Microorganisms and their role in soil formation. General lightsDenia. Soil formation is a biological process, and the most diverse groups of living organisms are directly involved in its development. Among them, microorganisms that are widely distributed in nature are of great importance. They are found in soil, air, on high mountains, on bare stone rocks, in deserts, in the depths of the Arctic Ocean, etc.
Microbes are especially widespread in the soil, which is the only natural environment where all the necessary conditions exist for their normal development.
Good soil always contains a sufficient amount of organic and mineral substances, often has the necessary moisture and reaction of the soil solution, is adequately supplied with oxygen and protects microorganisms from the harmful effects of direct sunlight.
The development of microorganisms in the soil is closely related to organic matter. The richer the soil is in plant residues, the more microbes it contains (Table 4).
IN 1 G soddy-podzolic soils contain about 500 million bacteria, 1 G chestnut-1 -1.5 billion; in chernozems, characterized by a high content of organic matter, the number of microorganisms reaches 2-3 billion in 1 G soil, and in well-cultivated chernozems there are much more microorganisms.
Despite the negligibly small size of microbes, their total weight in soils reaches a significant value. So, if we take the average cell size equal to 1X2 microns and their number in 1 g of soil is 5 billion, then in a 25-cm layer 1 ha soil, the live weight of microbes will be about 1-3 tons.
Cultural, well-cultivated and manure-fertilized soils are especially rich in microorganisms.
All this mass of microbes in the soil layer is unevenly distributed. The surface horizons are richest in microorganisms up to a depth of 25-35 cm; as you go deeper, the number of microbes becomes less and less, and at a considerable depth they are found in negligible numbers. The root system of plants has a great influence on the distribution of microflora in the soil environment. The roots constantly release into the external environment various kinds of organic compounds that serve as a good source of nutrition for microorganisms; in the root zone of plants, there are usually favorable conditions for microorganisms. This zone is called the rhizosphere. In the rhizosphere, as shown by many studies, the number of microbes is tens and hundreds, and sometimes thousands of times greater than outside the root zone. Microbes cover the root system of plants in an almost continuous layer.
Abundant microflora in the rhizosphere, as well as in the entire soil layer, plays an important role in the development of soil fertility. Microorganisms can develop intensively only under certain temperature conditions, with appropriate humidity and environmental reaction.
The temperature regime is of great importance for their life activity.
Experiments show that the minimum temperature at which the vital activity of most soil microbes is still possible is approximately + 3°. Below this temperature, their development usually stops. The maximum temperature is about +45°. As for the optimum temperature, it is most often in the range of + 20-35 °.
The effect of temperature on the vital activity of microorganisms is closely related to humidity. Moisture is as much a necessary factor for the development of microbes as heat. If the temperature of the decomposing mass is quite favorable, but the humidity is insufficient or excessive, then decomposition will be difficult.
Similarly, decomposition will be difficult if moisture conditions are optimal but temperature conditions are unfavourable. Decomposition processes usually reach the highest intensity when the soil moisture is about 60% of the total moisture capacity. In accordance with this, the decomposition of plant residues in nature proceeds unevenly throughout the year.
The most vigorous decomposition takes place most often in the first half of summer, when thermal conditions and humidity are in the most favorable combination. In the hot summer months, when the soil dries out, the vital activity of microorganisms decreases and the decomposition process is minimized. Decomposition also slows down as the heat decreases in the autumn period, and with the onset of frost, this process completely stops.
With regard to the reaction of the environment, different groups of microorganisms in this respect have different requirements. So, all bacteria can develop only in a neutral, slightly acidic or slightly alkaline environment. The acidic reaction has a depressing effect on bacteria. The strongest obstacle to the vital activity of bacteria are also tannins contained in woody vegetation.
Mushrooms, on the contrary, freely put up with a pronounced acid reaction. Unlike bacteria, fungi thrive on plant debris containing tannins.
The vital activity of microorganisms is associated with the decomposition of dead plants and animals and their transformation into humus, or humus, the processes of mineralization of organic matter, the fixation of atmospheric nitrogen, the processes of ammonification, nitrification, denitrification, and the processes of synthesis of complex organic compounds.
Microorganisms are of great importance in the destruction and synthesis of minerals, as well as in the regulation of redox conditions in the soil.
The composition of the huge microscopic population of the soil includes bacteria, actinomycetes, fungi, algae, protozoa (protozoa) and various ultramicroscopic creatures - phages, bacteriophages and actinophages.
bacteria. Bacteria constitute the most abundant and diverse group of soil microflora; these are microscopic unicellular organisms that have a cell membrane, are rich in nucleoproteins and lack chlorophyll and plastids. Bacteria do not have a cell nucleus and reproduce by simple division. Bacteria are very small in size, usually no larger than a few microns. They have a different shape - round, rod-shaped and curved.
By type of nutrition, bacteria are divided into two groups - autotrophic and heterotrophic.
In relation to air, bacteria are divided into aerobic and anaerobic. Aerobic bacteria can develop only in conditions of free air access, anaerobic bacteria do not require molecular oxygen for respiration. Among the anaerobes there are conditional, facultative, bacteria that can develop both without oxygen and in its presence, and unconditional, obligate, microbes that can live and multiply only in the absence of free air access.
Autotrophic bacteria use only carbon from carbon dioxide for nutrition and do not need complex organic substances. To convert carbon dioxide into organic compounds of their body, they use either solar energy (photosynthesis) or the chemical energy of the oxidation of a number of mineral substances (chemosynthesis).
The category of bacteria with the ability of photosynthesis includes only colored, green and purple sulfur bacteria. The nutrition of microbes by means of chemosynthesis is much more widely developed in nature. The most common chemotrophic bacteria in the soil are nitrifying, iron bacteria, thionic and hydrogen bacteria.
Of great importance in soil formation are nitrifying bacteria, which are associated with the process of nitrification.
The process of nitrification, that is, the process of converting ammonia into salts of nitric acid, is carried out under the influence of two kinds of bacteria. One of them (Nitrosomonas, Nitrocystus, Nitrosospira) oxidize ammonia to nitrous acid: 2NH+3 O 2 =2 HNO 2 +2 H 2 O + 158 kcal. Other bacteria (Nitrobacter) continue the oxidation reaction, resulting in the formation of nitric acid: 2HNO 2 + O 2 = 2 HNO 3 + 48 kcal.
Nitric acid, meeting in the soil with various bases, immediately gives a number of nitrate salts: NaNO 3 , KNO 3 and Ca( NO 3 ) 2 . Nitric acid salts are the most convenient form of nitrogen nutrition for plants, so the nitrification process is of great industrial importance.
It should be noted that nitrification in soils proceeds with the joint, and not sequential, activity of the nitrifying microbes noted above, therefore, it is not possible to detect a significant content of nitrous acid salts in soils.
The nitrification process develops best in well-aerated soils with a neutral or alkaline reaction (pH 6.2 to 9) with a significant amount of humus and sufficient moisture content. Anaerobic conditions and an acidic environment are detrimental | for nitrifying bacteria.
Rational mechanical tillage, liming of acidic soils, fertilization are the most important measures that can be used to create the most favorable conditions for nitrification. Nitrification is an oxidative process, so aeration is a necessary condition for the intensive formation of nitrogen salts in the soil.
Sulfur bacteria, which include Thiobacillus thiooxydans, Thiobacillus thioparusand others, cause the process of sulfification, i.e., the oxidation of hydrogen sulfide to sulfuric acid. The sulfification process is carried out in two stages - the oxidation of hydrogen sulfide to sulfur and the oxidation of sulfur to sulfuric acid:
The sulfuric acid formed in this process, meeting in the soil with various bases, passes into salts of sulfuric acid, from which plants take sulfur for nutrition.
All sulfur bacteria are aerobes; therefore, conditions favorable for the nitrification process also contribute to the sulfification process. The looser the soil and the more favorable the gas exchange conditions in it, the more vigorously the transformation takes place. H 2 Sinto sulfuric acid. In poorly aerated, compacted, and airless soils, the sulfification process gives way to the so-called desulfurization process, in which a special kind of anaerobic bacteria reduce sulfuric acid salts back to H 2 S.
Iron bacteria are present in soils mainly as filamentous (Crenothrix, Leptothrix) and unicellular (Gallionella, siderocapsa) bacteria. The process of oxidation of iron ferrous salts into oxide salts is associated with the vital activity of iron bacteria:
Some iron bacteria are also capable of oxidizing manganese salts, thus forming ferromanganese nodules in the soil.
Heterotrophic bacteria absorb carbon from organic compounds, so they can develop only in the presence of organic matter. They are represented in soils by various physiological groups, which in their totality carry out the process of destruction of all organic compounds to the stage of their complete mineralization. The processes of ammonification, butyric acid fermentation, fermentation of pectin substances, cellulose, protein decomposition, denitrification and desulphurization are associated with the vital activity of heterotrophic bacteria.
This category of microorganisms also includes nitrogen-fixing bacteria, which play a huge role in the nitrogen cycle in nature. In relation to atmospheric oxygen, heterotrophs are divided into aerobic and anaerobic bacteria.
Ammonification, i.e., the process of decomposition of organic nitrogenous substances with the formation of ammonia, is caused by the vital activity of very diverse groups of microorganisms. Ammonia is released during the decomposition of proteins, peptones, amino acids, urea, uric and hypuric acids.
Typical representatives of ammonifying bacteria are Bact. vulgare, Bact. putidum, Bact. subtilis, Bact. mesentericus and Bact. mycoides.
The first step in protein breakdown is hydrolysis to form free amino acids; some of them are used by microbes to build the body, the other part can undergo further decomposition with the elimination of nitrogen in the form of ammonia.
Chemically, this process can be expressed as follows:
The process of protein ammonification can proceed both under aerobic and anaerobic conditions. The hydrolytic decomposition of urea proceeds mainly under aerobic conditions under the influence mainly of the following bacteria: Micrococcus ureae, Saroina ureae, Urobacterium pasteurii, Urobacillus miqueliiand etc.
Schematically, the process of ammonia fermentation of urea can be represented as follows:
The resulting ammonium carbonate, as a chemically fragile substance, then easily decomposes into carbon dioxide, water and ammonia:
Meeting in soil conditions with various acids, ammonia reacts with them and forms ammonium salts. So, for example, in the case of the interaction of ammonia with sulfuric acid, ammonium sulfate can be formed:
Nitrogen in the form of ammonia compounds is quite available for plant nutrition. Since the ammonification process is carried out by aerobic and anaerobic microorganisms, the formation of ammonia nitrogen can occur both in well-aerated soils and in compacted soils with difficult gas exchange.
It should be noted that the accumulation of ammonia in the soil and the further process of its oxidation or nitrification take place when the ratio of C to N in the decomposing material is less than 20:1; when the ratio of C to N is more than 20:1, all the ammonia formed is intercepted by microorganisms that decompose nitrogen-free organic substances and used by them to build their plasma protein. The presence in the soil of a large amount of undecomposed organic matter rich in carbohydrates (for example, straw) inhibits the accumulation of ammonia in the soil (LN Aleksandrova).
The breakdown of carbohydrates occurs under the influence of butyric acid bacteria. Clostridium pasteurianum, Clostridium butricumand etc.
Butyric fermentation is accompanied by the formation of butyric acid, carbon dioxide and hydrogen:
Cellulose fermentation is caused by the vital activity of specific cellulose-decomposing bacteria, typical representatives of which are Cytophaga hutchinsonii, You. omelianskii and etc.
The biochemical process of the breakdown of cellulose or fiber occurs both under aerobic and anaerobic conditions.
Fermentation of pectin substances, which are intercellular substances of plant tissues, proceeds under aerobic and anaerobic conditions under the influence of pectin-decomposing bacteria. Clostridium pectinovorumand etc.
The hydrolytic breakdown of fats occurs under the influence of microorganisms that have the enzyme lipase. The most energetic fat destroyers are Pseudomonas. fluorescens and Bact. pyocyaneum.
Very common microorganisms in the soil are denitrifying bacteria that cause the process of denitrification - the reduction of nitrates to free nitrogen.
The most vigorous denitrifiers are predominantly non-spore-bearing bacteria.Pseudomonas fluorescens, Bact. stutzeri, Bact. denitrificans and etc.
Denitrifying bacteria belong to facultative anaerobes, which, although they can develop in the presence of atmospheric oxygen, develop more intensively with difficult access to air or even in its complete absence. Not receiving air oxygen or receiving it in a limited amount, these bacteria take it away from nitrates and nitrites and oxidize nitrogen-free organic substances with it. Part of the nitrogen released in this process irrevocably evaporates into the atmosphere, while the other part goes to build the denitrifier plasma.
For agriculture, denitrification is in most cases harmful, since it is associated with the loss of nitrogen, the most important nutrient for plants. However, this process can develop intensively only in soils with poor air permeability, compacted and waterlogged. In cultivated and well-cultivated soils, the vital activity of denitrifying bacteria is strongly inhibited and their negative role is not manifested.
Bacteria that assimilate atmospheric nitrogen. Of great importance in the accumulation of nitrogen compounds in soils is the process of fixing, or binding, atmospheric nitrogen.
The essence of this process lies in the fact that a certain group of bacteria, the so-called nitrogen fixers, binds the free nitrogen of the atmosphere and, turning it into complex compounds of its body, thereby enriches the soil layer with it. Thus, along with the processes of decomposition of complex organic nitrogen substances in the soil, the processes of creation, or synthesis, of nitrogen compounds due to the free nitrogen of the atmosphere also occur.
Note that the reserves of nitrogen in the atmosphere are practically inexhaustible. A column of gaseous nitrogen weighing 8 tons hangs over each square meter of the earth's surface. Meanwhile, atmospheric nitrogen is completely inaccessible to higher plants directly; it can be used only after it has been previously bound by special nitrogen-fixing microorganisms.
There are two groups of nitrogen-fixing microbes in the soil. One of them, the so-called nodule bacteria (Bacterium radicicola), are able to develop only on the roots of various leguminous plants, while others live freely in the soil environment.
Of the free-living microbes, some are aerobic (Azotobacter chroococcum), others are anaerobic organisms (Clostridium pasteurianum).
Nodule bacteria are of the greatest importance in agriculture and from free-living - AzotobacterAs for bacteria of a different kind - Clostridium pasteurianum, then they, being anaerobic, are usually inhibited in cultivated, well-cultivated soils, as a result of which their role in the accumulation of nitrogen in the soil is relatively insignificant.
Nodule bacteria, which can live only in symbiosis with leguminous plants, are represented in soils by several species. Each species of nodule bacteria can develop only on one specific species or on several species of leguminous plants. Under favorable conditions, as observations show, the amount of nitrogen bound by nodule bacteria can reach 100 and even 120 kg per hectare per growing season.
What about free-living bacteria?Azotobacter), then the most necessary condition for their existence is the presence of humus substances in the soil as a source of carbon compounds, from which these organisms draw the energy they need.
The total amount of nitrogen that can be accumulated in the soil by Azotobacter during the summer reaches an average of 30-35 kg per hectare. These figures speak very eloquently of the enormous role that nitrogen-fixing bacteria play in soil fertility. The nitrogen accumulated in the bodies of microorganisms undergoes the same transformations in the soil as the nitrogen of other organic compounds. After the death of nitrogen-fixing bacteria, their bodies decompose under the influence of the processes of ammonification and nitrification, and the nitrogen enclosed in them passes into ammonium and then into nitrate compounds, which serve as food for plants.
Mushrooms. Along with bacteria, fungi, which are heterotrophic saprophytic organisms that feed on ready-made organic matter, take a large part in soil-forming processes.
The fungal microflora in soils is very diverse and is represented by a large number of species. The most common of these are fungi that reproduce either by the formation of conidia from conidiophores, or sporangia, on special thickened cells. Representatives of the genera belong to the group of mold fungi Penicillium, Trichoderma, Aspergillus, Cladosporium, Rhizopus.
Algae fungi are also widely distributed in soils (Phycomycetes), marsupials (Ascomycetes), including yeast fungi (Saccharomycetes), and then higher (Basidio– mycetes) and imperfect mushrooms (Fungi imperfecti).
Many species of fungi are able to form mycorrhiza on the roots of green plants, causing a special mycotrophic type of root nutrition of plants.
Mycorrhiza is usually called the cohabitation of many plants with special soil fungi, called mycorrhizal fungi. There are ectotrophic, or external, mycorrhiza and endotrophic, or internal, mycorrhiza; hyphae of the fungus of ectotrophic mycorrhiza spread mainly on the surface of the root, forming around it, as it were, a special sheath; hyphae of the fungus of endotrophic mycorrhiza penetrate the root, spreading in its tissues.
In this symbiosis, mycorrhiza fungi use carbohydrates, in particular sugar, as well as some hydroxy acids and amino acids that come from the leaves to the roots of plants, and at the same time supply green plants with nitrogen, since fungi are able to absorb nutrients, including nitrogen, directly from organic compounds of soil humus, forest litter and semi-decomposed peat residues.
Mycorrhizal fungi are the most widespread among woody plants, and each plant species is characterized by a specific type of fungus. Yes, mushroom. Boletus elegausgives mycorrhiza in larch and is found only where this tree grows; Boletus luteussettles on pine roots, etc.
All fungal microflora is characterized by a rather high need for oxygen, therefore, the surface layers of the soil are the richest in fungi. Most fungi develop at temperatures from 5 to 40°C, with an optimum around 25-30°C. An essential feature of mushrooms is that they develop well in both neutral and acidic environments, therefore, the decomposition of woody residues in the forest, which are acidic, occurs mainly under the influence of fungal microflora.
Various processes of decomposition of cellulose, fats, lignin, proteins and other organic compounds are associated with the vital activity of the fungal microflora in the soil. In the decomposition of fiber, fungi from the genera take the greatest part Trichoderma, Aspergillus, Fusariumother; from mushrooms that decompose pectin can be called Mucor stolonifer, Aspergillus niger, Cladosporiumother; many molds (Oidium lactis, different types Aspergillus and Penicillium) vigorously decompose fats,
Hydrocarbons with an open chain, as well as aromatic hydrocarbons, under the influence of a number of fungi, are oxidized to CO 2 and H 2 O; Many mold and imperfect fungi cause ammonification of proteins. Fungi play a particularly important role in the formation and decomposition of humus substances, which make up the most significant part of the soil.
Actinomycetes. Actinomycetes, or radiant fungi, are widely distributed in soils (Actynomycetes), which are a transitional form between bacteria and fungi (Table 5).
A characteristic feature of actinomycetes is a unicellular branched mycelium, which has two parts: one of them is immersed in a nutrient substrate, and the other rises up in the form of aerial mycelium, on which spores are formed. Colonies of actinomycetes are often pigmented and colored in pink, red, greenish, brown and black colors.
All actinomycetes are typical aerobes and develop best at a temperature of 30-35°C. Among them, antagonists are widely distributed, which inhibit the development of bacteria by isolating antibiotics.
The role of actinomycetes in soil-forming processes is very significant. They take an active part in the decomposition of nitrogen-free and nitrogenous organic substances, including the most persistent compounds that make up soil humus, or humus.
Seaweed. Algae occupy a significant place among the soil microflora. The most common flagellate algae found in soilFlagellatae), green algae (Chlorophyceae), blue-green (Cyanophyceae) and diatoms (diatomeae). On the soil surface, as well as in the arable layer with a depth of 30 cm the number of algae cells can reach 100 thousand in 1 G soil.
Algae are actively involved in the processes of weathering of rocks and minerals, such as kaolinite, decomposing it into free oxides of silicon and aluminum.
Being organisms containing chlorophyll, they are capable of photosynthesis and, during their development, enrich the soil layer with a certain amount of organic matter.
Blue-green algae (Nostoc, Phormidium) are capable of assimilating nitrogen gas. In this regard, they are of interest to agriculture. At the same time, the abundant development of algae enriches the soil with carbohydrates and stimulates the development of nitrogen-fixing bacteria such as Azotobacter in it.
Lichens. Along with bacteria, fungi and algae, lichens, which are complex symbiotic organisms consisting of a fungus and algae, take a significant part in soil-forming processes.
Lichens are able to grow directly on rocks and rocks, so they are usually the pioneers of plant life on exposed rock surfaces. The most common of them are scale, or crusty, then foliose and fruticose lichens. Most lichens have the ability to penetrate into the rock mass with the help of fungal hyphae and cause active destruction of all rocks that come to the surface. They belong to Rhizocarpon geographicum, different kinds Lecarona, Aspicilia, Halmatommaand others. Lichens of the genera Cladonia, Alectoriaand others in the tundra, in the forest zone and high mountain areas.
Developing on igneous, especially on silica-rich rocks, lichens form on their surface very characteristic, variegated covers of red, yellow, black, gray, brown and other colors.
Lichens emit carbon dioxide and specific lichen acids that cause the destruction of minerals; many lichens form antibiotics that inhibit the development of bacteria.
As a result of the vital activity of lichens, a thin layer of primitive soil is formed on the surface of rocks, in which a certain amount of humus, as well as phosphorus, potassium, sulfur and other elements accumulate. On this primitive soil rocky mosses settle, and later on some higher green plants.
Protozoa ( Protozoa). Representatives of the simplest animal organisms, which received the general name Protozoa. These include roots
( rhizopoda), flagella (Flagellata) and ciliary, or ciliates (Ciliata). Most protozoa are aerobes and only a few are anaerobes.
The most favorable temperature conditions for their development lie in the range of 18-22 °, the best reaction is neutral, however, a good development of protozoa is also observed with an acidic reaction. By way of nutrition, protozoa are mostly heterotrophs; they feed mainly on other organisms - bacteria, algae, as well as fungal germs and other microorganisms.
Among the protozoa there are saprophytic organisms, in particular flagellates and some ciliates, feeding on soluble organic substances. Among the flagella there are autotrophic protozoa. Some representatives of protozoa live in symbiosis with green algae. Protozoa are distributed mainly in the surface 15-cm soil layer. IN 1 G their soils number up to 1.5 million. The richer the soil is in organic matter, the more protozoa it contains, especially amoebae.
In the process of vital activity, protozoa transform complex organic compounds into simpler ones and thereby contribute to an increase in the supply of substances more accessible to higher plants in the soil. Often, in soils rich in amoebas, more soluble nitrogen compounds are found than in similar soils less populated by amoebas.
Animals and their role in soil formation. V soil lives a large number of invertebrates and vertebrates that are constantly and actively involved in soil-forming processes.
In this regard, representatives of invertebrates are primarily of importance - the larvae of various insects, ants, and especially earthworms, which, grinding organic residues and passing them along with mineral soil particles through the digestive apparatus, often produce very profound changes in chemical and physical properties. soils.
The importance in the soil-forming process of various kinds of animals inhabiting the soil is eloquently indicated, for example, by the fact that earthworms alone are able to pass several tons of soil mass annually through their bodies in area 1 ha. It follows from this that long before the cultivation of the soil with agricultural tools, it was continuously "plowed" by worms. These low organized creatures play an important role in the development of soils. In cultivated irrigated gray soils, according to the research of N. A. Dimo, earthworms are thrown annually to the surface 1 ha about 123 T processed soil.
Worm feces, or coprolites, are well-glued, water-resistant soil lumps enriched with microorganisms, organic matter, nitrogen, calcium and other elements. Thus, earthworms not only improve the physical properties of the soil - porosity, aeration, water permeability, but to a certain extent, its chemical composition.
Other animals also do significant work in this respect. Moles, mice, hamsters, ground squirrels and others, making various moves in the soil - molehills - and mixing organic substances with minerals, significantly increase the water and air permeability of the soil, which undoubtedly enhances and accelerates the decomposition of plant residues, and creates a kind of tubercular microrelief, very characteristic for steppe regions.
Thus, burrowing and digging animals constantly loosen, mix and move the soil, which, undoubtedly, most noticeably affects the intensification of the processes of decomposition of organic residues, as well as the weathering of its mineral part.
The idea of the participation of animals in the decomposition of organic matter will become even more complete if we take into account that vegetation serves as food for various herbivores and that, before getting into the soil, a significant part of the organic residues undergoes significant processing in the digestive organs of animals.
Green plants and their role in soil formation. Mainthe role in soil formation belongs to green plants, which, using solar energy, synthesize organic matter by assimilating carbon dioxide from air, water, nitrogen compounds and ash elements of the soil. The remains of dead plants entering the soil become food for microorganisms, which in the process of life synthesize soil humus and form mineral and organo-mineral compounds, which in turn serve as a food source for new generations of green plants.
The division of the soil profile into horizons is closely related to vegetation.
Due to the ability to release carbon dioxide and a number of organic acids by their roots, plants enhance the process of weathering of hardly soluble minerals and thereby contribute to the formation of easily mobile compounds in the soil layer.
Vegetation cover is also of great importance as a factor capable of changing climatic conditions in the smallest spaces and to a large extent hindering the development of erosion processes, i.e., soil erosion and blowing.
Thus, as a result of the vital activity of green vegetation on the continents of the globe, soils develop that contain humus, or humus, mineral and organomineral compounds.
Green plants are divided into woody and herbaceous.
Woody plants are perennial, their life expectancy is often measured in tens of years, and sometimes for many centuries.
A characteristic feature of woody plants is that only a part of the organic mass formed during the summer dies off in them every year. The other part, often more significant, remains in a living plant, being the material for the growth of the stem, branches and roots. Dead remains in the form of leaves, needles and branches are deposited mainly on the soil surface, forming a layer of forest litter. In the soil layer, trees leave a relatively small part of the dead organic matter, since their root system is perennial.
Herbaceous vegetation has a large network of thin, densely penetrating roots, after the death of which the soil mass is enriched with a significant amount of organic matter. In annual herbaceous plants, all vegetative organs usually exist for only one year, the plants die off completely every year, with the exception of only ripened seeds.
Dying plants deposit dead organic matter both on the soil surface and in its mass at various depths. Due to this, decomposition processes take place directly in the soil column, and the soil is annually enriched with humus and elements of ash and nitrogen food.
Mosses, which are widely found under the forest canopy and in swamps, play a peculiar role in soil formation. Mosses do not have a root system and assimilate nutrients with the entire surface of organs, attaching to the substrate with fibrous formations, or rhizoids.
Mosses have a huge moisture capacity. Where they settle, anaerobiosis is created, the processes of decomposition of organic residues slow down, and bogging and accumulation of peat begin.
The considered features inherent in one or another group of green plants directly affect the soil-forming process, and, consequently, the nature and quality of the resulting soils.
But no matter how different individual groups of green plants differ in one way or another, their main significance in soil formation always comes down to the synthesis of organic matter from mineral compounds. Organic matter, which plays an important role in soil fertility, can only be created by green plants.
The decomposition of organic residues of various plant formations is carried out by different microorganisms. In one case, this process is caused mainly by the vital activity of fungi, in the other - by bacteria.
Thus, wood residues in the forest decompose mainly with the dominant participation of mold fungi. Bacteria here develop somewhat weaker due to the fact that the wood pulp contains tannins and has an acidic reaction. Bacteria are usually included in the process of decomposition of wood residues after the fungi break down the tannins that retard the development of many groups of bacteria. Conditions are favorable for fungal decomposition in the forest, since elastic woody residues lie on the soil surface and the air flow to them is not limited.
An essential feature of the fungal decomposition of woody plant residues is that a significant amount of fulvic acids are formed here, which play an important role in the development of soddy-podzolic soils.
The organic remains of meadow herbaceous vegetation are decomposed mainly by anaerobic bacteria in the absence of aeration. Only in the upper parts of the soil, where oxygen penetrates, aerobic decomposition processes occur.
Anaerobic decomposition proceeds very slowly. This explains the fact that in meadows under herbaceous vegetation very often a rather powerful, entwined with roots, slightly decomposed turf is formed.
In the same way, under the action of anaerobic microorganisms, significant accumulations of peat are gradually formed in swamps and on swampy soils, which are widespread in the northern and central parts of our country.
Unlike meadows and swampy areas, all dead remains of steppe plants are decomposed for the most part by aerobic bacteria.
This is explained, firstly, by the fact that the steppe vegetation dies off in the summer, when the soil is most dry and well aerated; secondly, the herbaceous vegetation dying off in the summer in the steppe does not form a continuous dense felt, but usually lies in a loose layer, which also cannot serve as an obstacle to the penetration of oxygen into the soil.
The process of aerobic decomposition of all organic substances proceeds very quickly and completely; this explains the situation that from plants of the steppe formation, especially in the conditions of the dry steppe, after their death, large deposits of humus usually do not remain in the soil.
- A source-
Garkusha, I.F. Soil science / I.F. Garkusha. - L .: Publishing house of agricultural literature, magazines and posters, 1962. - 448 p.
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The rocks from which the soil is formed are called soil-forming, or parent.
Soil-forming rocks are characterized by their origin, composition, structure and properties. The soil-forming rock is the material basis of the soil and transfers to it its mechanical, mineralogical and chemical composition, as well as physical and chemical properties, which in the future gradually change to varying degrees under the influence of the soil-forming process.
The properties and composition of parent rocks affect the composition of the settling vegetation, its productivity, the rate of decomposition of organic residues, the quality of the resulting humus, the features of the interaction of organic substances with minerals, and other aspects of the soil-forming process.
The main soil-forming rocks are loose sedimentary.
Sedimentary rocks - deposits of weathering products of massively crystalline rocks or remains of various organisms. They are subdivided into detrital, chemical and biogenic sediments.
The most common sedimentary rocks include continental Quaternary deposits: glacial, water-glacial, loess and loess-like loams, eluvial, alluvial, deluvial, proluvial, eolian, lacustrine, marine are less common. They differ in the nature of composition, moisture capacity, water permeability, porosity, which determines the water-air and thermal regimes.
Biological factor of soil formation
The biological factor of soil formation is understood as the diverse participation of living organisms and their metabolic products in the soil formation process.
The most powerful factor influencing the direction of the soil-forming process are living organisms. The beginning of soil formation is always associated with the settlement of organisms on a mineral substrate. Representatives of all four kingdoms of living nature live in the soil - plants, animals, fungi, prokaryotes. Pioneers in the development and transformation of inert mineral matter in the soil are various types of microorganisms, lichens, algae. They do not yet create soil, they prepare biogenic fine earth - a substrate for the settlement of higher plants - the main producers of organic matter. It is they, the higher plants, as the main accumulators of matter and energy in the biosphere, that play the leading role in the processes of soil formation.
The role of woody and herbaceous, forest and steppe or meadow vegetation in the processes of soil formation is significantly different.
Under the forest, the litter, which is the main source of humus, comes mainly to the soil surface. To a lesser extent, the roots of woody vegetation are involved in humus formation.
In a coniferous forest, litter, due to the specifics of its chemical composition and high mechanical strength, is very slowly subjected to decomposition processes. Forest litter, together with coarse humus, forms a "mor"-type litter of one thickness or another. The process of decomposition in the litter is carried out mainly by fungi; humus has a fulvic character.
In mixed and, especially, in broad-leaved forests, deciduous litter is softer, contains a high amount of bases, and is rich in nitrogen. The process of mineralization of the annual litter is mainly carried out during the annual cycle. In forests of this type, the litter of herbaceous vegetation takes a large part in humus formation. The bases released during the mineralization of the litter neutralize the acidic products of soil formation, and more calcium-saturated humus of the humate-fulvate type is synthesized.
A different nature of the input of organic residues and chemical elements into the soil is observed under the canopy of herbaceous steppe or meadow vegetation. The main source of humus formation is the mass of dying root systems and, to a much lesser extent, the above-ground mass (steppe felt, plant seeds, etc.). This is explained by the fact that the root biomass of herbaceous vegetation (in contrast to woody vegetation) usually significantly predominates over aboveground biomass. The litter of herbaceous vegetation, in contrast to the litter of tree species, is characterized by a finer structure, lower mechanical strength, high ash content, and richness in nitrogen and bases.
The soil-forming process that occurs under the influence of herbaceous vegetation is called sod process.
Along with higher vegetation, soil formation processes are greatly influenced by numerous representatives of the soil fauna - invertebrates and vertebrates, inhabiting various soil horizons and living on its surface.
The functions of invertebrates and vertebrates are important and varied; one of them is the destruction, grinding and eating of organic residues on the surface of the soil and inside it.
The second function of soil animals is expressed in the accumulation of nutrients in their bodies and mainly in the synthesis of nitrogen-containing protein compounds. After the completion of the life cycle of an animal, tissue decay occurs and the substances and energy accumulated in the bodies of animals return to the soil.
The activity of burrowing animals has a great influence on the movement of soil and soil masses, on the formation of a kind of micro- and nanorelief. In some cases, soil burrowing and emissions to the surface reach such proportions that it becomes necessary to introduce special definitions into the nomenclature of soils (for example, calcareous burrowed chernozem). The profile of such soils has a loose, cavernous structure; soil horizons are often displaced and transformed.
Thus, three groups of organisms participate in soil formation - green plants, microorganisms and animals that form complex biocenoses on land. At the same time, the functions of each of these groups as soil formers are different.
Green plants are the only primary source of organic matter in the soil, and their main function as soil formers should be considered the biological cycle of substances - the supply of nutrients and water from the soil, the synthesis of organic matter and its return to the soil after the end of the life cycle.
The main functions of microorganisms as soil formers are the decomposition of plant residues and soil humus to simple salts used by plants, participation in the formation of humic substances, in the destruction and neoformation of soil minerals.
The main functions of soil animals are loosening the soil and improving its physical and water properties, enriching the soil with humus and minerals.
Lecture course "Soil Science"
LECTURE 3. Soil properties and its structure
1. Morphological features of soils 34
1.1.Soil structure 34
1.2. Soil coloring 38
1.3. Granulometric composition of soils and its agronomic significance 40
2. Organic and organo-mineral substances in soils 43
2.1. Influence of soil formation conditions on humus formation 43
2.2 Composition of humus 44
2.3. Humus status of soils 48
Brief summary Lectures 3 49
1. Morphological features of soils
In the process of soil formation, the rock acquires a multilevel morphological organization. There are morphones of 1.2, 3, 4.5 orders. To isolate morphones, there is a system of morphological features of the soil.
Morphological features of the soil - a system of indicators that allows you to distinguish morphological elements from one another.
External morphological features include:
structure,
thickness of the profile and individual horizons,
grading,
structure,
addition,
neoplasms,
inclusions.
1.1. The structure of the soil
Any soil is a system of successively replacing each other vertically. genetic horizons- layers into which the original parent rock in the process of soil formation.
This vertical sequence of horizons is called soil profile.
A soil profile is a certain vertical sequence of genetic horizons within a soil individual, specific for each type of soil formation.
The soil profile represents the first level of the morphological organization of the soil as a natural body, the soil horizon is the second.
The soil profile characterizes the change in its properties along the vertical, associated with the influence of the soil-forming process on the parent rock. The main factors in the formation of a soil profile, i.e., differentiation of the original soil-forming rock into genetic horizons, are
these are, firstly, vertical flows of matter and energy (descending or ascending depending on the type of soil formation and its annual, seasonal or long-term cyclicity)
and, secondly, the vertical distribution of living matter (root systems of plants, microorganisms, soil-dwelling animals).
The structure of the soil profile, i.e., the nature and sequence of its constituent genetic horizons, is specific to each type of soil and serves as its main diagnostic characteristic. This means that all horizons in the profile are mutually connected and conditioned.
The soil horizon, in turn, is also not homogeneous and consists of morphological elements of the third level - morphones, which are understood as intra-horizon morphological elements.
At the fourth level of morphological organization, soil aggregates, into which the soil naturally breaks down within genetic horizons.
The next, fifth level of soil morphological organization can be detected only with the help of a microscope. This is the microstructure of the soil, studied in the framework of soil micromorphology.
Vegetation (higher and lower) creates a biological cycle of ash substances in nature and enriches the soil with organic residues. It is the main factor in soil formation.
The essence of the process of soil formation is manifested in nature through plant formations. Plant formations are combinations of higher and lower plants interacting under certain environmental conditions.
On the territory of Russia, the following groupings of plant formations are distinguished (N. N. Rozov): 1) woody (taiga forests, broad-leaved forests, forests of humid subtropics); transitional woody-herbaceous (xerophyte forests); herbaceous (dry and marshy meadows, temperate steppes, subtropical shrub steppes); 4) desert; 5) lichen-moss (tundra, raised bogs).
Each group of plant formations is characterized by its own characteristics.: the composition of organic substances, the characteristics of their entry into the soil and decomposition, as well as the interaction of decay products with the mineral part of the soil.
Differences in plant formations- the main reason for the diversity of soils in nature. Under the same conditions of the taiga-forest zone, podzolic soils develop under coniferous dense forests, and soddy soils form in meadows.
Depending on the biological characteristics in terms of the quantity and quality of the generated biomass, the impact on the soil formation process, green plants are divided into woody and herbaceous.
woody plants(trees, shrubs, semi-shrubs) - perennial, living tens and hundreds of years. Every year, only a part of the ground mass (needles, leaves, branches, fruits) dies off, and it is deposited on the soil surface in the form of litter or forest litter. Woody plants are characterized by the creation of a huge biomass, mainly terrestrial, but their annual litter is less than growth, and therefore a relatively small amount of ash elements and nitrogen returns to the soil with the litter. The litter of trees, especially conifers, contains a lot of fiber, lignin, tannins, and resins. The decomposition products of the forest litter interact with the soil in solution when the soil is washed out by precipitation.
Lifespan of herbaceous plants ranges from a few weeks (ephemera) to 1-2 years (cereals) and 3-5 years (legumes). However, roots and rhizomes live up to 7-15 years or more.
In soil formation processes, the effect of herbaceous plants is greater than that of woody ones, although the amount of biomass created by herbaceous associations is less. This is due to the short life span of herbaceous plants and the rapid turnover of all components involved by them in the biological cycle in the plant-soil system. The soil is annually enriched with organic remains of grasses in the form of ground mass (provided that it is not alienated) and roots. Root residues, in contrast to the ground mass, decompose directly in place, in the soil, and the products of their decomposition interact with its mineral part.
Remains of herbaceous plants compared to forest litter, they contain less fiber, more proteins, ash elements and nitrogen. Herbaceous residues are characterized by a neutral or slightly alkaline reaction.
mosses- plant organisms, devoid of a root system and assimilating nutrients throughout the surface of the organs. They are widely found under the forest canopy and in swamps. Mosses are attached to any substrate by rhizoids. They can absorb and retain a large amount of moisture, so the process of decomposition of plant residues proceeds slowly, with a gradual accumulation of peat and waterlogging. In the formation of raised bogs, the role of sphagnum (white) mosses should be especially noted.
Microorganisms. Of the microorganisms in the soil, bacteria, fungi, actinomycetes, algae and protozoa are widely represented. The largest number of microorganisms is found in its upper layers, where the bulk of organic matter and the roots of living plants are concentrated.
Microorganisms contribute to the decomposition of organic residues in the soil.
In relation to air, microorganisms are aerobic and anaerobic. Aerobic - these are organisms that consume oxygen in the process of life; anaerobes - live and develop in an oxygen-free environment. They receive the energy necessary for life as a result of coupled redox reactions. The decomposition and synthesis reactions taking place in the soil are influenced by various enzymes produced by microorganisms. Depending on the type of soil, the degree of their cultivation, the total number of microorganisms in 1 g of soddy-podzolic soils can reach 0.6-2.0 billion, chernozems - 2-3 billion.
bacteria- the most common type of soil microorganisms. According to the way they feed, they are divided into autotrophic, assimilating carbon from carbon dioxide, and heterotrophic, using carbon from organic compounds.
Aerobic bacteria oxidize various organic substances in the soil, including the process of ammonification - the decomposition of nitrogenous organic substances to ammonia, the oxidation of cellulose, lignin, etc.
Decomposition of organic residues heterotrophic anaerobic bacteria is called the fermentation process (fermentation of carbohydrates, pectins, etc.). Along with fermentation under anaerobic conditions, denitrification occurs - the reduction of nitrates to molecular nitrogen, which can lead to significant losses of nitrogen in soils with poor aeration.
green plants
Different groups of plants determine the unequal course of the biological cycle. Lower plants have a short lifespan and, therefore, determine the rapid circulation of elements in the biological cycle. . higher plants have a developed root system, providing a large area of contact of the organism with the soil. The cycle is carried out within one year in herbaceous vegetation and for several years (tens, hundreds, thousands) in woody vegetation. At the same time, different elements are not retained by plant organisms for the same time. In nature, a combination of the considered groups of plants is often observed. The following groups are distinguished:
lichen-moss formations occupy the tundra and swamps;
tree formations are taiga and broad-leaved forests, humid subtropical forests and tropical (rain) forests;
xerophytic forests belong to the group of transitional woody-herbaceous formations; this group of plants is typical for the forest-steppe and savanna;
the group of herbaceous formations includes dry and swampy meadows, prairies, steppes of the temperate zone, subtropical shrub steppes;
the desert formation is divided in turn into subboreal, subtropical, tropical.
Each formation is characterized by its own special composition and properties of organic matter, processes of decomposition of organic matter. The biomass of each plant formation also has its own differences, which is reflected in the composition of soil organic matter.
Seaweed distributed in all soils, in their surface layer. Diatoms, blue-green and green algae are common in the soil. Their number depends on soil moisture. They are all autotrophs. Synthesize organic matter through photosynthesis. Algae, when dying, enrich the soil with organic matter, which is easily decomposed by microorganisms. Participate in the processes of weathering of rocks.
Microorganisms participate in the transformation of organic residues, turning them either into humus, or destroying organic matter to final products, while complex organic compounds decompose to mineral salts available to vegetation . bacteria assimilate atmospheric nitrogen and supply it to higher plants, synthesize complex organic compounds, building their body from them. They participate in redox processes in the soil, changing the degree of oxidation of various organic and mineral compounds. Thus, almost all links of the soil-forming process are associated with the vital activity of microorganisms. All these processes are carried out by microorganisms with the help of enzymes.
Mushrooms are saprophytic heterotrophic organisms. It should be noted the great role of fungi, which develop better in soils with low pH values. These organisms have a wide range of hydrolytic enzymes, through which they decompose all types of organic substances. Among other things, they decompose compounds resistant to hydrolysis and oxidation such as lignin, phenols, quinones, aromatic hydrocarbons, waxes.
The role of worms in soil formation is great, as well as mammals living in the soil, making passages in the soil with a diameter of several millimeters to 4 to 12 cm, mixing the soil to different depths, mainly to a depth of 1 meter, releasing enzymes, organic acids, increasing soil biomass when dying.
The leading role in soil formation and the formation of soil fertility belongs to three
groups of living organisms - terrestrial plants, microorganisms and soil animals. Each of these groups
organisms fulfills its role, but only with their joint activity does the soil-forming rock turn into soil. The dominant position in soil formation belongs to green plants, which extract ash elements and nitrogen from the rock, synthesize organic matter during photosynthesis, which, together with ash elements, enters the soil through the litter. The role of different types of vegetation differs significantly, and this is the main reason for the diversity of soils in nature. Microorganisms (bacteria, fungi, algae and lichens) are the first to settle on the rock, actively participating in its biological weathering. They play the main role in the processes of decomposition of plant residues of green plants and their mineralization to simple salts available to plants. They participate in the processes of humification and mineralization of humus, in the destruction and soil formation of soil minerals, affect the composition of soil air, regulating the ratio between O 2 and CO 2 in it.
The number, species composition and activity of microorganisms depend on soil fertility and hydrothermal conditions. The most common bacteria in the soil, the number of which can reach up to 3 billion pieces. in 1 g of soil. Soil animals also participate in soil formation, represented by nematodes, insects, earthworms, ants, moles, rodents, etc. All of them use organic residues in the form of food, contribute to its decomposition, accelerate the humification of plant residues, and improve the physical properties of the soil. Invertebrates (nematodes, insects, worms, etc.) predominate among the soil fauna. A special role is played by earthworms, which pass through themselves up to 600 tons of fine earth per year. It has been established that many soils are 50, sometimes 89% composed of dilapidated aggregates created by worms.
Soil formation process- the process of soil formation, the essence of which is the interaction of organisms and their decay products with rocks and their weathering products.
Thus, the soil-forming process occurs at the contact between the lithosphere and the biosphere as a result of their interpenetration. Along with the lithosphere and biosphere, the source of substances involved in the soil-forming process is the atmosphere and hydrosphere. The main source of energy for the soil-forming process is solar energy, both direct and condensed in the remains of organisms, water seeping through the soil, etc. The soil-forming process is very complex, it includes a variety of chemical, physical and biological phenomena occurring simultaneously and in different directions . These phenomena can be grouped into 3 groups - decomposition, synthesis and movement. In the soil, there is a decay of plant and animal organisms, various minerals and fragments of rocks; it synthesizes special forms of organic matter (humus) and various secondary minerals (mainly clay minerals, oxide minerals and simple salts); products of decomposition and synthesis in the form of true and colloidal solutions, as well as suspensions, move down the profile, and in the case of a close occurrence of soil and groundwater, they move up with their capillary and film currents. These main groups of processes, in turn, are diverse.
