HOME      AG LIBRARY      KRASILNIKOV TABLE OF CONTENTS



Part II

THE SOIL AS AN ENVIRONMENT FOR MICROORGANISMS

 

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

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

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

 

Size of particle,
diameter in mm

rocky

above 3

coarse sand

3-1

medium sand

1-0.25

fine sand

0.25-0.05

coarse dust

0.05-0.01

medium dust

0.01-0.005

fine dust

0.005-0.001

silt

less than 0.001

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

 

percentage of particles smaller than 0.001 mm

heavy clays

80%

medium and light clays

80-50%

heavy loams

50-40%

medium loams

40-30%

light loams

30-20%

sandy loams

20-10%

compact sand

10-5%

loose sand

less than 5%

  The quality of the soil as an environment for microorganisms is determinednot only by the size of the particles but also by the character of their distribution,in other words, by the structure of the soil.

 

Soil Structure

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

  The structured soil consists of aggregates of different sizes, from1 to 20 mm and more in diameter. The aggregates in the soil are more or less loose,the intervals between them are of different sizes and forms. These intervals or poresensure the presence of water, air permeability and various foodstuffs for plantsand microorganisms. In such a soil the evaporation of moisture is diminished.

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

 

In structureless soil,
%

In structured soil,
%

Porosity

50

55-60

General porosity

45-48

20-25

Capillary porosity

2-5

30-35

Noncapillary porosity

5

30-40

Air content

3-5

20-25

Water permeability
(in mm/hr)

1.6

0.7

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

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

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

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

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

  According to Godroits, the binding of the smallest elementary particlesinto microaggregates precedes the formation of aggregates; these can be destroyedonly by chemical means.

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

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

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

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

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

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

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

  When studying the growth of microorganisms on decomposing organiccompounds, Kostychev noticed that when only bacteria develop the substrate neverdarkens. The substrate, according to his data, becomes colored only in the presenceof fungi. On the basis of these findings, the author concluded that fungi play themost important role in the darkening of the substrate and the bacteria have no partin it. At the same time, Kostychev did not preclude the role of bacteria in thisprocess. He pointed out that when the access of oxygen to the substrate is arrested,fungi stop their activity and their growth ceases, but the process of decompositioncontinues, though at a lower rate. (Kostychev--The formation of chernozem. Thesoils of the chernozem belt of Russia, their origin, composition and properties,1889).

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

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

  The ideas of Kostychev were developed and furthered by Vil'yams. Vil'yamspaid special attention to humus in the structural formation of soils, pointing outthat not every type of humus is capable of forming a stable soil structure; onlyfreshly precipitated humus of given properties may contribute to the stable soilaggregate (Vil'yams-Lectures on the soil science given at the Moscow AgriculturalInsitute in 1895-1898, 1897). Vil'yams (1897) thought that physicochemical agentsand rain water play a considerable part in the destruction of soil structure. Henoticed that rain water containing ammonia destroys soil aggregates liberating calcium,and, as a result, humus loses its cementing properties. According to him, the mainfactor in the regeneration of soil structure is the sowing of grasses. Later. Vil'yamssuggested that biological processes, caused by the metabolism of microorganisms,are the most important factor in the formation of structure. The process of organicresidue decomposition varies according to the degree of aeration and is caused bydifferent types of microorganisms. Vil'yams distinguishes two entirely differentprocesses in the decomposition of plant residues which are of great importance inthe structural formation of soils and these are: aerobic (caused by fungi and bacteria)and unaerobic processes.

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

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

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

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

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

  Some scientists (Kanivets, 1951, and others) suggest that fungi suchas Trichoderma lignorum, Mucor intermedius, and Martierella isabellinaare the most important factor in the process of structure formation. Foreign investigatorstested Trichoderma köningii, Aspergillus niger and other fungi with positiveresults (Martin and Andersen, 1924; Martin and Waksman, 1940; Martin, 1945, 1946;Peels, 1940; Peels and Beale, 1944). Gel'tser (1940) suggests that lysates of fungalcultures formed by bacterial action and also colloidal protein compounds synthesizedby bacteria represent the cementing substance.

  Kononova (1951) assumes that the cementing substance consists of amixture of cellulose decomposition products and protoplasm of the decomposed cellulosebacteria.

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

  Considerable work was carried out by Mishustin et al. He studied theaction of fungi, actinomycetes, and bacteria on the decomposition of organic compounds(peptone, albumin, saccharose, starch, malic acid) artificially introduced into thesoil on the formation of soil structure. The author concludes that the aggregationof soil particles is accomplished most vigorously by fungi and actinomycetes of mycelialstructure. Thirty-two to fifty-one per cent of aggregates of 0.25 mm and more insize is obtained in experiments with fungi. Experiments with actinomycetes yield25-30%. This value never exceeded 2.8% in the control soil. The various kinds offungi have unequal powers of causing aggregation. Various sporogenous and asporogenousbacteria were studied including representatives of the genera Azotobacter, PseudomonasRhizobium, Bacterium, Bacillus, and others. Their cementing ability was veryweak. The number of aggregates did not exceed 2-3%. more often 1-1.5%, and only insporadic cases were there more than 10%. The cementing of soil particles proceedsconsiderably weaker under the influence of the simultaneous development of fungiand bacteria than in experiments with pure cultures of fungi. The opinion of theauthor is that bacteria in mixed cultures lower the aggregating action of fungi andactinomycetes. This is apparently caused by the action of bacteria in supressingthe development of fungi or by their destroying the cementing compound formed bythe fungi. Mishustin distinguishes two types of structure formations: biological,caused directly by microorganisms, and the so-called type of "humus structure",formed by humus compounds. The first type yields an unstable structure and is consideredas an original and indispensable stage in the natural circumstances of structureformation (Mishustin and Pushkinakaya, 1942; Mishustin, 1945a). Rudakov (1951) assumesthat the cementing of soil particles into aggregates in carried out by active humus,which comprises a complex of compounds of uronic acids and bacterial proteins, orproducts of their lysis. The uronic acids are formed, in his opinion, by pectin-destroyingor protopectinase bacteria, which are found in greater or lesser quantities in thesoil and on decomposing plant residues. Bacteria possessing protopectinase grow onplant roots, penetrate intracellular spaces, destroy intermediate compounds suchas protopectin, and in so doing, form sugars and uronic acids. The latter, by combiningwith bacterial proteins, form uroprotein complexes capable of cementing soil particles.The galacturonic and other uronic acids are formed at the expense of living rootsof vegetating plants as well as at the expense of dead root residues.

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

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

 

%

In the control

11.48

Bac. polymyxa

20.51

Bac. mycoides

10.11

Bac. coli

4.989

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

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

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

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

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

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

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

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

 

Soil Porosity

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

Figure 49. Soil porosity:

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

 

  The total volume of all the pores in 1 cm3of soil taken in its natural surroundings is designated as soil porosity. The porosityof soils is one of the most important factors determining their fertility. The porosityvolume varies in different soils and depends on the type of soil, its state, external,seasonal, climatic influences and on the vegetation. However, the most importantfactors in determining soil porosity is the degree of soil structure, and the sizeof soil aggregates. The smaller the aggregates the less the spaces between them andthe greater the total porosity. The size of the spaces between soil aggregates andparticles varies from several microns to 2-3 mm and more (Kachinskii, Vadyunina andKorchagina, 1950).

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

  P = (1 - vol/sp) x 100

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

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

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

Soils

Depth of the layer, cm

Total porosity, %

Intra-aggregate total porosity, %

inter-aggregate porosity, %

Lixiviated chernozem with slght clay admixture, Kurak Oblast', steppe .

 

 

 

 

A--0-4

63.86

40.54

23.32

 

A1--10-14

61.17

39.30

21.87

 

A2--40-44

58.75

--

--

 

B1--55-59

58.93

36.43

21.82

 

B2--80-84

57.85

36.03

21.82

         
Nutty-lumpy solonets with slight clay admixtures, Sverdlovak Oblast', virgin soil

 

 

 

 

 

A1--10-14

50.00

29.55

20.45

 

B1--15.19

50.18

20.27

29.91

 

C--60-64

44.40

--

--

         
Medium podsolic loam, Moscow Oblast'

 

 

 

 

 

A1--0-12

49.05

31.61

16.43

 

A2--20-32

47.55

32.27

15.28

 

B1--32-35

41.70

--

--

 

B2--55-85

36.76

--

--

 

B2--85-110

34.10

23.78

10.32

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

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

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

 

The liquid phase of soils

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  Apart from the aforementioned two categories of water, chemicallybound water is also present in the soil. It enters into the composition of mineralsand represents an integral part of compounds appearing as water of crystallizationand hydration. This water can be removed only upon prolonged heating, whereby essentialchanges of the properties of the heated compounds take place. This water is unavailableto organisms.

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

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

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

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

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

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

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

  Capillary water exists in three states:

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

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

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

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

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

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

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

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

 

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

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

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

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

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

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

 

The soil solution

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

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

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

Table 8
Water solubility of mineral salts present in the soil (g/l)
(according to Vilenskii, 1954)

Salt

0°C

10°C

20°C

30°C

40°C

50°C

K2CO3

10.53

10.83

11.05

11.37

11.69

12.13

KCl

276

310

340

370

400

426

KNO3

133

209

316

458

639

426

K2S04

74

92

111

130

148

166

Na2C03

7

125

215

388

485

--

NaHCO3

69

82

96

111

138

--

NaCl

357

358

360

363

366

370

NaNO3

721

779

845

916

984

1,041

NaSO4

50

90

194

408

--

--

CaCO3

--

--

0.0145

--

--

0.0152

Ca(HCO3)2

161.5

--

166

--

--

--

CaCl2

595

650

745

1,020

--

--

Ca(NO3)2

1,021

1,153

1,293

1,526

--

--

Ca(H2PO4)2

--

--

153

--

170.5

--

CaSO4

1.759

1.928

--

2.090

2.097

--

MgCl2

528

535

545

--

575--

--

MgSO4

408

423

445

454

--

504

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

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

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

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

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

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

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

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

  The results are given in Table 9.

Table 9
The growth of bacteria in various types of soil solutions
(millions of bacteria per 1 ml)
Soil solution

Pseudomonas
No. 11

Pseudomonas
No. 23

Bacterium
No. 1

Bacterium
No. 5

Azcroococum

Rhizob.
meliloti

Solution of podsol,
garden soil

 

 

 

 

 

 

autoclaved

350

41

640

86

105

60

filtered

113

20

280

50

88

40

natural

52

20

100

20

5.0

0.1

 

 

 

 

 

 

 

Solution of podsol,
slightly cultivated
field soil

 

 

 

 

 

 

autoclaved

1.5

0.1

3.2

0

0

0.1

filtered

0.6

0

1.5

0

0

0

natural

0.2

0.

0.6

0

0

0

 

 

 

 

 

 

 

Solution of chernozem

 

 

 

 

 

 

autoclaved

120

900

170

1,200

450

600

filtered

50

600

100

800

200

500

natural

20.2

200

40

400

50

400

 

 

 

 

 

 

 

Solution of serozem

 

 

 

 

 

 

autoclaved

100

450

50

20

100

250

filtered

80

300

40

5

60

200

natural

20

50

5

0

20

200

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

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

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

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

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

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

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

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

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

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

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

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




HOME      AG LIBRARY      GO TO PART II, section 2