The scientific basis of soil science as a naturalscience was established by the classical works of Dokuchaev. Previously, soil hadbeen considered a product of physicochemical transformations of rocks, a dead substratefrom which plants derive nutritious mineral elements. Soil and bedrock were in factequated.

   Dokuchaev considers the soil as a natural bodyhaving its own genesis and its own history of development, a body with complex andmultiform processes taking place within it. The soil is considered as different frombedrock. The latter becomes soil under the influence of a series of soil-formingfactors--climate, vegetation, country, relief and age. According to him, soil shouldbe called the "daily" or outward horizons of rocks regardless of the type;they are changed naturally by the common effect of water, air and various kinds ofliving and dead organisms.

   The outstanding soil-science specialist, P.A.Kostychev (1892), in developing the theory of soil, attaches special importance tobiological factors. He considers the soil as a botanist and not as a geologist. Onthe question of the origin of the chernozem, Kostychev attributes the most essentialrole to plants and microbes. He writes that chernozem formation is involved withthe geography and physiology of higher plants as well an that of lower ones whichperform the decomposition of organic matter. The accumulation of soil humus dependson the intensity and completeness of the decomposition of plant residues, the rootsand the parts which are above the ground. In these processes Kostychev identifiesthe most important role with microscopic creatures--fungi and bacteria. Being anexcellent microbiologist he carried out interesting experiments on the decompositionof organic matter and the formation of humus. The experiments showed that in differentcases the decay of plant residues began in a different way. Sometimes bacteria inhabitdecaying matter first, and sometimes fungi emerge first. Various parts of the samedecaying matter decompose differently, in one part one organism multiplies and nearit an entirely different organism may be found. It is further pointed out that variousforms of decomposition change consecutively according to changes in the propertiesof the decaying substance.

   Kostychev (1889) for the first time establishedthat humus is formed by soil fungi, Moreover, the discovery of the regularity ofthe relationship between carbon and nitrogen (C:N) in soil and of its importanceIn the development of plants and microbes must be attributed to him. Kostychev revealedthe essence of the enrichment of soil humus by nitrogen. During the process of thedecomposition of plant residues, which are known to contain no more than 1.5-2% nitrogen,humus with 4-5% of nitrogen is obtained. This transformation of organic matter asmay be seen from Kostychev's data, occurs with the aid of microorganisms (see Kostychev,1951).

   With his works Kostychev laid the foundationof soil microbiology. He also outlined a vast program of investigations, the problemsof which are at present being solved by Soviet microbiologists.

   Vernadskii (1927), who expounded the theory of"bio-inert" natural bodies, wrote that the entire soil is a characteristic"bio-inert" body. All physicochemical properties of the soil would appearconsiderably different if the living substance in it were not taken into, account.In this way Vernadskii formulated the dependence of the fundamental property of soilupon the organisms lodged in it.

Vil'yams (Russian soil scientist (1863-1939) whose name is thusspelled when transliterated from the Russian, though it may appear as Williams inforeign publications) developing the teachings of Dokuchaev and Kostychev. introducedmany new principal elements into the science of soil.

   He considers the soil a natural body and meansof agricultural production, an attributes to it the new qualitative property of fertility,i. e., the ability to produce plant crops. According to him ,the notion of soil andits fertility are inseparable. He considers fertility an essential property, a qualitativeindication of soil independent of the degree of its quantitative expression. We opposethe idea of fertile soil to the idea of sterile stone, in other words to the notionof massive rock (Vil'yams, 1949).

   It must be noted, that rocks also possess fertilityto some degree. Investigations show that even the hardest rock massifs a are inhabitedby various organisms. Lichens develop on their surfaces and often cover large spaceson mountain tops. The upper layer of rock massifs, the so-called weathering crust,is saturated with bacteria and algae, as well as fungi, actinomycetes, protozoa andother organisms. According to our data, in the upper layer of basalt rocks, the bacteriain one grain of substrate number from some tens of thousands to millions. Similardata are also given by other investigators (Novogrudskii, 1950; Glazovskaya, 1950;Parfenova, 1955; Krasil'nikov, 1949 and others).

   Rocks differ in their fertility. Some of themare densely overgrown with lichens and microorganisms, others are sparsely covered.There are rocks, or more properly parts of one and the same rock, which are not overgrownwith lichens, but contain only certain microbial forms--bacteria, actinomycetes andfungi (Krasil'nikov, 1949b).

   Even under arctic conditions, on the islandsof the northern Arctic Ocean (Franz Josef Land, Novaya Zemlya, Severnaya Zemlya andother areas) rocks and loose calcareous soils contain a considerable number of microorganisms.Tens of thousands to hundreds of millions were counted in one gram. Moreover, theseorganisms live actively and carry out biochemical and chemical transformations (Krasil'nikovand Artarnonova, 1958).

   In order to study the properties which determinethe crop capacity of soil and to increase the soil's fertility by exercising an influenceon it, "it is first of all necessary to know these properties, to enumeratethem, and to choose from the large number of properties and qualities of soil justthose which determine the capacity of soil to produce the products necessary formankind" (Vil'yams, 1949, p 138). Consequently, Vil'yams inseparably relatedthe theory of soil with the theory of its fertility and its crop capacity.

   The principal property of the soil fertilityis determined by biological factors, mainly by microorganisms. The development oflife in soil endows it with the property of fertility. "The notion of soil isinseparable from the notion of the development of living organisms in it". Soilis created by microorganisms. "Were this life dead or stopped, the former soilwould become an object of geology" (Vi'lyams, 1950, p 204). Kostychev and Vil'yamstransferred the science of soil from the chapter of geology to the chapter of biology.

   The new understanding of the biological essenceof the soil-creating processes, which was established by Kostychev and Vil'yams,has given the majority of Soviet microbiologists the principal leading landmarksin their investigations. Microbiology also contributed considerably to the developmentof this new direction in the science of soil, successfully solving many essentialquestions of soil fertility.

   During the last two decades, microbiology hasshown that life processes taking place in soil, were larger and deeper than it hadbeen assumed earlier. Whereas earlier, hundreds of thousands and millions of microbialcells were counted in one gram of soil, at present, with more improved methods ofinvestigation, hundreds of millions and billions are determined. The total bacterialmass on one hectare of the surface layer of fertile soils amounts to five to seventons. This mass to composed of single cells which live, develop and multiply.

   Along with bacteria, a very large number of fungi,actinomycetes, algae, ultramicrobes, phages, protozoa, insects, worms and other livingcreatures. inhabit the soil. Fungi and actinomycetes are counted in hundreds of thousandsand millions in one gram, algae--in thousands and tens of thousands. Quite oftentheir number even amounts to a hundred thousand in one gram of soil. The total massof these organisms in the upper surface layer of the soil may amount to two-threetons in one hectare.

   Analyses show that there is in the soil a greatnumber of phages--actinophages and bacteriophages, which have very intense activity.

   No less important is the soil fauna. Accordingto different authors, amoebae, ciliata, and other protozoa are numbered in tens andhundreds of thousands in one gram (Brodskii, 1935, Nikolyuk, 1949; Dogel', 1951).In one square meter of the surface layer of soil some tens to hundreds of largerinvertebrates may be found--earth worms, myriapods, larvae of various beetles, etc.The population of small nonmicroscopic arthropods (ticks, Collembola, and others)is numbered in tens and hundreds of thousands in 1 m2 of cultivated soil layer, and in forest soils their number often amountsto a million individuals.

   The number of nematodes is sometimes countedin millions per 1 m2. Accordingto the counts of Gilyarov (1949, 1953), the total mass of this fauna comprises severaltons (3-4) per hectare of soil.

   As seen from the data cited, every particle ofsoil is saturated with living creatures. The enormous mass of these creatures isin a state of continuous activity during the whole vegetative period. Separate individualsor cells of simpler creatures multiply rapidly and attain astronomical numbers. Inthe process of life activity the whole soil population carries out work of cosmicimportance. It transforms enormous masses of organic and mineral compounds, and continuouslysynthesizes new organic and inorganic substances.

   Various active biocatalysts--metabolites of microbesand other living creatures are found in the soil, including enzymes, vitamins, auxins,antibiotics, toxins and many other compounds. All these substances together withliving organisms lend particular properties to the soil, differentiating it froma geological body or a mineral rock.

   The biogeny of soil is the most significant indicatorof its fertility. As soon as the activity of a microbial population begins in a rock,the first signs of fertility are manifested. The degree of soil fertility is determinedby the intensity of the life processes of the microbial population.

   Quantitative manifestations of biological processesare diverse and depend upon climatic, and geographic al or topographical conditions,as well as the seasons of the year and other external factors.

   The knowledge of life in soil, investigationof biological and biochemical processes taking place in it, are inseparably connectedwith the knowledge of living organisms inhabiting the soil. Consequently, investigationof the microbial world of the soil is one of the basic problems of microbiology andagrobiology. The knowledge of biological processes, caused by the living populationof the soil, should be one of the most important problems of pedology and agriculture.

   This work deals with present knowledge of microorganisms,mainly of bacteria, actinomycetes and partly of fungi, inhabiting the soil, of theirrelationships with higher plants, of the importance of different groups and microbialspecies in the life of higher plants, and of the effect of metabolic products ongrowth and yield of agricultural crops.

   It should be pointed out that in recent years,in the investigation of the problem of soil fertility. more and more attention inpaid to biological factors. A growing interest in microbial population is displayedby plant growers as well as by soil scientists. This to not surprising, since itis impossible to solve problems of pedology, not to speak of agriculture and plantgrowing, without taking into account the microflora of soil.

   The importance of microorganisms in the lifeof plants, as shown by present data, in very great but still little investigated.A lesser influence on the development and life activity of microbes in the soil ismanifested by plants.

   The interaction of soil microorganisms with higherplants is very complex and multiform. The effect of the former on the latter maybe positive or negative. Depending upon the plant cover on the same soil under equalexternal climatic conditions, the composition of the microflora changes sharply.Plants are a very strong ecological factor. selecting certain species of bacteria,fungi, actinomycetes and other inhabitants of soil. As a result of wrong agriculturalpractices and crop rotation, the soil becomes infested with harmful microbial forms.By use of suitable plants in the crop rotation, one may change the microflora ofsoil in the desired direction, and eliminate harmful organisms, in other words--restorethe health of soil.

   The influence of soil microorganisms on growthand development of higher plants is of great diversity. The role of microbes is notby any means limited to the mineralization of substances in nature. At present, microorganismsshould not be considered as only a link in the circulation of substances, as agentsdelivering sources of mineral nutrition to plants. Microorganisms of soil displaya direct, very essential influence on plants, a positive or negative one dependingupon the species and external conditions.

   The positive role of symbiotic root-nodule bacteria,mycorhizal fungi and others is well known though little investigated. Great attentionin paid to free-living nitrogen-fixing organisms--Azotobacter, Clostridiumand others. Fertilizers comprising bacteria and fungi are largely used (azotogen,nitrogen, silicogen, phosphorobacterin and others).

   A large chapter of science is devoted to thebiology of phytopathogenic bacteria and fungi.

   Little investigated and elucidated is the roleof the free-living microflora, particularly that which inhabits the root zone. Investigationof the rhizosphere microflora has begun comparatively recently. The data obtainedshow that its effect on plants to very considerable. Among the rhizosphere microflorathere is a large number of species which affect the growth of plants with productsof their metabolism. Some microbial species are active producers of various bioticsubstances vitamins, auxins, amino acids and other substances essential to the growthof plants. Other species are antagonists of phytopathogenic bacteria, fungi and protozoa.Microorganisms of these species strengthen the immunization properties of tissues,thus protecting the plants from infections.

   Among the soil microflora there are many organismswhich produce toxic substances and suppress the plants; they inhibit their growthand development. It must be assumed that there are in the soil other microbial forms,as yet unknown, exerting a positive or negative effect on plants.

   Representatives of useful and harmful microflorainhabit the rhizosphere. Their quantitative ratio in various soils and under variousconditions of treatment is different. Consequently, the total effect of the activityof the root zone microflora will not be the same. It may be positive or negative,depending on what microbial species predominates.

   In this work the latest information on the importanceof this microflora for plants is given.

   It is evident that in investigations of the activityof soil microflora exact knowledge of its species composition is necessary, as areexact conceptions of the soil climate and the ecological conditions of its developmentand accumulation. At present. investigations of soil biology in general, and questionsof interaction of microorganisms with higher plants in particular, cannot be resolvedwith a merely quantitative account of the general composition of the microbes. Itis necessary to establish the degree of dissemination of the different species, toinvestigate their biology and specificity of interaction with the plant, and theirinteraction with other species of soil microflora. The latter should be an importantproblem of soil microbiology. Microbial antagonists represent an important ecologicalfactor of development and formation of microbial associations in general and disseminationof individual species in particular.

   It is important to point out that the determinationof the microbial species and particularly of bacteria is very difficult. The similarityof structure and the lack of external marks of identification does not allow theinvestigator to recognize, immediately or precisely, the forms and species he isfaced with in the analysis of soil or other natural substrate.

   We shall not be able to reveal the specificityof the root-zone microflora until we learn to identify and differentiate bacterialspecies. Without this it is impossible to determine the specificity of biochemicaltransformations in soil, and, consequently, the site of its fertility is also indeterminable.