Part II, continued:
The gaseous phase of soil
The gaseous or air phase of the soil plays an essential role in lifeprocessed. The free space between the soil aggregates is filled with air, providedit is not occupied by soil solution.
The air in the soil exists in three states: a) free, filling the freespace 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 amountsbeing adsorbed by organic matter in the dry state. With the increase in moisture,the adsorption of air diminishes. When the moisture somewhat exceeds the maximalhygroscopicity, the adsorption of air is completely arrested. Water molecules areadsorbed 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 compositionof the colloidal fraction. Humus and iron hydroxide adsorb most strongly.
With increasing temperature, the capacity of the soil to retain gasesdecreases. The free air and the air dissolved in the soil solution in of the greatestimportance for the biology of the soil. The total free air content of the soil dependson 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 decreasein 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. risesto 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 chernozemcontains the following quantities of air per 1,000 CM3of soil under different plants (A. A. Shmuk, 1924): Virgin soil 80 cm3;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 thesame soil changes in relation to Its state and degree of cultivation, the vegetativecover, and in relation to other factors. There is less air in virgin (8%) than incultivated soil , and in soil under cereals there is less air than under intertilledcrops. The largest amount of air (32%) is found in fallow soil. Similar data wereobtained by studying sod-podsolic soils (experimental fields of the AgriculturalAcademy of Timiryazev). Their air content fluctuated from 15 to 30%; the bare fallowsoil contained the greatest amount of air (Vilenskil, 1954).
The air content in the soil changes but little during the whole growthperiod provided that soil moisture is kept constant. The amount of air diminishescorrespondingly with increasing moisture.
The composition of the soil air. The air in the soil neverhas the same composition as atmospheric air. It is more diverse in the qualitativeand 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 ofother gases (neon, krypton, argon, xenon, helium and others) can be detected in theearth's atmosphere.
Soil air differs from that of the atmosphere by its higher CO2content. The oxygen content varies within a smaller range, and that of nitrogen isalmost constant. Besides atmospheric gases, there are other gases in the soil formedby the metabolism of organisms and the respiration of the soil. Various volatileorganic and inorganic substances can be detected in the soil: ammonia. hydrogen sulfide,methane, organic acids, alcohols, esters, tars, and many other compounds, productsof the metabolism of bacteria, plants, and animals.
The composition of the soil air is not well known, The most importantcomponent of the soil atmosphere is carbon dioxide, the final decomposition productof organic matter. The intensity of the biochemical processes taking place in thesoil can be judged by the amount of carbon dioxide released.
The amount of carbonic acid in soil varies noticeably in relationto the composition and type of the soil, to the metabolic activities of the soilpopulation, and to climatic conditions and other factors. The formation of CO2depends to a large degree on microbial metabolism. Everything that favors growthof microorganisms increases the generation of CO2. Lundergardh (1924)assumes that 2/3 of the total amount of CO2 in the soil atmosphere isformed as a result of bacterial metabolism, the remaining 1/3 being formed by plantroots.
In soils rich in organic compounds of humus, the CO2 contentis, as a rule, greater than in soils poor in humus.
Vilenskii (1954) gives the following figures on the formation of CO2in various soils (CO2 in kg per hectare per hour): Nonfertilized claysoil, 1.25; Nonfertilized sandy soil, 2.0; Sandy soil, high in humus, 4.0; Sandyloam, not fertilized, 4.0.
Beneath the canopy of forests the air in more saturated with CO2than in the field (Zonn. 1954); in autumn (10-17 September), the following amountof CO2 was found to have been released: In an oat field between forestbelts, 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 (28years) 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 relationto the composition of plant residues. According to the data of Stoklasa (1906), onegram of dry root substance released the following quantities of CO2 ondecomposition: 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 thisgas by the soil fluctuates in the range of 400 to 800 kg/hectare in 24 hours. Infields under crop rotation the following amount of CO2 is released duringone season (in tons/hectare): fallow, 35; under winter rye, 05; under oats, 79; underfirst-year grass, 98.
The formation and release of CO2 under different vegetationvaries. 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; underrice, 0.285 g. All data refer to release of CO2 per 1 m2of soil (Reinan, 1927).
The largest amount of CO2 in releasedfrom soil under legumes, i.e. , clover, lucerne, etc. This can be explained by theactivity of the Rhizobium bacteria. According to the data of Bond (1941),respiration of Rhizobium bacteria on soya roots was 3 times higher than therespiration of the roots per unit of dry weight. The total mass of root nodules releasedmore CO2 than the root mass of the entire plant.
The CO2 in the surface layer of theair may reach 10% as a result of its release from the soil. In the deep layers ofthe soil the air contains more CO2 than in upperlayers.
The dynamics Of CO2 liberation changesaccording to the phase of plant vegetation. The liberation of CO2from soil under wheat is greatest during flowering but under other grasses it isgreatest before the harvest. Makarov connects the maximum CO2 releasewith the greatest development of the root system in the given plant phase. Accordingto our observations, the period of maximum CO2 liberation coincides withthe maximal growth of the microflora near the roots (Krasil'nikov, Rybalkina, andothers, 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 formationof CO2 in the soil twofold (Bunt and Rovina, 1955).
The dependence of soil respiration on some of these factors to shownin 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 reverseof that of the CO2, In the upper layer, to a depthof 30 cm, oxygen comprises 15-20% of the total amount of gases. With increasing depthits quantity decreases sharply. In the spring the amount of oxygen, at a depth of60-90 cm, comprises 0.3-0.8%. In the summer the amount of oxygen in deep layers ofthe soil rises and in July, at the same depth, reaches 15-19%; in August it comprises11-13%, 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 biologicalprocesses will be modified. not only quantitatively, but also qualitatively. In thepresence 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. Asalready mentioned, the soil solution always contains a certain amount of air andgases 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 proportionalto its pressure. If the liquid is in contact with a mixture of gases then each ofthe gases will dissolve, not under the influence of the total pressure, but accordingto its own partial pressure.
Of all the gases in the soil the most soluble are CO2,NH3, H2S, and someothers. Oxygen is less soluble; nitrogen dissolves with difficulty. The dependenceof the solubility of gases on temperature is shown in Table 10.
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 soilsolution of saline soils contains less gasses than that of nonsaline soils. The adsorptionof 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. aswell 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 ofthe 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 compoundswhich 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, cangrow satisfactorily in a drop of a medium in which soil vapors served as the onlysource of nutrition.
The presence of foodstuffs in the soil atmosphere was also establishedin 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 mediumof Chapek. However, this bacterium placed in a drop of this medium in a soil chamberbegins to grow and yields many generations. It follows that volatile substances werereleased from the soil and found their way into the drop of the medium, thus securingnormal 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. Accordingto Cholodny (1944c), vitamins given off into the air by plants are utilized by soilbacteria and by the plants themselves. The air of forests and meadows is the richestin volatile vitamins (Grummer, 1955),
Shavlovskii (1954) detected thiamine and nicotinic acid in the soilatmosphere of gray forest soil and podsol chernozem.
In the soil atmosphere, volatile compounds, toxic to certain microbialtypes, 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-podsolicsoil, 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 wellas the absence of such activity by the vapors of soil under clover.
Radioactive substances can be detected in the soil atmosphere, usuallyin the form of disintegration products of radium or other substances.
The composition of the soil air changes constantly. A continuous exchangebetween the atmospheric air and the soil air taken place, This exchange is of theutmost importance for the life of the soil. Without this exchange the CO2,H2S, methane and other gases formed would quicklyfill up all the pores of the soil, the oxygen would be exhausted, and many biochemicalprocesses would stop. The population of the soil--plants, animals, and microorganisms--wouldbe poisoned. Without the influx of atmospheric air, without replenishment of thesoil with oxygen, anaerobic conditions would be established.
The replenishment of the soil air is accomplished under the influenceof 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 certaingases by the living population of the soil.
The first method of air exchange is accomplished due to the propertyof gases to expand upon heating and to shrink upon cooling, With the rise in soiltemperature, 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 apparentlyare of less importance for the respiration of the soil. These periodic fluctuationsof the soil temperature are responsible for a regular gas exchange between the soiland the atmosphere. It appears as if the soil respires. The characteristic featureof this respiration is, as of any other respiration, the uptake of oxygen and therelease of carbon dioxide.
The respiratory activity of the soil can be increased or decreasedby various factors, such as humidity, wind, and others. The water and the air areantagonists, The humidity of the soil leads to a decrease in the amount of air inthe pores of the soil. During the rainy seasons the soil gets so wet that the airin almost completely supplanted. With the drying of the soil the reverse processtakes place.
The CO2 and oxygen content of thesoil air varies seasonally. The largest amount of CO2in 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 andwinter the amount of CO2 decreases considerably.Together with these findings there are observations which show that the activityof 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 ofactinomycetes and bacteria multiply abundantly. In one gram of soil 10-15 thousandactinomycetes Act. globisporus were counted in the spring and summer; in wintertheir number reached 100-500 thousand and more. Sauerland and Groetner (1953) foundthat the release of CO2 in the soil increases inwinter.
The barometric pressure strongly influences the respiration of thesoil. Observations have shown that with the change in pressure, the gas content indeep layers, at a depth of 2 m and more, also changes. With decreasing pressure thegas volume increases and the gases are released into the atmosphere; with risingpressure the picture is reversed.
Diffusion also plays an important role in the gas exchange of thesoil.
According to some authors, diffusion alone can secure the gas exchangeof the soil and maintain the composition of the soil air at a level sufficient tomaintain the life processes of the soil population.
In the soil (except when frozen), biological processes of synthesisand decomposition take place continuously. In this process various organisms formCO2, O2, esters,acids, alcohols, ammonia, hydrogen sulfide. methane, etc. These compounds serve annutrients for other organisms, especially microbes. The content of gases in the soilwill change according to the prevalence of these or other microorganisms In a givensoil and the direction of the biochemical processes.
There are indications that plant roots, not only release, but alsoactively absorb CO2. The amount of CO2taken up, from the soil may be of the same magnitude an that coming from the atmosphereor may even exceed it. The intensity of CO2 absorptionfrom the soil depends on its concentration. The higher the concentration of CO
The CO2 of the soil in taken up bymany microorganisms. The soil is known to be the habitat of many kinds of autotrophswhich use CO2 as a source of carbon for the synthesisof organic compounds. Besides, there are many kinds of heterotrophs in the soil whichcan also use CO2.
We have cited opinions of individual workers who maintained that theCO2 released mainly represents the product of metabolismof the soil microflora. We agree with this opinion. Experiments show that as soonas the activity of microorganisms is hindered, the release of CO2decreases. The reverse picture can be observed when compounds which increase thevital processes of microorganisms are introduced into the soil. Vincent and Nissen(1954) introduced into the soil small doses of antibiotics and obtained a noticeableincrease in CO2 output. In the control series therelease of CO2 amounted to 51.2-56.9 mg/40 g ofsoil. Upon introduction of penicillin, the amount of CO2released reached 112.6 mg, with chloromycetin, 85.2 mg; and with terramycin, 148.7mg.
Numerous investigators consider the release of gases from the soilas directly linked to microbial activity, The biological processes taking place inthe soil under the influence of microflora may be judged by the respiration of thesoil. 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 andbiochemical processes taking place in it, it can also serve as an index of soil fertilityas 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, 1955and others.
Thermal Regime of the Soil
The thermal regime is of an especially great importance in the lifeof the soil.
The main source of heat is the solar radiation. Other sources, suchas the internal heat of the planet and the heat obtained from chemical and biochemicalreactions, are negligible and are not taken into account. The heat effect of radioactivereactions 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 exerta great influence on its heat regime. The clearer the air and the less water vaporsit 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 ismost strongly heated at the equator and most weakly heated at the poles. The heatabsorption is conditioned, not only by the geographical location, but also by itsqualitative content, particularly by the color of the soil. Darkly colored soilsabsorb more heat than the lightly colored ones. The chernozems, for example, absorb86% 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 dependson various factors. Of greatest importance is humidity, since water possesses greaterheat capacity than the solid particles of the soil. Dry soils warm up more rapidlythan moist soils. Heat conductivity also depends on soil moisture. Dry soils conductheat slower than moist ones.
The surface of the soil becomes warm during the day and cools in thenight. This creates a diurnal fluctuation of soil temperature. The greatest amplitudeof these fluctuations can be observed in the summer, especially in places with asharp continental climate.
Heat waves are formed in the soil as a result of the alternation ofwarming 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. Indeeper layers, the temperature of the soil remains relatively constant.
Besides diurnal fluctuations, there also exist seasonal (annual) thermicfluctuations. The depth to which the soil freezes depends on regional and climaticpeculiarities of the locality. There are regions where the soil does not thaw inthe summer or it does so only in the upper shallow layer. This is the region of eternalfreeze. The snow cover strongly influences the thermal conditions in the soil. Itprotects the soil against the winter freeze. In forests the soil freezes to a lesserextent than in the fields. Vegetation slows the warming up of the soil in summerand lessens the degree of freezing in winter. In the same way, it eases the diurnaltemperature fluctuations in summer.
The freezing of the soil in winter exerts a definite effect on thebiological 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 temperatureof liquid air, In our experiments Azotobacter, and root-nodule bacteria surviveda month's storage at 180°C below the freezing point.
These are data on the increase of the metabolism of microorganismsunder the influence of frost. After a three-week storage at -15 to -20° C (infrozen state) Azotobacter, for example, grows and multiplies more rapidly,root-nodule bacteria become more active and virulant, and yeasts are more activeat fermentation of sugars, etc. This apparently is the explanation of the vigorousoutburst of metabolism in the soil in spring.
The spring outburst of microbial activity is sometimes observed inlaboratory conditions in bacteria grown in pure cultures. The periodicity of thechange of summer and winter temperature apparently manifests itself in hereditarycharacteristics, which become fixed to a certain degree and are transmitted fromgeneration to generation for some time. We have observed such an outburst of metabolicactivity in some cultures of azotobacter isolated from the soil near Moscow. Theinfluence of seasonal and meteorological conditions on the metabolic activity ofbacteria was noted by some other investigators (Bortels, 1942).
Under the influence of frost a noticeable change in the chemical andphysicochemical properties of the soil takes place. The concentration of the soilsolution varies; a number of compounds precipitate, for example, ulminic acid intoulmin. According to our observations, the toxic substances of the soil are decomposedand inactivated. Soils exhausted by clover become less toxic after frosts. Inactivationof antibiotics produced by the soil microbes is noticed after the soil has been frozenfor a prolonged period. It is assumed that many other organic and inorganic compoundsin the soil are subjected to sharp changes under the influence of frost, and thesoil as a whole becomes more fertile.
The insolation of the soil has as yet been little studied. The soilis irradiated by the sun's rays only on the surface. The thicker the vegetation theless is the solar radiation reaching the surface of the soil.
Most rays of the spectra do not penetrate into the deep layers ofthe 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 presenceof 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 mountainsummits has shown that biological processes are more vigorous there. According toour observations, the nitrogen-fixing activity of microbes on mountain summits ismore vigorous than in the valleys. Some nitrogen-fixing bacteria are powerful accumulatorsof molecular nitrogen, in high places (Krasil'nikov, 1956b). On comparing the biochemicalactivity of soil bacteria in mountain soils with that of bacteria from valley soilswe 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 climateand especially insolation affect the natural characteristics of bacteria. These characteristicsare not lost for some time after their transfer to valleys. These as yet isolatedobservations give grounds to assume that insolation strongly affects the life ofsoil microbes.
Summarizing, it can be said that the sites occupied by microorganismsin the soil are all the spaces between the soil particles and the aggregates. Microbesflourish in large and small pores. They inhabit microscopically small pores and capillaries.The soil solution is their nutrient medium. Its nutritional value varies, dependingon the concentration of the nutrients, the presence or absence of toxic and bioticsubstances, the gaseous phase, intensity of air exchange in the pores, the incomeof atmospheric oxygen, and on the elimination Of CO2from the soil,
The soil solution with its nutrient properties is to a certain degreean index of the fertility of the soil and the capability of microorganisms to growin it. It determines, not only the composition of the microbial population, but alsothe qualitative distribution of the individual genera and groups.
Organic Matter of the Soil
Organic matter is one of the main components of the soil and conditionsits fertility. According to their composition, the organic compounds of the soilare unique and complex. They are formed from plant and animal residues as a resultof 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 aremetabolized by the living cells of microbes which form various substances. Thesesubstances, in their turn, are subject to biochemical transformations, as a resultof which specific, relatively stable, and complex compounds called humus are formed.
Higher plants supply the soil with organic compounds during the periodof vegetation, releasing various nitrogenous, or nonnitrogenous compounds from theirroots. They also shed dead fractions of roots and parts growing above the surface.
The total mass of plant residues entering the soil may reach considerableproportions. For example, in forests, the annual fall of leaves and twigs comprises1.5-7 tons/hectare according to the type of the forest, its age, and the climatean soil conditions. Various woods yield different amounts of residue. Annual fallof 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 leavesand twigs shed, with pinewood, oak trees, and deciduous forests following.
The amount of the forest litter formed varies. The largest amountis to be found in fir-tree forests (50 tons/hectare and more).
According to the data given, the amount of organic compounds in soilsof various kinds of forests varies. The amount of organic compounds shed in fir-treeforests reaches 5.85 tons/hectare; in pinewood forests, 3.96 tons/ hectare; and inoak forests, 3.5 tons/hectare (Zonn, 1954),
As should be expected, the "sheds" of different forestsdiffer qualitatively, too. According to the data of Zonn, the fall of fir trees ismore acid than that of pine or oak trees. According to our observations, the leavesof birch and lime trees in June are decomposed in the soil more rapidly than theleaves of oak, aspen, or the needles of pine trees.
Meadow vegetation yields dry mass (from the parts growing above thesurface) amounting to about 2-6 tons/hectare and roots, 7-11 tons/hectare. In thechernozem meadows of the steppes about 7 tons/hectare of dry mass (parts grown abovethe surface) were found and 25 tons of roots. In steppes on solonets soils 5 tons/hectareof the dry mass of the parts growing above the surface and 13 tons of roots werefound (Savvinov and Pankova, 1942). In the desert steppes on serozem, about 1 ton/hectareof 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 massand, 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 (C6H
The cellulose is decomposed by special cellulose microbes--bacteria,myxobacteria, actinomycetes, and fungi. Various intermediate compounds are formedin 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 whichreaches 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 aromaticcompounds. The chemical structure of lignin has not been ascertained. Lignin in thesoil 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 thecells of plants, animals, and microbes. They are present in protoplasm, nuclei, andin various protein reserve substances (metachromatin, protein crystals, aleuron grains,etc). Complex proteins are known--proteids and proteins proper such as globulinswhich are insoluble in water but soluble in dilute salt solutions; water-solublealbumins; prolamins--proteins of the gluten of the wheat grain (gliadin) which aresoluble in 80% alcohol; glutelins--plant proteins, soluble in dilute alkaline solutions;sclero-proteins--insoluble proteins of horny tissues such as collagen, keratin, andothers.
Many complex protein compounds are known, such as phosphoproteidscontaining phosphorus, nucleoproteids--proteins of the cellular nuclei and nuclearinclusions (which upon hydrolysis are decomposed into simple proteins and nucleicacids 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 microbialcells are: albumoses and peptones, which are protein compounds forming colloidalsolutions and giving biuret reaction, and amino acids, which are colorless watersolublecompounds containing amino groups (-NH2) and carboxylgroups (OH-C=O).
Plant residues as well as the dead cells of microbes and animal organismsfind their way into the soil, where they are subject to physicochemical and biologicalprocesses.
The main transformations of plant residues are carried out under theinfluence of biotic factors. The dead parts of plants in the soil begin to decomposeimmediately, 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 decomposewaxes, tars, and many other stable compounds. It can be said that no organic compoundexists which cannot be decomposed by microorganisms. Some of them are decomposedrapidly (carbohydrates, proteins, etc.), and others, slowly (tars, waxes, etc. ).
The decomposition of organic compounds may be carried out to the finalproducts, CO2 and water, or may stop with the formationof intermediate compounds. The latter may be in the form of organic acids, alcohols,amino acids, etc.
Simultaneously with the decomposition of organic compounds, syntheticprocesses are taking place in the soil. The so-called autotrophs are known to synthesizeorganic compounds by assimilation of CO2. The firstof these are the photoautotrophic algae, which may be present in considerable numbers.Many colorless chemitrophs and pigmented bacteria possess the same capability. Theyassimilate carbon dioxide and synthesize organic compounds at the expense of chemicalor light energy. Among these are the nitrate, sulfur, iron, hydrogen, and methane-oxidizingbacteria. 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 heterotrophicmicroorganisms are capable of assimilation of CO2and of synthesizing organic compounds. This capability was detected in the representativeof the genera Pseudomonas and Azotobacter, in sporogenous and asporogenousbacteria, in yeasts, fungi, and in actinomycetes. The synthesis of organic compoundsmay reach considerable dimensions, 5% and more of the CO2supplied during the experiment (Liener and Buchanan, 1951). Cells dividing in thelogarithmic phase of growth assimilate ten times more CO2than 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, autotrophsand 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 areutilized as nutrients by microbes and plants.
Shmuk (1930) noted the presence of the following compounds in thesoil: 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 paperchromatography, 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 inthe soil: 6-7% aspartic acid, 5% glutamic acid, and 18% of other amino acids, totaling31.9% total nitrogen. According to the author, 66-75% of soil nitrogen is not inthe humus but in microbial proteins.
Such compounds as polyuronic acids which comprise either componentsof plant tissue (hemicellulose) or products of microbial synthesis (slimy compoundsconstituting bacterial capsules) can also be detected In the soil.
Schreiner and Reed (1907) isolated various organic nitrogen and carbonproducts from fertile soils. Creatine, xanthine, hypoxanthine, adenine, and cysteinewere among the first detected.
Rudakov and Birkel' (1949) found uronic acids among the metabolitesof plant roots. The liberation of these acids takes place with the participationof bacteria possessing protopectinase.
Shori isolated allantoin from the soil and Enders obtained methylglyoxalate, a compound which, according to Neiberg, is an intermediate in hexosefermentation 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
Investigations show that enzymes exist in the soil in the active state.Their quantity varies in accordance with the soil composition, season, and climaticconditions. In fertile cultivated soils there are more enzymes than in poor nonfertilesoils. The more organic compounds in the soil, the more active is the growth of microbesand the greater the enzymatic activity of the soil (Hoffmann, 1952). The upper layerscontain more enzymes than the deeper ones.
The liberation of CO2 has been observedto 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 increasesproportionally to the amount of organic compounds introduced (Table 11). The enzymaticactivity of the soil is more pronounced in May than in October, when the microbiologicalprocesses diminish.
500 tons*/ hectare
1,000 tons/ hectare
2,000 tons/ hectare
500 tons/ hectare
1,000 tons/ hectare
2,000 tons/ hectare
*The organic compounds were introduced with sewage waters.
Kuprevich (1949) detected the presence of the following enzymes in thesoil: catalase, tyrosinase, phenolase, asparaginase, urease, invertase, amylase,and protease, noting that their accumulation depends on soil cultivation. The quantitativefigures for catalase, invertase, and urease present in soils, according to his data,are given in Table 12.
|The soil of the garden of the Botanical Institute of the USSR Academy of Sciences in Leningrad|| |
|The soil of a pine forest|| |
|The soil of an orchard|| |
|Washed river sand under barley|| |
Sorensen noted greater activity of xylanase in cultivated soils thanin noncultivated soils. The enzymatic activity increased six times and more whenstraw or xylan were applied to the soil (Sorensen, 1955).
Scheffer and others (1953) and Seegerer (1953) pointed out the increasedactivity of invertase and urease after the application of organic fertilizers, especiallymanure.
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 invertasewas greater than after the plowing in of green lupine (Table 13).
|Fallow (control)|| |
|After introduction of lupine|| |
|After introduction of serradella|| |
As can be seen from the given data, the enzymatic activity is closelycorrelated with the activity of microorganisms. Any increase In the amount of thelatter leads to enhancement of enzymatic soil processes. Hoffmann (1951) considersthat the enzymatic activity of the soil is an index of its fertility.
There are data in the literature indicating that plant roots excretevarious enzymes into the soil, such as catalase, tyrosinase, amylase, protease, lipaseand others.
All these organic compounds comprise only 10-15% (approximately) ofthe total organic mass of the soil. However, owing to their great activity, theyare of considerable importance. Many of these organic compounds (vitamins, auxins,certain amino acids) are catalysts of biological and biochemical processes in thesoil.
The part played by free extracellular enzymes is not yet clear, butthey may be assumed to be important in transformations of many types of organic compoundsand, in particular, in the synthesis of humus compounds.
We should note the considerable role of antibiotics in the life ofthe soil. These substances influence the composition of the microbial populationsand this affects many soil properties.
Humic substances of the soil. Humus comprises the bulk of theorganic 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 anyof the groups of compounds known to organic chemistry. These substances are synthesizedin the soil, apparently exogenically, by the action of extracellular enzymes. Thecomposition of humus is more complex than that of many compounds of plant and microbialorganisms. 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 tobe the three acids: ulmic, humic, and crenic.
According to Vil'yams, ulmic acid is formed during the anaerobic decompositionof organic compounds by anaerobic microbes. It is easily soluble in water impartinga dark brown color. It forms water-soluble salts with monovalent cations (potassiumand sodium) and insoluble salts with bi- and trivalent cations. Under the influenceof external factors, such as low temperature (freezing) or drying, ulmic acid isconverted to water-insoluble ulmin.
Humic acid is formed under aerobic conditions and is considered tobe a product of bacterial and fungal metabolic activity. Its properties are closeto those of ulmic acid. It is less soluble in water than ulmic acid and gives thesoil 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 withbiand trivalent cations.
Humic acid has been studied in more detail. The following organicgroups were detected: carboxyl (COOH), hydroxyl (OH), carbonyl (CO), and methoxyl(CH20), (Kononova, 1951).
Humus contains from 10-40% humic acids. The largest amount can befound 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 theavailable data, more than one humic acid exists. Dragunov (1948) found that two samplesof humic acid, one obtained from peat and the other from chernozem, differed fromeach other in their chemical composition, in the amount and structural type of theirfunctional groups, as well as in the structure of their nuclei,
Bremmer (1955) subjected samples of humic acids, obtained by him fromnine different soils, to chemical analyses. Each sample of the acid was analyzedfor total nitrogen, ammonia-nitrogen, amino-nitrogen, and a- amino-acid nitrogen.The solutions obtained after hydrolysis were analyzed by paper chromatography foramino acids.
It was found that the samples of humic acids studied differed fromeach other in the composition of their nitrogen compounds and amino acids. Alkaliextracts 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 identifiedby 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, especiallyby actinomycetes. Many actinomycetes grow well, bear fruit, and form antibiotic compoundson media containing humic acids as a sole source of carbon and nitrogen. Many formsof 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 forestvegetation and forest litter. Its properties differ sharply from other humic acids.It is colorless, highly soluble in water and acids, is not subject to denaturationand forms salts which can be crystallized.
Crenic acid possesses sharply pronounced acidic properties. Accordingto Vil'yams, it can raise the soil acidity to such an extent that the activity andgrowth of many microorganisms is arrested.
It is difficult to accept this assumption. Organic acids as such areby 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 numberof microbes.
Owing to its solubility, crenic acid penetrates deep layers of soiland there, combining with bases, forms crenates. They are harmless to microorganismsand are utilized by them as nutrients. Crenates are highly soluble in water, areeasily leached from the soil, and may find their way either into ground or surfacewater. Thus, due to the high solubility of crenic acid and its salts, their accumulationin large concentrations is prevented.
Crenic acid may be reduced by nascent hydrogen with the formationof apocrenates. The reduction is carried out with the participation of anaerobicbacteria. Apocrenates are the salts of apocrenic acids. They have not been obtainedin pure form. The salts of monobasic cations are highly soluble in water. Calciumapocrenate is slightly soluble in water and apocrenates of trivalent metals--iron,mangane se, and aluminum--are completely insoluble. These compounds are depositedin the soil in the form of voluminous amorphous sediments.
Crenic and apocrenic acids (fulvo acids) are widely distributed insoils. Their properties vary according to the soil. Kononova (1953) found that theacids from podsols differ from those of krasnozems.
The diversity of the natural conditions of soil formation both ofa geographical and an ecological character, influence humus formation as a wholeand, in particular, the composition of its individual components: humic, ulmic, andfulvo acids and other organic and organomineral compounds.
V. V. Dokuchaev was the first to point out the regular nature of theformation 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 theregularity of humus-compound formation in soils, in relation to the vegetative coverarid biochemical activity of the microflora.
Later investigations proceeding from chernozems to podsol soils, provedthe 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 theresults of his own investigations, showed that the geographical regularity of humusformation 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 USSRand, in general, of the podsol soils is of a bright color, it contains few stablehumic and ulmic acids but many compounds highly soluble in water which are easilyleached from the soil, e. g. , crenic acid and apocrenates. Their concentration inpodsol 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 humuswith a different ratio of humic and fulvo acids.
The composition of humus in various soils also differs. Chernozem-typesoils 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 typesof the USSR. She found variations in the elementary composition of the acids, theiroptical density, and their distribution. The humic acids of chernozem soils are themost highly condensed, they are followed by the humic acids of the dark-gray forestsoils, chestnut soils, and the bright-gray soils of the serozem; the humic acidsof podsol soils and krasnozems are but weakly condensed, By applying the methodsof X-ray structural analysis, the author determined the main structural outlinesof humic and fulvo acids, which varied in relation to the type of the soil. Theseinvestigations disclosed the unity of the soil-forming process. While studying thegenesis of humus and its components in various soils, Ponomareva (1956) reached analogousconclusions.
Investigators express three different points of view on the mechanismof humic-acid synthesis (see Kononova, 1951). The majority of workers consider thatthe formation of these compounds is outside the activity of microorganisms. Thiswas criticized by Kostychev and then by Vil'yams.
At present, microorganisms are considered to play an increasinglyimportant role in the process of humus formation. Reistric et al., (1938,1941), bymeans of molds, detected the formation of compounds of the aromatic quinone seriesfrom sugars.
These investigations stimulated the study of products of microbialmetabolism; products which could serve as building material for the synthesis ofhumic acid. At present, many foreign (especially German) and Soviet investigatorsare busy studying microorganisms, their metabolic products, and the synthesis ofhumus compounds.
Great attention has been drawn to molds, actinomycetes, and heterotrophicbacteria as producers of humus-like compounds. While studying the products of bacterialmetabolism, Martin, J. (1945) found that about 30% of the humus is synthesized atthe expense of bacterial polysaccharides of the uronic type. The most stable of them,"levan", is formed by sporogenous bacteria Bac. mesentericus andBac. 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 similarin color and certain chemical properties to humin substances. Laatsch. Hoops, andBieneck (1952) found that the fungus Spicaria and certain actinomycetes, whengrown on artificial protein media, are capable of forming a compound closely relatedto humin. Scheffer and Twaditmann (1953), Plotho (1950), Laatsch and others succeededin finding a medium in which, under given conditions, fungi or actinomycetes formedsubstances of the phenol type. These investigators assumed that the oxidation-reductionsystems--quinones < = > polyphenols--are in a state of continuous activityin the living cell, being oxidized by polyphenol oxidases and reduced by dehydrases.With the cessation of respiration the quinones are released from the cell, beingirreversibly oxidized; they then combine with organic nitrogen compounds (proteindecomposition products) to form humic acids. Consequently, according to the above-mentionedauthors, the reaction of quinones with microbial nitrogen compounds is the basisof humus formation.
Wilts (1952) noticed that humic substances are formed from variousorganic compounds. The building blocks of the humus particles may be products ofdecomposition of lignin and of tannic compounds--aromatic compounds of the phenylpropansseries, easily hydrolyzed carbohydrates (cellulose and others), and proteins whichare 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 substancesupon 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 vegetativeresidues. Tepper (1949, 1952) has shown that humin substances are formed at the expenseof pigments formed by fungi and actinomycetes (see Rudakov, 1949 and 1951).
Kononova (1951), in her monograph, proposes that various plant residuesand products of resynthesis, as well as the microbial protoplasm participating inthe 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 aromaticcompounds with an amino acid or polypeptide. This process takes place with the participationof microorganisms under the conditions of biocatalysis maintained by the oxidativebacterial enzymes. As a result, nitrogen-containing compounds of a cyclic structureare formed.
Radioactive Substances of the Soil
Among the mineral elements of the soil a special place is occupiedby radioactive substances: radium, uranium, thorium, and others. According to Baranovand Tseitlin (1941) their content (weight %) in different soils in as follows:
Ra (x 10 -11)
U (x 10 -5)
Th (x 10 -4)
|Krasnozem, Batumi|| |
|Desert serozem|| |
|Medium podsol loams, Moscow Oblast'|| |
|Dark forest|| |
|Podsol, Leningrad Oblast'|| |
|Loamy chernozem|| |
|Mountainous tundra, Khibiny|| |
|Marshy tundra, peat|| |
The biological significance of natural radioactive elements remainsunknown. 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 substancesin small concentrations activate biological processes, increase metabolism, and exerta positive influence on the growth of plants. The natural radioactive substancesof 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 indetail some of the biological processes of yeasts, fungi, and bacteria caused byradium, radium emanations. X-rays, etc. These authors were the first to establishthe effect of radium and other sources of radiation energy in promoting genetic mutations.
We (Krasil'nikov, 1938) have shown that various types of luminescentactinomycetes react differently to the radiation of radon. Certain species were moresensitive than others. Radon rays have a stimulating or suppressing effect on thegrowth 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 thesesubstances from the soil and accumulate them in their cells in considerable quantitieswhich many times exceed the concentrations of these substances in the soil.
Attention should be drawn to the fact that the degree of accumulationof radioactive substances in cells varies in different kinds of bacteria. Some kindsof bacteria, especially Azotobacter, accumulate radium in large quantities,others, in small quantities, or are completely devoid of this capacity. Even in thesame genus different strains accumulate natural-radioactive substances to a varyingextent.
The radioactive substances inside the bacterial cell stimulate growthand metabolism. Nitrogen fixation by Azotobacter is enlarged under the influenceof radium and thorium. The ability of root-nodule bacteria to penetrate the rootsof 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 cannotbe detected by ordinary electronic counters (radiomer B-2 and others). The microorganismsare sensitive to irradiation by radioactive substances in doses which cannot be detectedby modern instruments.
The microbial population of the soil as well as plants are adjustedto small concentrations of radioactive elements. High doses given to them artificiallyunder experimental conditions are harmful. Minimal concentrations of radium, uraniumor thorium which can be detected by electronic counters damage even the least sensitivespecies of bacteria. Under the influence of such doses the cells undergo degeneration,increase in size and deform, their protoplasm becomes coarsely-granular, vacuolesappear, and their reproduction slows down and eventually stops altogether (Figure53). Similar changes were observed by Filippov, Shtern, Rokhlina , and other collaboratorsof 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 actionof large doses of radium and other sources of radioactive irradiation, was notedby Meisel' (1955). His investigations led to the emergence of a scheme of consecutivedamage 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 possessfree energy and continuously carry out considerable chemical activity in the soil.The energy of radioactive elements affects chemical and biochemical processes ofmicrobes and organisms. Vernadskii stresses the fact that life in the biosphere originatesfrom two energy sources: solar radiation and atomic radioactive energy. Accordingto his calculations, only three radioactive elements, uranium, thorium, and radiumsupply the earth with heat, the quantity of which exceeds a thousand times that receivedby the earth's surface.
The biosphere of the earth accumulates dispersed radioactive elementsand concentrates them on the surface, thus essentially changing the energetics ofthe whole population. It should be assumed that plants, animals, and microbes have,during their long evolution, acquired the ability to utilize these powerful energysources. Analyses show that radioactive substances are present, to a larger or smallerextent, in all organisms and almost always in concentrations exceeding those in thesurrounding environment. In many instances, plants contain ten times or hundredsof 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 substancesremains 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 dosesof radium, uranium or thorium stimulate the growth and increase the dry mass yield.Studies on the importance of natural-radioactive substances in soil fertility andin the life of plants and microbes are still inadequate. There are a number of observationswhich give reason to believe that these substances play an essential role in nitrogenfixation. A question arises as to the energy source needed to fix 100 to 150 kg andmore of molecular nitrogen per one hectare of soil in one season. To fix such amountsof nitrogen and they are actually of this magnitude, Azotobacter, the mostpowerful nitrogen-fixing organism, requires 5-10 tons of glucose. Such quantitiesof this energy-yielding material are hardly to be found even in the most fertilesoils.
Perhaps in this case the radioactive soil substances constitute theenergy source which is indispensable for nitrogen fixation, as well an for many otherprocesses taking place in the natural environment.
'The natural-radioactive substances deserve the most painstaking studiesas biocatalysts on the earth's surface. When they enter into the chemical compositionof living organisms, it should be assumed that they are not destroyers but creators,participating in many transformations and stimulating various enzymatic processes.