find their way after death to the soil, where they are metabolized by the living cells of microbes which form various substances. These substances, in their turn, are subject to biochemical transformations, as a result of which specific, relatively stable, and complex compounds called humus are formed.

  Higher plants supply the soil with organic compounds during the period of vegetation, releasing various nitrogenous, or nonnitrogenous compounds from their roots. They also shed dead fractions of roots and parts growing above the surface.

  The total mass of plant residues entering the soil may reach considerable proportions. For example, in forests, the annual fall of leaves and twigs comprises 1.5-7 tons/hectare according to the type of the forest, its age, and the climate an soil conditions. Various woods yield different amounts of residue. Annual fall of leaves and twigs according to Zonn (1954), is as follows (average figures):

Deciduous forests 2.7 ton/hectare;
Oak forests 3.9 ton/hectare;
Pinewood forests 4.1 ton/hectare;
Fir tree forests 6.0 ton/hectare.

  Thus, fir-tree forests rank first according to the amount of leaves and twigs shed, with pinewood, oak trees, and deciduous forests following.

  The amount of the forest litter formed varies. The largest amount is to be found in fir-tree forests (50 tons/hectare and more).

  According to the data given, the amount of organic compounds in soils of various kinds of forests varies. The amount of organic compounds shed in fir-tree forests reaches 5.85 tons/hectare; in pinewood forests, 3.96 tons/ hectare; and in oak forests, 3.5 tons/hectare (Zonn, 1954),

  As should be expected, the "sheds" of different forests differ qualitatively, too. According to the data of Zonn, the fall of fir trees is more acid than that of pine or oak trees. According to our observations, the leaves of birch and lime trees in June are decomposed in the soil more rapidly than the leaves of oak, aspen, or the needles of pine trees.

  Meadow vegetation yields dry mass (from the parts growing above the surface) amounting to about 2-6 tons/hectare and roots, 7-11 tons/hectare. In the chernozem meadows of the steppes about 7 tons/hectare of dry mass (parts grown above the surface) were found and 25 tons of roots. In steppes on solonets soils 5 tons/hectare of the dry mass of the parts growing above the surface and 13 tons of roots were found (Savvinov and Pankova, 1942). In the desert steppes on serozem, about 1 ton/hectare of the mass growing above the surface and 15 tons of roots were found (Kul'tiasov, 1925). According to Kononova (1951), grass yields about 21 tons/hectare of root mass and, according to Belyakova (1953), the weight of roots of lucerne reaches 40 tons/hectare. Annual grasses yield less root mass than perennials (Vilenskii, 1954).

  Plant tissues are composed of various carbon and nitrogen compounds. They contain sugars, dextrins, starch, pectic and tannic substances, organic acids. fats, waxes, tars, and many other compounds.

  The main component of the plant material is cellulose (C₆H₁₀O₅)n. It constitutes the cell wall. Cellulose comprises 85-90% of the total weight of cottonseed fibers, and about 50% of bark.

  The cellulose is decomposed by special cellulose microbes--bacteria, myxobacteria, actinomycetes, and fungi. Various intermediate compounds are formed in the decomposition process: organic acids, alcohols, sugars, and others.

  Hemicellulose, in addition to cellulose, also appears in plant cells. Hemicellulose is easily hydrolyzed by acids and alkalis with the formation of sugars, uronic acids, and other compounds.

  In wood, cellulose is impregnated with lignin, the content of which reaches 34%. Lignin differs from cellulose by its higher content of carbon (62-69%, in cellulose only 49.4 %) and lower content of oxygen. Upon oxidation it yields aromatic compounds. The chemical structure of lignin has not been ascertained. Lignin in the soil is decomposed by microbes with the formation of final decomposition products, CO₂ and water, or intermediate products.

  Proteins are the most common nitrogenous compounds present in the cells of plants, animals, and microbes. They are present in protoplasm, nuclei, and in various protein reserve substances (metachromatin, protein crystals, aleuron grains, etc). Complex proteins are known--proteids and proteins proper such as globulins which are insoluble in water but soluble in dilute salt solutions; water-soluble albumins; prolamins--proteins of the gluten of the wheat grain (gliadin) which are soluble in 80% alcohol; glutelins--plant proteins, soluble in dilute alkaline solutions; sclero-proteins--insoluble proteins of horny tissues such as collagen, keratin, and others.

  Many complex protein compounds are known, such as phosphoproteids containing phosphorus, nucleoproteids--proteins of the cellular nuclei and nuclear inclusions (which upon hydrolysis are decomposed into simple proteins and nucleic acids containing phosphorus), chromoproteids--proteins containing pigments (e.g., blood hemoglobin and some antibiotics formed by microbes), and glucoproteids (mucoproteins), which are proteins containing carbohydrates.

  Other complex proteins which are present in plant, animal and microbial cells are: albumoses and peptones, which are protein compounds forming colloidal solutions and giving biuret reaction, and amino acids, which are colorless watersoluble compounds containing amino groups (-NH₂) and carboxyl groups (OH-C=O).

  Plant residues as well as the dead cells of microbes and animal organisms find their way into the soil, where they are subject to physicochemical and biological processes.

  The main transformations of plant residues are carried out under the influence of biotic factors. The dead parts of plants in the soil begin to decompose immediately, at first under the action of their own enzymes and then quite rapidly (perhaps simultaneously) under the action of microbial enzymes.

  The first to be decomposed are the easily assimilated organic compounds: sugars, organic acids, and alcohols; then follow proteins, amino acids, fats, pectins, gums, hemicellulose, and lastly cellulose and lignin. The soil microbes also decompose waxes, tars, and many other stable compounds. It can be said that no organic compound exists which cannot be decomposed by microorganisms. Some of them are decomposed rapidly (carbohydrates, proteins, etc.), and others, slowly (tars, waxes, etc. ).

  The decomposition of organic compounds may be carried out to the final products, CO₂ and water, or may stop with the formation of intermediate compounds. The latter may be in the form of organic acids, alcohols, amino acids, etc.

  Simultaneously with the decomposition of organic compounds, synthetic processes are taking place in the soil. The so-called autotrophs are known to synthesize organic compounds by assimilation of CO₂. The first of these are the photoautotrophic algae, which may be present in considerable numbers. Many colorless chemitrophs and pigmented bacteria possess the same capability. They assimilate carbon dioxide and synthesize organic compounds at the expense of chemical or light energy. Among these are the nitrate, sulfur, iron, hydrogen, and methane-oxidizing bacteria. The sole source of carbon for these organisms is CO₂. Their energy requirements are satisfied by the following simple compounds: ammonia, nitrates, sulfurous and ferrous compounds, hydrogen, methane, and others. Many heterotrophic microorganisms are capable of assimilation of CO₂ and of synthesizing organic compounds. This capability was detected in the representative of the genera Pseudomonas and Azotobacter, in sporogenous and asporogenous bacteria, in yeasts, fungi, and in actinomycetes. The synthesis of organic compounds may reach considerable dimensions, 5% and more of the CO₂ supplied during the experiment (Liener and Buchanan, 1951). Cells dividing in the logarithmic phase of growth assimilate ten times more CO₂ than in other stages of growth (Mac Lean et al. 1951; Shaposhnikov, 1952; Rabotnova, 1950; Linsh and Calvin, 1952; Citterman and Knight, 1952, and others).

  According to Werkman and Wilson (1954), all microorganisms, autotrophs and heterotrophs, are endowed with the ability to assimilate CO₂, but to a different extent according to the type and conditions of culture growth.

  The synthesized compounds and decomposition products of plant residues, as well as other organic compounds, find their way into the soil solution and are utilized as nutrients by microbes and plants.

  Shmuk (1930) noted the presence of the following compounds in the soil: nitrogenous compounds (methylamine, choline, histidine, arginine, lysine, cytosine, xanthine), fats, organic acids (oxalic, succinic, crotonic, acrylic, benzoic, etc. esters (glycerides of caprylic and oleic acids), carbohydrates (pentoses, pentosans, hexoses, cellulose and its decomposition products), alcohols, aldehydes, tars, paraffins, and other compounds.

  Davidson, Sowden, and Atkinson (1951), employing the method of paper chromatography, detected about 30 compounds in the organic fraction of the soil: such as arginine, histidine, lysine, alanine, leucine, proline, isoleucine, valine, aminovaleric acid, aspartic acid, tyrosine, threonine, glutamic acid, and others.

  According to Kejima (1947), the following acids can be detected in the soil: 6-7% aspartic acid, 5% glutamic acid, and 18% of other amino acids, totaling 31.9% total nitrogen. According to the author, 66-75% of soil nitrogen is not in the humus but in microbial proteins.

  Such compounds as polyuronic acids which comprise either components of plant tissue (hemicellulose) or products of microbial synthesis (slimy compounds constituting bacterial capsules) can also be detected In the soil.

  Schreiner and Reed (1907) isolated various organic nitrogen and carbon products from fertile soils. Creatine, xanthine, hypoxanthine, adenine, and cysteine were among the first detected.

  Rudakov and Birkel' (1949) found uronic acids among the metabolites of plant roots. The liberation of these acids takes place with the participation of bacteria possessing protopectinase.

  Shori isolated allantoin from the soil and Enders obtained methyl glyoxalate, a compound which, according to Neiberg, is an intermediate in hexose fermentation and, according to Gebert, a primary structural element of protolignin. It is assumed that methyl glyoxalate is an intermediate compound, "a bridge" which links the lignin and cellulose theories of the origin of humic acids (Kononova, 1951). There are various biologically active compounds in the soil: vitamins (B_l, B₂, B₆, B₁₂), auxins, pantothenic, nicotinic, folic, and para-aminobenzoic acids, biotin, and other compounds activating the growth of plants and microbes, There are also inhibitors that may suppress the growth of plants (toxins) and microbes (antibiotics), and free enzymes--catalase, peroxidase, invertase, amylase, tyrosinase, and others.

  Investigations show that enzymes exist in the soil in the active state. Their quantity varies in accordance with the soil composition, season, and climatic conditions. In fertile cultivated soils there are more enzymes than in poor nonfertile soils. The more organic compounds in the soil, the more active is the growth of microbes and the greater the enzymatic activity of the soil (Hoffmann, 1952). The upper layers contain more enzymes than the deeper ones.

  The liberation of CO₂ has been observed to depend on the enzymatic activity of the soil (Seegerer, 1953; Ukhtomskaya, 1952). According to the data of Ukhtomskaya, the amount of enzymes in the soil increases proportionally to the amount of organic compounds introduced (Table 11). The enzymatic activity of the soil is more pronounced in May than in October, when the microbiological processes diminish.

*The organic compounds were introduced with sewage waters.

  Kuprevich (1949) detected the presence of the following enzymes in the soil: catalase, tyrosinase, phenolase, asparaginase, urease, invertase, amylase, and protease, noting that their accumulation depends on soil cultivation. The quantitative figures for catalase, invertase, and urease present in soils, according to his data, are given in Table 12.

  Sorensen noted greater activity of xylanase in cultivated soils than in noncultivated soils. The enzymatic activity increased six times and more when straw or xylan were applied to the soil (Sorensen, 1955).

  Scheffer and others (1953) and Seegerer (1953) pointed out the increased activity of invertase and urease after the application of organic fertilizers, especially manure.

  The amount of enzymes in the soil also depends on the vegetative cover. When a green crop of serradella was plowed in, the amount of catalase and invertase was greater than after the plowing in of green lupine (Table 13).

  As can be seen from the given data, the enzymatic activity is closely correlated with the activity of microorganisms. Any increase In the amount of the latter leads to enhancement of enzymatic soil processes. Hoffmann (1951) considers that the enzymatic activity of the soil is an index of its fertility.

  There are data in the literature indicating that plant roots excrete various enzymes into the soil, such as catalase, tyrosinase, amylase, protease, lipase and others.

  All these organic compounds comprise only 10-15% (approximately) of the total organic mass of the soil. However, owing to their great activity, they are of considerable importance. Many of these organic compounds (vitamins, auxins, certain amino acids) are catalysts of biological and biochemical processes in the soil.

  The part played by free extracellular enzymes is not yet clear, but they may be assumed to be important in transformations of many types of organic compounds and, in particular, in the synthesis of humus compounds.

  We should note the considerable role of antibiotics in the life of the soil. These substances influence the composition of the microbial populations and this affects many soil properties.

  Humic substances of the soil. Humus comprises the bulk of the organic soil compounds and is responsible for the dark coloration.

  Humus is a mixture of various and very complex natural compounds. The uniqueness of these compounds does not allow for their classification into any of the groups of compounds known to organic chemistry. These substances are synthesized in the soil, apparently exogenically, by the action of extracellular enzymes. The composition of humus is more complex than that of many compounds of plant and microbial organisms. Humus comprises 85-90% of the total organic matter of the soil.

  The chemical composition and origin of humus is not as yet clear. Characterization and subdivision of humic soil substances is based on external features.. color, and its relation to solvents. The main components of humus are assumed to be the three acids: ulmic, humic, and crenic.

  According to Vil'yams, ulmic acid is formed during the anaerobic decomposition of organic compounds by anaerobic microbes. It is easily soluble in water imparting a dark brown color. It forms water-soluble salts with monovalent cations (potassium and sodium) and insoluble salts with bi- and trivalent cations. Under the influence of external factors, such as low temperature (freezing) or drying, ulmic acid is converted to water-insoluble ulmin.

  Humic acid is formed under aerobic conditions and is considered to be a product of bacterial and fungal metabolic activity. Its properties are close to those of ulmic acid. It is less soluble in water than ulmic acid and gives the soil a black color. It is also denatured and converted into an insoluble compound, humin. It forms water-soluble salts with monovalent cations and insoluble salts with biand trivalent cations.

  Humic acid has been studied in more detail. The following organic groups were detected: carboxyl (COOH), hydroxyl (OH), carbonyl (CO), and methoxyl (CH₂0), (Kononova, 1951).

  Humus contains from 10-40% humic acids. The largest amount can be found in chernozems.

  Humic acid contains 3.5-5% nitrogen. After acid hydrolysis about 50-60% of the nitrogen goes into solution in the form of amides and mono-and diamino acids. The molecular structure of humic acids has not been determined. According to the available data, more than one humic acid exists. Dragunov (1948) found that two samples of humic acid, one obtained from peat and the other from chernozem, differed from each other in their chemical composition, in the amount and structural type of their functional groups, as well as in the structure of their nuclei,

  Bremmer (1955) subjected samples of humic acids, obtained by him from nine different soils, to chemical analyses. Each sample of the acid was analyzed for total nitrogen, ammonia-nitrogen, amino-nitrogen, and a- amino-acid nitrogen. The solutions obtained after hydrolysis were analyzed by paper chromatography for amino acids.

  It was found that the samples of humic acids studied differed from each other in the composition of their nitrogen compounds and amino acids. Alkali extracts contain much of the nitrogen in the form of acid-soluble nitrogen compounds. About 20-60% of the nitrogen does not dissolve after acid hydrolysis. From 3-10% of the nitrogen is in the form of amino sugars. Nineteen amino acids were identified by means of paper chromatography: phenylalanine, leucine, threonine, isoleucine, valine, alanine, serine, aspartic acid, glutamic acid, lysine, arginine, histidine, proline, hydroxyproline, a- amino-butyric acid and others.

  Humin and humic acids are decomposed by bacteria and fungi, especially by actinomycetes. Many actinomycetes grow well, bear fruit, and form antibiotic compounds on media containing humic acids as a sole source of carbon and nitrogen. Many forms of bacteria also grow on humic acid substrates.

  Crenic acid was first found in spring water. According to Vil'yams, it is formed by fungi under aerobic conditions during the decomposition of forest vegetation and forest litter. Its properties differ sharply from other humic acids. It is colorless, highly soluble in water and acids, is not subject to denaturation and forms salts which can be crystallized.

  Crenic acid possesses sharply pronounced acidic properties. According to Vil'yams, it can raise the soil acidity to such an extent that the activity and growth of many microorganisms is arrested.

  It is difficult to accept this assumption. Organic acids as such are by themselves nutrients for many forms of microorganisms. It is quite clear, therefore, that their accumulation in the soil will be accompanied by an increase in the number of microbes.

  Owing to its solubility, crenic acid penetrates deep layers of soil and there, combining with bases, forms crenates. They are harmless to microorganisms and are utilized by them as nutrients. Crenates are highly soluble in water, are easily leached from the soil, and may find their way either into ground or surface water. Thus, due to the high solubility of crenic acid and its salts, their accumulation in large concentrations is prevented.

  Crenic acid may be reduced by nascent hydrogen with the formation of apocrenates. The reduction is carried out with the participation of anaerobic bacteria. Apocrenates are the salts of apocrenic acids. They have not been obtained in pure form. The salts of monobasic cations are highly soluble in water. Calcium apocrenate is slightly soluble in water and apocrenates of trivalent metals--iron, mangane se, and aluminum--are completely insoluble. These compounds are deposited in the soil in the form of voluminous amorphous sediments.

  Crenic and apocrenic acids (fulvo acids) are widely distributed in soils. Their properties vary according to the soil. Kononova (1953) found that the acids from podsols differ from those of krasnozems.

  The diversity of the natural conditions of soil formation both of a geographical and an ecological character, influence humus formation as a whole and, in particular, the composition of its individual components: humic, ulmic, and fulvo acids and other organic and organomineral compounds.

  V. V. Dokuchaev was the first to point out the regular nature of the formation and transformation of humus under the varying conditions of different soils. climates. and zones. P. A. Kostychev and V. R. Vil'yams conceived the idea of the regularity of humus-compound formation in soils, in relation to the vegetative cover arid biochemical activity of the microflora.

  Later investigations proceeding from chernozems to podsol soils, proved the regularity in the formation of the individual components of humus. Tyurin. (1949). developing the thesis of Dokuchaev on the basis of data from the literature and the results of his own investigations, showed that the geographical regularity of humus formation manifests itself not only quantitatively but also qualitatively. As a rule, the humus of the coniferous forests of the northern and central belt of the USSR and, in general, of the podsol soils is of a bright color, it contains few stable humic and ulmic acids but many compounds highly soluble in water which are easily leached from the soil, e. g. , crenic acid and apocrenates. Their concentration in podsol soils is 2-3 times higher than that of humic and ulmic acids (Kachinskii, 1956). In southern steppe regions having a grass vegetation, the soils contain humus with a different ratio of humic and fulvo acids.

  The composition of humus in various soils also differs. Chernozem-type soils contain humic acids of different properties from those of podsol soils. Kononova (1956) showed the regularity in the variations of humic acids in the main soil types of the USSR. She found variations in the elementary composition of the acids, their optical density, and their distribution. The humic acids of chernozem soils are the most highly condensed, they are followed by the humic acids of the dark-gray forest soils, chestnut soils, and the bright-gray soils of the serozem; the humic acids of podsol soils and krasnozems are but weakly condensed, By applying the methods of X-ray structural analysis, the author determined the main structural outlines of humic and fulvo acids, which varied in relation to the type of the soil. These investigations disclosed the unity of the soil-forming process. While studying the genesis of humus and its components in various soils, Ponomareva (1956) reached analogous conclusions.

  Investigators express three different points of view on the mechanism of humic-acid synthesis (see Kononova, 1951). The majority of workers consider that the formation of these compounds is outside the activity of microorganisms. This was criticized by Kostychev and then by Vil'yams.

  At present, microorganisms are considered to play an increasingly important role in the process of humus formation. Reistric et al., (1938,1941), by means of molds, detected the formation of compounds of the aromatic quinone series from sugars.

  These investigations stimulated the study of products of microbial metabolism; products which could serve as building material for the synthesis of humic acid. At present, many foreign (especially German) and Soviet investigators are busy studying microorganisms, their metabolic products, and the synthesis of humus compounds.

  Great attention has been drawn to molds, actinomycetes, and heterotrophic bacteria as producers of humus-like compounds. While studying the products of bacterial metabolism, Martin, J. (1945) found that about 30% of the humus is synthesized at the expense of bacterial polysaccharides of the uronic type. The most stable of them, "levan", is formed by sporogenous bacteria Bac. mesentericus and Bac. subtilis.

  Flaig (1952) isolated 42 cultures of actinomycetes from the soil which, under certain conditions, form a dark-brown or almost black humus-like compound. Kuster (1950-1952) concentrated his attention on fungi which produce compounds similar in color and certain chemical properties to humin substances. Laatsch. Hoops, and Bieneck (1952) found that the fungus Spicaria and certain actinomycetes, when grown on artificial protein media, are capable of forming a compound closely related to humin. Scheffer and Twaditmann (1953), Plotho (1950), Laatsch and others succeeded in finding a medium in which, under given conditions, fungi or actinomycetes formed substances of the phenol type. These investigators assumed that the oxidation-reduction systems--quinones < = > polyphenols--are in a state of continuous activity in the living cell, being oxidized by polyphenol oxidases and reduced by dehydrases. With the cessation of respiration the quinones are released from the cell, being irreversibly oxidized; they then combine with organic nitrogen compounds (protein decomposition products) to form humic acids. Consequently, according to the above-mentioned authors, the reaction of quinones with microbial nitrogen compounds is the basis of humus formation.

  Wilts (1952) noticed that humic substances are formed from various organic compounds. The building blocks of the humus particles may be products of decomposition of lignin and of tannic compounds--aromatic compounds of the phenylpropans series, easily hydrolyzed carbohydrates (cellulose and others), and proteins which are subject to complex transformations as a result of bacterial metabolism.

  The works of Soviet investigators Mishustin, Gel'tser, Rudakov, Kononova, and others should be mentioned. Mishustin (1938) studied the formation of humus substances upon self-heating of grain; Gel'tser (1940), upon the decomposition of fungi; Rudakov (1949) ascribes the main role in humus formation to pectin compounds. Troitskii (1943) assumes that humic acids are formed by microbes from decomposition products of vegetative residues. Tepper (1949, 1952) has shown that humin substances are formed at the expense of pigments formed by fungi and actinomycetes (see Rudakov, 1949 and 1951).

  Kononova (1951), in her monograph, proposes that various plant residues and products of resynthesis, as well as the microbial protoplasm participating in the process of humus formation, may serve as sources of humus. According to her. the primary molecule of humic acid emerges as a result of the condensation of aromatic compounds with an amino acid or polypeptide. This process takes place with the participation of microorganisms under the conditions of biocatalysis maintained by the oxidative bacterial enzymes. As a result, nitrogen-containing compounds of a cyclic structure are formed.

  Among the mineral elements of the soil a special place is occupied by radioactive substances: radium, uranium, thorium, and others. According to Baranov and Tseitlin (1941) their content (weight %) in different soils in as follows:

  The biological significance of natural radioactive elements remains unknown. It should be assumed that it is of considerable importance for the plant, animal, and microbial population of the soil. Existing data show that these substances in small concentrations activate biological processes, increase metabolism, and exert a positive influence on the growth of plants. The natural radioactive substances of soil find their way into plants, may concentrate there, and cause definite effects (Drobkov, 1951; Vlasyuk, 1955; Popov, 1956, and others).

  The biological action of radioactive substances (radium, uranium, radium emanations and others) has been studied for a long time by microbiologists. Nadson et al. (1920, 1932), Filippov (1932). and Rokhlina (1930, 1954) studied in detail some of the biological processes of yeasts, fungi, and bacteria caused by radium, radium emanations. X-rays, etc. These authors were the first to establish the effect of radium and other sources of radiation energy in promoting genetic mutations.

  We (Krasil'nikov, 1938) have shown that various types of luminescent actinomycetes react differently to the radiation of radon. Certain species were more sensitive than others. Radon rays have a stimulating or suppressing effect on the growth of mycobacteria, actinomycetes, and proactinomycetes. According to our observations, pigmented cultures are more sensitive to radon than nonpigmented ones.

  In recent years we have studied soil bacteria--Azotobacter, root-nodule, and some others and their relation to certain radioactive substances, such as radium, thorium, and uranium. It was found that the bacteria absorb these substances from the soil and accumulate them in their cells in considerable quantities which many times exceed the concentrations of these substances in the soil.

  Attention should be drawn to the fact that the degree of accumulation of radioactive substances in cells varies in different kinds of bacteria. Some kinds of bacteria, especially Azotobacter, accumulate radium in large quantities, others, in small quantities, or are completely devoid of this capacity. Even in the same genus different strains accumulate natural-radioactive substances to a varying extent.

  The radioactive substances inside the bacterial cell stimulate growth and metabolism. Nitrogen fixation by Azotobacter is enlarged under the influence of radium and thorium. The ability of root-nodule bacteria to penetrate the roots of legumes and to form nodules is also increased (Krasil'nikov, Drobkov, Shirokov, and Shevyakova, 1955).

  As a rule, the activating doses of the substances studied by us cannot be detected by ordinary electronic counters (radiomer B-2 and others). The microorganisms are sensitive to irradiation by radioactive substances in doses which cannot be detected by modern instruments.

  The microbial population of the soil as well as plants are adjusted to small concentrations of radioactive elements. High doses given to them artificially under experimental conditions are harmful. Minimal concentrations of radium, uranium or thorium which can be detected by electronic counters damage even the least sensitive species of bacteria. Under the influence of such doses the cells undergo degeneration, increase in size and deform, their protoplasm becomes coarsely-granular, vacuoles appear, and their reproduction slows down and eventually stops altogether (Figure 53). Similar changes were observed by Filippov, Shtern, Rokhlina , and other collaborators of Nadson, in yeasts, fungi, and certain plants when irradiated by X-rays, radon, or ultraviolet rays. The same picture of degeneration in yeasts, under the action of large doses of radium and other sources of radioactive irradiation, was noted by Meisel' (1955). His investigations led to the emergence of a scheme of consecutive damage to the structure and function of cells.

Figure 53. The effect of radioactive compounds (U) applied in the minimum doses detectable by the electronic counter (B-2) on the culture of Aspergillus niger:

a) control; growth on a medium without radioactive substances; b) growth on a medium containing uranium. Swollen hyphae of the mycelium with degenerative coarsely-granular protoplasm.

  As noted by Vernadskii (1926, 1929), radioactive substances possess free energy and continuously carry out considerable chemical activity in the soil. The energy of radioactive elements affects chemical and biochemical processes of microbes and organisms. Vernadskii stresses the fact that life in the biosphere originates from two energy sources: solar radiation and atomic radioactive energy. According to his calculations, only three radioactive elements, uranium, thorium, and radium supply the earth with heat, the quantity of which exceeds a thousand times that received by the earth's surface.

  The biosphere of the earth accumulates dispersed radioactive elements and concentrates them on the surface, thus essentially changing the energetics of the whole population. It should be assumed that plants, animals, and microbes have, during their long evolution, acquired the ability to utilize these powerful energy sources. Analyses show that radioactive substances are present, to a larger or smaller extent, in all organisms and almost always in concentrations exceeding those in the surrounding environment. In many instances, plants contain ten times or hundreds of times higher concentrations of radioactive substances than the surrounding substrate (Vinogradov, 1932; Baranov and Tseitlin, 1941; Drobkov, 1951, and others).

  The problem as to whether the organisms require the radioactive substances remains experimentally unsolved. Opinions are held according to which these substances, in small doses, do not play any role in the life of organisms and, in large doses, are harmful. Recently, data have accumulated which prove the reverse: small doses of radium, uranium or thorium stimulate the growth and increase the dry mass yield. Studies on the importance of natural-radioactive substances in soil fertility and in the life of plants and microbes are still inadequate. There are a number of observations which give reason to believe that these substances play an essential role in nitrogen fixation. A question arises as to the energy source needed to fix 100 to 150 kg and more of molecular nitrogen per one hectare of soil in one season. To fix such amounts of nitrogen and they are actually of this magnitude, Azotobacter, the most powerful nitrogen-fixing organism, requires 5-10 tons of glucose. Such quantities of this energy-yielding material are hardly to be found even in the most fertile soils.

  Perhaps in this case the radioactive soil substances constitute the energy source which is indispensable for nitrogen fixation, as well an for many other processes taking place in the natural environment.

  'The natural-radioactive substances deserve the most painstaking studies as biocatalysts on the earth's surface. When they enter into the chemical composition of living organisms, it should be assumed that they are not destroyers but creators, participating in many transformations and stimulating various enzymatic processes.

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