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Chapter 4. SOIL ORGANIC MATTER AND ITS COMPOSITION

§1. Sources of organic matter and its composition

The most important component of the soil is organic matter, which is a complex combination of plant and animal remains in various stages of decomposition, and specific soil organic substances called humus.

All components of the biocenosis that fall on or into the soil (dead microorganisms, mosses, lichens, animals, etc.) are considered a potential source of organic matter, but green plants, which are annually left in the soil and on it, are the main source of humus accumulation in soils. surface a large amount of organic matter. The biological productivity of plants varies widely and ranges from 1–2 t/year of dry organic matter (tundra) to 30–35 t/year (humid subtropics).

Plant litter differs not only quantitatively, but also qualitatively (see Chapter 2). The chemical composition of organic substances entering the soil is very diverse and largely depends on the type of dead plants. Most of their mass is water (75 - 90%). The composition of the dry matter includes carbohydrates, proteins, fats, waxes, resins, lipids, tannins and other compounds. The vast majority of these compounds are macromolecular substances. The main part of plant residues consists mainly of cellulose, hemicellulose, lignin and tannins, while tree species are the richest in them. Protein is most found in bacteria and legumes, the smallest amount is found in wood.

In addition, organic residues always contain some amount of ash elements. The bulk of the ash is calcium, magnesium, silicon, potassium, sodium, phosphorus, sulfur, iron, aluminum, manganese, which form organomineral complexonates in the composition of humus. The content of silica (SiO 2) ranges from 10 to 70%, phosphorus - from 2 to 10% of the mass of ash. The name of the ash elements is due to the fact that when plants are burned, they remain in the ash, and do not volatilize, as happens with carbon, hydrogen, oxygen and nitrogen.

In a very small amount, microelements are found in the ash - boron, zinc, iodine, fluorine, molybdenum, cobalt, nickel, copper, etc. Algae, cereals and legumes have the highest ash content, the least ash is found in coniferous wood. The composition of organic matter can be represented as follows (Fig. 6).

§2. Transformation of organic matter in soil

The transformation of organic residues into humus is a complex biochemical process that takes place in the soil with the direct participation of microorganisms, animals, air oxygen and water. In this process, the main and decisive role belongs to microorganisms that are involved in all stages of humus formation, which is facilitated by the huge population of soils with microflora. Animals inhabiting the soil are also actively involved in the transformation of organic residues into humus. Insects and their larvae, earthworms crush and grind plant residues, mix them with the soil, swallow, process and discard the unused part in the form of excrement into the soil.

When dying, all plant and animal organisms undergo decomposition processes to simpler compounds, the final stage of which is complete mineralization organic matter. The resulting inorganic substances are used by plants as nutrients. The rate of decomposition and mineralization of various compounds is not the same. Soluble sugars and starch are intensively mineralized; proteins, hemicelluloses and cellulose decompose quite well; resistant - lignin, resins, waxes. Another part of the decomposition products is consumed by the microorganisms themselves (heterotrophic) for the synthesis of secondary proteins, fats, carbohydrates, which form the plasma of new generations of microorganisms, and after the death of the latter, it is again subjected to the process of decomposition. The process of temporary retention of organic matter in a microbial cell is called microbial synthesis. Some of the decomposition products are converted into specific complex macromolecular substances - humic substances. The set of complex biochemical and physico-chemical processes of transformation of organic matter, as a result of which a specific organic matter of the soil, humus, is formed, is called humification. All three processes take place in the soil simultaneously and are interconnected with each other. The transformation of organic matter occurs with the participation of enzymes secreted by microorganisms, plant roots, under the influence of which biochemical reactions of hydrolysis, oxidation, reduction, fermentation, etc. are carried out. and humus is formed.

There are several theories of humus formation. The first in 1952 appeared to condensation theory developed by M.M.Kononova. In accordance with this theory, the formation of humus proceeds as a gradual process of polycondensation (polymerization) of intermediate decomposition products of organic substances (fulvic acids are formed first, and humic acids are formed from them). Concept biochemical oxidation developed by L.N. Alexandrova in the 70s of the XX century. According to it, the reactions of slow biochemical oxidation of decomposition products, which result in the formation of a system of high molecular weight humic acids of variable elemental composition, have a leading role in the process of humification. Humic acids interact with the ash elements of plant residues released during the mineralization of the latter, as well as with the mineral part of the soil, forming various organo-mineral derivatives of humic acids. In this case, a single system of acids is split into a number of fractions that differ in the degree of solubility and the structure of the molecule. The less dispersed part, which forms water-insoluble salts with calcium and sesquioxides, is formed as a group of humic acids. A more dispersed fraction, giving predominantly soluble salts, forms a group of fulvic acids. Biological the concepts of humus formation suggest that humic substances are products of the synthesis of various microorganisms. This point of view was expressed by V.R. Williams, it was developed in the works of F.Yu. Geltser, S.P. Lyakh, D.G. Zvyagintsev and others.

In various natural conditions character and speed humus formation is not the same and depends on the interrelated conditions of soil formation: the water-air and thermal regimes of the soil, its granulometric composition and physicochemical properties, the composition and nature of the supply of plant residues, the species composition and intensity of vital activity of microorganisms.

The transformation of residues occurs under aerobic or anaerobic conditions, depending on the water-air regime. AT aerobic conditions with a sufficient amount of moisture in the soil, a favorable temperature and free access to O 2, the process of decomposition of organic residues develops intensively with the participation of aerobic microorganisms. The most optimal conditions are a temperature of 25 - 30 ° C and humidity - 60% of the total moisture capacity of the soil. But under the same conditions, mineralization of both intermediate decomposition products and humic substances proceeds rapidly, therefore, relatively little humus accumulates in the soil, but many elements of ash and nitrogen nutrition of plants (in gray soils and other subtropical soils).

Under anaerobic conditions (with a constant excess of moisture, as well as at low temperatures, lack of O 2), the processes of humus formation proceed slowly with the participation, mainly, of anaerobic microorganisms. In this case, many low molecular weight organic acids and reduced gaseous products (CH 4 , H 2 S) are formed, which inhibit the vital activity of microorganisms. The decomposition process gradually fades, and the organic remains turn into peat - a mass of weakly decomposed and undecomposed plant remains, partially retaining the anatomical structure. The combination of aerobic and anaerobic conditions in the soil with alternating periods of drying and moistening is most favorable for the accumulation of humus. This regime is typical for chernozems.

The species composition of soil microorganisms and the intensity of their vital activity also affect the formation of humus. Northern podzolic soils, as a result of specific hydrothermal conditions, are characterized by the lowest content of microorganisms with low species diversity and low vital activity. The consequence of this is the slow decomposition of plant residues and the accumulation of weakly decomposed peat. In the humid subtropics and tropics, an intensive development of microbiological activity and, in connection with this, an active mineralization of residues are noted. Comparison of humus reserves in different soils with different numbers of microorganisms in them indicates that both very low and high soil biogenicity does not contribute to the accumulation of humus. The greatest amount of humus accumulates in soils with an average content of microorganisms (chernozems).

The granulometric composition and physicochemical properties of the soil have no less significant influence. In sandy and sandy loamy, well-heated and aerated soils, the decomposition of organic residues proceeds rapidly, a significant part of them is mineralized, there are few humic substances and they are poorly fixed on the surface of sand particles. In clayey and loamy soils, the process of decomposition of organic residues under equal conditions is slower (due to lack of O 2), humic substances are fixed on the surface of mineral particles and accumulate in the soil.

The chemical and mineralogical composition of the soil determines the amount of nutrients necessary for microorganisms, the reaction of the environment in which humus is formed, and the conditions for fixing humic substances in the soil. Thus, soils saturated with calcium have a neutral reaction, which is favorable for the development of bacteria and the fixation of humic acids in the form of calcium humates insoluble in water, which enriches it with humus. In an acidic environment, when soils are saturated with hydrogen and aluminum, soluble fulvic acids are formed, which have increased mobility and lead to a large accumulation of humus. Clay minerals such as montmorillonite and vermiculite also contribute to the fixation of humus in the soil.

Due to the difference in the factors affecting the formation of humus, the quantity, quality and reserves of humus are not the same in different soils. Thus, the upper horizons of typical chernozems contain 10–14% humus, gray dark forest soils 4–9%, soddy-podzolic soils 2–3%, dark chestnut, yellow soils 4–5%, brown and gray-brown semidesert soils 1 - 2%. The reserves of organic matter in natural areas are also different. The largest reserves, according to I.V. Tyurin, have various subtypes of chernozems, peatlands, gray forest, medium - dark chestnut, red soils, low - podzolic, sod-podzolic, typical gray soils. The arable soils of the Republic of Belarus contain humus: clayey– 65 t/ha, in loamy– 52 t/ha, in sandy - 47 t/ha, in sandy– 35 t/ha. The soils of the Republic of Belarus, depending on the content of humus in the arable layer, are divided into 6 groups (Table 3). In the soils of other natural zones, there are gradations depending on the humus content.

Table 3

Soil grouping of the Republic of Belarus by humus content

In the Republic of Belarus, most of the land belongs to soils of groups II and III, about 20% - to soils of group IV (Fig. 7).

§3. Composition and classification of humus

Humus is a specific high-molecular nitrogen-containing organic substance of an acidic nature. It makes up the main part of the organic matter of the soil, which has completely lost the features of the anatomical structure of dead plant and animal organisms. Soil humus consists of specific humic substances, including humic acids (HA), fulvic acids (FA), and humin (see Fig. 6), which differ in solubility and extractability.

Humic acids- these are dark-colored high-molecular nitrogen-containing substances insoluble in water, mineral and organic acids. They dissolve well in alkalis with the formation of colloidal solutions of dark cherry or brown-black color.

When interacting with metal cations, humic acids form salts - humates. Humates of monovalent metals are highly soluble in water and are washed out of the soil, while humates of divalent and trivalent metals do not dissolve in water and are well fixed in soils. The average molecular weight of humic acids is 1400. They contain C - 52 - 62%, H - 2.8 - 6.6%, O - 31 - 40%, N - 2 - 6% (by weight). The main components of the humic acid molecule are the core, side chains and peripheral functional groups. The core of humic substances consists of a number of aromatic cyclic rings. Side chains can be carbohydrate, amino acid and other chains. Functional groups are represented by several carboxyl (–COOH) and phenolhydroxyl groups, which play an important role in soil formation, as they determine the processes of interaction of humic acids with the mineral part of the soil. Humic acids are the most valuable part of humus, they increase the absorption capacity of the soil, contribute to the accumulation of soil fertility elements and the formation of a water-resistant structure.

Fulvic acids is a group of humic acids remaining in solution after precipitation of humic acids. These are also high-molecular organic nitrogen-containing acids, which, unlike humic acids, contain less carbon, but more oxygen and hydrogen. They have a light color (yellow, orange), are readily soluble in water. Salts (fulvates) are also soluble in water and weakly fixed in the soil. Fulvic acids have a strongly acidic reaction, vigorously destroy the mineral part of the soil, causing the development of soil podzoobrazovaniya process.

The ratio between humic acids and fulvic acids in different soils is not the same. Depending on this indicator (C HA: C FA), the following types of humus are distinguished: humate(> 1,5), humate-fulvate (1,5 – 1), fulvatno-humate (1 – 0,5), fulvic (< 0,5). Качество гумуса, плодородие почвы зависят от преобладания той или иной группы. К северу и к югу от черноземов содержание гуминовых кислот в почвах уменьшается. Относительно высокое содержание фульвокислот наблюдается в гумусе подзолистых почв и красноземов. Можно сказать, что условия, благоприятствующие накоплению гумуса в почвах, способствуют и накоплению устойчивой и наиболее агрономически ценной его части – гуминовых кислот. Соотношение С ГК: С ФК имеет наибольшее значение (1,5 – 2,5) в гумусе черноземов, снижаясь к северу и к югу от зоны этих почв. При интенсивном использовании пахотных земель без достаточного внесения органических удобрений наблюдается снижение как общего содержания гумуса (дегумификация), так и гуминовых кислот.

Gumin- this is a part of humic substances that do not dissolve in any solvent, are represented by a complex of organic substances (humic acids, fulvic acids and their organo-mineral derivatives), firmly associated with the mineral part of the soil. It is an inert part of soil humus.

The specificity and composition of humus complexes serve as the basis for the classification of humus types. R.E. Muller proposed a classification of forest forms of humus as a biological system of interaction between organic substances, microbiota and vegetation. Among these complexes, 3 types of humus are distinguished.

Soft humus - mul It forms in deciduous or mixed forests with intensive activity of soil fauna under favorable hydrothermal conditions and the presence of a sufficient amount of bases, primarily calcium, in litter and soils; Litter almost does not accumulate in mule soils, since the incoming litter is vigorously decomposed by the microbiota. The composition of humus is dominated by humic acids.

Coarse humus - pestilence, containing a large amount of semi-decomposed residues, is characteristic of coniferous forests, is formed with a low content of ash elements in the litter, a lack of bases and a high content of silica in the soil, has an acidic reaction, is resistant to microorganisms, and mineralizes slowly with the participation of fungi. As a result of the slow development of humification and mineralization processes in soils, a powerful litter peat-like horizon A 0 is formed, consisting of 3 layers: a) a layer of weakly decomposed organic matter (L), which is fresh litter, b) a semi-decomposed fermentation layer (F), c) a humified layer ( H).

Intermediate form - moder develops under conditions of fairly rapid mineralization of plant residues, where the functional activity of soil animals, which grind plant residues, plays a significant role, which greatly facilitates their subsequent decomposition by soil microflora.

§four. Importance and balance of soil humus

The accumulation of humus is the result of the soil formation process, while the humus substances themselves have a great influence on the further direction of the soil formation process and soil properties. The functions of humus in the soil are very diverse:

1) the formation of a specific soil profile (with horizon A), the formation of soil structure, the improvement of the water-physical properties of the soil, the increase in the absorption capacity and buffer capacity of soils;

2) a source of mineral nutrients for plants (N, P, K, Ca, Mg, S, trace elements), a source of organic nutrition for heterotrophic soil organisms, a source of CO 2 in the surface layer of the atmosphere and biologically active compounds in the soil, which directly stimulates growth and plant development, mobilizes nutrients, affects the biological activity of the soil;

3) performs sanitary and protective functions - accelerates the destruction of pesticides, fixes pollutants, reducing their entry into plants.

In connection with the diverse role of organic matter in soil fertility, the problem of the humus balance of arable soils is of current importance. Like any balance, the humus balance includes items of income (inflow of organic residues and their humification) and expenditure (mineralization and other losses). Under natural conditions, the older the soil, the more fertile: the balance is positive or zero, in arable soils it is more often negative. On average, arable soils lose about 1 t/ha of humus per year. To regulate the amount of humus, a systematic introduction of a sufficient amount of organic matter in the form of manure is used (from 1 ton of manure, ≈ 50 kg of humus is formed), peat composts, sowing of perennial grasses, the use of green fertilizers (green manure), liming acidic soils and alkaline gypsum.

The humus state of soils is an important indicator of fertility and is determined by a system of indicators, including the level of content and reserves of organic matter, its profile distribution, enrichment with nitrogen (C: N) and calcium, the degree of humification, types of humic acids and their ratio. Some of its parameters serve as an object of environmental monitoring.

organic part soil represented by living organisms (living phase, or biophase), undecomposed, organic residues and humic substances (Fig. 1)

Organic part of the soil

Rice. 1. Organic part of the soil

Living organisms have been discussed above. Now it is necessary to define organic residues.

organic remains- these are organic substances, tissues of plants and animals, partially retaining their original shape and structure. It should be noted the different chemical composition of various residues.

Humic substances are all organic substances of the soil, with the exception of living organisms and their remains, which have not lost their tissue structure. It is generally accepted to subdivide them into specific humic substances proper and nonspecific organic substances of an individual nature.

Nonspecific humic substances contain substances of an individual nature:

a) nitrogenous compounds, for example, simple and complex, proteins, amino acids, peptides, purine bases, pyrimidine bases; carbohydrates; monosaccharides, oligosaccharides, polysaccharides;

b) lignin;

c) lipids;

e) tannins;

f) organic acids;

g) alcohols;

h) aldehydes.

Thus, non-specific organic substances are individual organic compounds and intermediate decomposition products of organic residues. They make up approximately 10-15% of the total humus content of mineral soils and can reach 50-80% of the total mass of organic compounds in peat horizons and forest litter.

Actually humic substances represent a specific system of high-molecular nitrogen-containing organic compounds of cyclic structure and acidic nature. According to many researchers, the structure of the humus compound molecule is complex. It has been established that the main components of the molecule are the core, side (peripheral) chains and functional groups.

It is believed that the core is aromatic and heterocyclic rings, consisting of five- and six-membered compounds of the type:

benzene furan pyrrole naphthalene indole

Side chains extend from the core to the periphery of the molecule. They are represented in the molecule of humic compounds by amino acid, carbohydrate and other chains.

The composition of humic substances contains carboxyl (-COOH), phenolhydroxyl (-OH), methoxyl (-CH3O) and alcohol hydroxyl. These functional groups determine the chemical properties of humic substances. A characteristic feature of the system of humic substances proper is heterogeneity, i.e. the presence in it of components of various stages of humification. Three groups of substances are distinguished from this complex system:

a) humic acids;

b) fulvic acids;

c) humins, or, more precisely, non-hydrolysable residue.

Humic acids (HA)- a dark-colored group of humic substances, extracted from the soil with alkaline solutions and precipitated with mineral acids at pH = 1-2. They are characterized by the following elemental composition: C content from 48 to 68%, H - 3.4-5.6%, N - 2.7-5.3%. These compounds are practically insoluble in water and mineral acids; they are easily precipitated from HA solutions by acids H+, Ca2+, Fe3+, A13+. These are humus compounds of an acidic nature, which is due to carboxyl and phenol hydroxyl functional groups. The hydrogen of these groups can be replaced by other cations. The ability to replace depends on the nature of the cation, the pH of the medium, and other conditions. In a neutral reaction, only hydrogen ions of carboxyl groups are replaced. The absorption capacity due to this property of HA is from 250 to 560 meq per 100 g of HA. With an alkaline reaction, the absorption capacity increases to 600-700 mg·eq/100 g of HA due to the ability to replace hydrogen ions of hydroxyl groups. The molecular weight of HA when determined by various methods varies from 400 to hundreds of thousands. In the HA molecule, the aromatic part is most clearly represented, the mass of which prevails over the mass of the side (peripheral) chains.

Humic acids do not have a crystalline structure; most of them are found in the soil in the form of gels, which are easily peptized by the action of alkalis and form molecular and colloidal solutions.

When HA interacts with metal ions, salts are formed, which are called humates. Humates NH4+, Na+, K+ are highly soluble in water and can form colloidal and molecular solutions. The role of these compounds in the soil is enormous. For example, Ca, Mg, Fe, and Al humates are basically poorly soluble, can form water-resistant gels, while passing into a stationary state (accumulation), and are also the basis for the formation of a water-resistant structure.

Fulvic acids (FA) - a specific group of humic substances, soluble in water and mineral acids. Characterized by the following chemical composition: C content from 40 to 52%; H - 5-4%, oxygen -40-48%, N - 2-6%. Fulvic acids, unlike HA, are highly soluble in water, acids and alkalis. Solutions are yellow or straw-yellow in color. From here these compounds got their name: in Latin fulvus - yellow. Aqueous solutions of FA are strongly acidic (pH 2.5). The molecular weight of fulvic acids, determined by various methods, ranges from 100 to several hundreds and even thousands of conventional mass units.

The molecule of fulvic acid has a simpler structure compared to humic acids. The aromatic part of these compounds is less pronounced. The structure of the FA molecule is dominated by side (peripheral) chains. Active functional groups are carboxyl and phenolhydroxyl groups, the hydrogen of which enters into exchange reactions. The exchange capacity of FA can reach 700-800 mg·eq per 100 g of fulvic acid preparations.

When interacting with the mineral part of the soil, fulvic acids form organo-mineral compounds with metal ions, as well as minerals. Fulvic acids, due to their strong acid reaction and good solubility in water, actively destroy the mineral part of the soil. In this case, salts of fulvic acids are formed, which have high mobility in the soil profile. Organo-mineral compounds of fulvic acids are actively involved in the migration of matter and energy in the soil profile, in the formation, for example, of individual genetic horizons.

Non-hydrolyzable residue (humins) - a group of humic substances, which is a residue of alkali-insoluble organic compounds in the soil. This group consists of both proper humic substances, for example, humins consist of humic acids, strongly associated with minerals, and of strongly associated individual substances and organic residues of varying degrees of decomposition with the mineral part of the soil.

Soil is a complex set of components that are in combination with each other. The composition of the soil includes:

• mineral elements.
• organic compounds.
• soil solutions.
• soil air.
• organo-mineral substances.
• soil microorganisms (biotic and abiotic).

To analyze the composition of the soil and determine its parameters, it is necessary to have the values ​​​​of the natural composition - depending on this, an assessment is made for the content of certain impurities.

Most of the inorganic (mineral) part of the soil is crystalline silica (quartz). It can be from 60 to 80 percent of the total number of mineral elements.

A fairly large number of inorganic components are occupied by such aluminosilicates as mica and feldspars. This also includes clay minerals of a secondary nature, for example, montmorillonites.

Montmorillonites are of great importance for the hygienic qualities of the soil due to the ability to absorb cations (including heavy metals) and thereby chemically disinfect the soil.

Also, the mineral part of soil components includes such chemical elements (mainly in the form of oxides) as:

• aluminum
• iron
• silicon
• potassium
• sodium
• magnesium
• calcium
• phosphorus

In addition, there are other components. Often they can be in the form of sulfuric, phosphoric, carbonic and hydrochloric salts.

Soil organic components

Most organic components are found in humus. These are, to one degree or another, complex organic compounds, having in their composition such elements as:

• carbon
• oxygen
• hydrogen
• phosphorus

A significant part of organic soil components is found dissolved in soil moisture.

As for the gas composition of the soil, it is air, with approximately the following percentage:

1) nitrogen - 60-78%

2) oxygen - 11-21%

3) carbon dioxide - 0.3-8%

Air and water determine such an indicator as soil porosity and can range from 27 to 90% of the total volume.

Determination of the granulometric composition of the soil

The granulometric (mechanical) composition of the soil is the ratio of soil particles of various sizes, regardless of their origin (chemical or mineralogical). These groups of particles are combined into fractions.

The granulometric composition of the soil is of decisive importance in assessing the level of fertility and other key soil indicators.

Depending on the dispersion, soil particles are divided into two main categories:

1) particles with a diameter of more than 0.001 mm.

2) particles with a diameter less than 0.001 mm.

The first group of particles originates from all kinds of mineral formations and rock fragments. The second category occurs during the weathering of clay minerals and organic components.

Factors affecting soil formation

When determining the composition of the soil, attention should be paid to soil-forming factors - they have a significant impact on the structure and composition of the soil.

It is customary to distinguish the following main soil-forming factors:

• the origin of the parent rock of the soil.
• soil age.
• surface topography of the soil.
• climatic conditions of soil formation.
• composition of soil microorganisms.
• human activities affecting the soil.

Clarke as a unit of measurement of the chemical composition of the soil

Clark is a conventional unit that determines the normal amount of a certain chemical element in ideal (uncontaminated) soil. For example, one kilogram of naturally pure soil should contain about 3.25% calcium - this is 1 clarke. The level of a chemical element of 3-4 clarks or more indicates that the soil is heavily contaminated with this element.

The soil is a complex system consisting of mineral and organic components. It serves as a substrate for plant development. For successful farming, it is necessary to know the features and ways of soil formation - this helps to increase its fertility, that is, it is of great economic importance.

The composition of the soil includes four main components:
1) mineral substance;
2) organic matter;
3) air;
4) water, which is more correctly called a soil solution, since certain substances are always dissolved in it.

Mineral matter of the soil

By soil consists of mineral components of different sizes: stones, crushed stone and "fine earth". The latter is usually subdivided in the order of coarsening of particles into clay, silt and sand. The mechanical composition of the soil is determined by the relative content of sand, silt and clay in it.

The mechanical composition of the soil strongly influences the drainage, nutrient content and temperature regime of the soil, in other words, the structure of the soil from an agronomic point of view. Medium- and fine-textured soils such as clays, loams and silts are usually more suitable for plant growth, as they contain sufficient nutrients and are better able to retain water with dissolved salts. Sandy soils drain faster and lose nutrients through leaching, but are beneficial for early harvests; in spring they dry out and warm up faster than clay ones. The presence of stones, i.e. particles with a diameter of more than 2 mm, is important from the point of view of wear of agricultural implements and the effect on drainage. Usually, as the content of stones in the soil increases, its ability to retain water decreases.

soil organic matter

organic matter, as a rule, makes up only a small volume fraction of the soil, but it is very important, since it determines many of its properties. It is the main source of such plant nutrients as phosphorus, nitrogen and sulfur; it contributes to the formation of soil aggregates, i.e., a finely cloddy structure, which is especially important for heavy soils, since as a result water permeability and aeration increase; it serves as food for microorganisms. Soil organic matter is divided into detritus or dead organic matter (MOB) and biota.

Humus(humus) is the organic material resulting from the incomplete decomposition of MOB. A significant part of it does not exist in a free form, but is associated with inorganic molecules, primarily with clay soil particles. Together with them, humus constitutes the so-called absorption complex of the soil, which is extremely important for almost all physical, chemical and biological processes occurring in it, in particular for the retention of water and nutrients.

Among soil organisms a special place is occupied by earthworms. These detritivores, together with MOB, ingest large amounts of mineral particles. Moving between different layers of soil, the worms constantly mix it. In addition, they leave passages that facilitate its aeration and drainage, thereby improving its structure and associated properties. Earthworms thrive best in a neutral and slightly acidic environment, rarely occurring at a pH below 4.5.

soil organic matter- this is a complex system of all organic substances present in the profile in a free state or in the form of organomineral compounds, excluding those that are part of living organisms.

The main source of soil organic matter is the remains of plants and animals at various stages of decomposition. The largest volume of biomass comes from fallen plant residues, the contribution of invertebrates and vertebrates and microorganisms is much less, but they play an important role in the enrichment of organic matter with nitrogen-containing components.

Soil organic matter is divided into two groups according to its origin, character and functions: organic residues and humus. As a synonym for the term "humus", the term "humus" is sometimes used.

organic remains are represented mainly by ground and root litter of higher plants, which has not lost its anatomical structure. The chemical composition of plant remains of different cenoses varies widely. Common to them is the predominance of carbohydrates (cellulose, hemicellulose, pectin), lignin, proteins and lipids. All this complex complex of substances, after the death of living organisms, enters the soil and transforms into mineral and humic substances, and is partially removed from the soil with groundwater, possibly to oil-bearing horizons.

The decomposition of organic soil residues includes mechanical and physical destruction, biological and biochemical transformation and chemical processes. Enzymes, soil invertebrates, bacteria and fungi play an important role in the decomposition of organic residues. Enzymes are structured proteins with many functional groups. The main source of enzymes are; plants. Acting as catalysts in the soil, enzymes accelerate the processes of decomposition and synthesis of organic substances millions of times.

Humus is a collection of all organic compounds found in the soil, except for those that are part of living organisms and organic residues that have retained the anatomical structure.

In the composition of humus, non-specific organic compounds and specific - humic substances are isolated.

Non-specific called a group of organic substances of known nature and individual structure. They enter the soil from decaying plant and animal remains and with root secretions. Non-specific compounds are represented by almost all components that make up animal and plant tissues and intravital secretions of macro- and microorganisms. These include lignin, cellulose, proteins, amino acids, monosaccharides, wax, and fatty acids.

In general, the share of nonspecific organic compounds does not exceed 20% of the total amount of soil humus. Nonspecific organic compounds are products of varying degrees of decomposition and humification of plant, animal and microbial material entering the soil. These compounds determine the dynamics of rapidly changing soil properties: the redox potential, the content of mobile forms of nutrients, the abundance and activity of soil microorganisms, and the composition of soil solutions. Humic substances, on the contrary, determine the stability over time of other soil properties: exchange capacity, water-physical properties, air regime and color.

Specific organic part of the soil - humic substances- represent a heterogeneous (heterogeneous) polydisperse system of high-molecular nitrogen-containing aromatic compounds of acidic nature. Humic substances are formed as a result of a complex biophysical and chemical process of transformation (humification) of the decomposition products of organic residues that enter the soil.

Depending on the chemical composition of plant residues, factors of their decomposition (temperature, humidity, composition of microorganisms), two main types of humification are distinguished: fulvate and humate. Each of them corresponds to a certain fractional-group composition of humus. The group composition of humus is understood as a set and content of various substances related in structure and properties of compounds. The most important groups are humic acids (HA) and fulvic acids (FA).

Humic acids contain 46 - 62% carbon (C), 3 - 6% nitrogen (N), 3-5% hydrogen (H) and 32-38% oxygen (O). In the composition of fulvic acids, there is more carbon - 45-50%, nitrogen - 3.0-4.5% and hydrogen - 3-5%. Humic and fulvic acids almost always contain sulfur (up to 1.2%), phosphorus (tens and hundreds of percent) and cations of various metals.

As part of the HA and FA groups, fractions are distinguished. The fractional composition of humus characterizes the set and content of various substances included in the groups of HA and FA, according to the forms of their compounds with the mineral components of the soil. The following fractions are of the greatest importance for soil formation: brown humic acids (BHA) associated with sesquioxides; black humic acids (CHA) associated with calcium; fractions I and Ia of fulvic acids associated with mobile forms of sesquioxides; HA and FA, strongly associated with sesquioxides and clay minerals.

The group composition of humus characterizes the quantitative ratio of humic acids and fulvic acids. A quantitative measure of the type of humus is the ratio of the carbon content of humic acids (C HA) to the carbon content of fulvic acids (C FA). According to the value of this ratio (С gk / С fk), four types of humus can be distinguished:

• - humate - more than 2;
• - fulvate-humate - 1-2;
• - humate-fulvate - 0.5-1.0;
• - fulvate - less than 0.5.

The group and fractional composition of humus naturally and consistently changes in the zonal genetic series of soils. In podzolic and soddy-podzolic soils, humic acids are almost not formed and little accumulates. The C gk / C fc ratio is usually less than 1 and most often is 0.3-0.6. In gray soils and chernozems, the absolute content and proportion of humic acids is much higher. The ratio С gk / С fk in chernozems can reach 2.0-2.5. In soils located south of chernozems, the proportion of fulvic acids gradually increases again.

Excessive moisture, carbonate content of the rock, salinity leave an imprint on the group composition of humus. Supplemental hydration usually promotes the accumulation of humic acids. Increased humicity is also characteristic of soils formed on carbonate rocks or under the influence of hard groundwater.

The group and fractional compositions of humus also change along the soil profile. The fractional composition of humus in different horizons depends on the mineralization of the soil solution and the pH value. Profile changes in the group composition of humus in most

Soil soils are subject to one general pattern: the proportion of humic acids decreases with depth, the proportion of fulvic acids increases, the ratio of C ha / C fc decreases to 0.1-0.3.

The depth of humification, or the degree of conversion of plant residues into humic substances, as well as the ratio C GC / C FC depend on the speed (kinetics) and duration of the humification process. The kinetics of humification is determined by soil-chemical and climatic characteristics that stimulate or inhibit the activity of microorganisms (nutrients, temperature, pH, humidity), and the susceptibility of plant residues to transformation depending on the molecular structure of the substance (monosaccharides, proteins are more easily converted, lignin, polysaccharides are more difficult) .

In the humus horizons of temperate climate soils, the type of humus and the depth of humification, expressed by the C HA /C FA ratio, correlate with the duration of the period of biological activity.

The period of biological activity is a period of time during which favorable conditions are created for the normal vegetation of plants, active microbiological activity. The duration of the period of biological activity is determined by the duration of the period during which the air temperature steadily exceeds 10 ° C, and the reserve of productive moisture is at least 1-2%. In the zonal series of soils, the C HA /C ph value, which characterizes the depth of humification, corresponds to the duration of the period of biological activity.

Simultaneous consideration of two factors - the period of biological activity and saturation of soils with bases, makes it possible to determine the areas of formation of various types of humus. Humate humus is formed only with a long period of biological activity and a high degree of soil saturation with bases. This combination of conditions is typical for chernozems. Strongly acidic soils (podzols, sod-podzolic soils), regardless of the period of biological activity, have fulvate humus.

Soil humic substances are highly reactive and actively interact with the mineral matrix. Under the influence of organic substances, unstable minerals of the parent rock are destroyed and chemical elements become more accessible to plants. In the process of organo-mineral interactions, soil aggregates are formed, which improves the structural state of the soil.

Fulvic acids most actively destroy soil minerals. Interacting with sesquioxides (Fe 2 O 3 and Al 2 O 3), FAs form mobile aluminum- and iron-humus complexes (iron and aluminum fulvates). These complexes are associated with the formation of illuvial-humus soil horizons, in which they are deposited. Fulvates of alkaline and alkaline earth bases are highly soluble in water and easily migrate down the profile. An important feature of FAs is their inability to fix calcium. Therefore, liming of acidic soils has to be carried out regularly, every 3-4 years.

Humic acids, in contrast to FA, form poorly soluble organomineral compounds (calcium humates) with calcium. Due to this, humus-accumulative horizons are formed in soils. Soil humic substances bind ions of many potentially toxic metals - Al, Pb, Cd, Ni, Co, which reduces the dangerous effect of chemical soil pollution.

The processes of humus formation in forest soils have their own characteristics. The vast majority of plant litter in the forest enters the soil surface, where special conditions are created for the decomposition of organic residues. On the one hand, it is the free access of oxygen and the outflow of moisture, on the other hand, a humid and cool climate, a high content of hardly decomposable compounds in the litter, a rapid loss due to washing out of the bases released during the mineralization of the litter. Such conditions affect the vital activity of soil animals and microflora, which plays an important role in the processes of transformation of organic residues: grinding, mixing with the mineral part of the soil, biochemical processing of organic compounds.

As a result of various combinations of all factors of decomposition of organic residues, three types (forms) of forest soil organic matter are formed: mulle, moder, mor. The form of organic matter in forest soils is understood as the totality of organic substances contained both in the forest litter and in the humus horizon.

When moving from mora to moder and mulle, the properties of soil organic matter change: acidity decreases, ash content increases, the degree of saturation with bases, nitrogen content, and the intensity of decomposition of the forest litter. In the soil with the mulle type, the litter contains no more than 10% of the total reserve of organic matter, while in the case of the mora type, the litter accounts for up to 40% of its total reserve.

During the formation of organic matter of the mora type, a thick three-layer litter is formed, which is well separated from the underlying mineral horizon (usually horizons E, EI, AY). In the decomposition of the litter, mainly fungal microflora takes part. Earthworms are absent, the reaction is strongly acidic. The forest floor has the following structure:

O L - upper layer about 1 cm thick, consisting of litter that has retained its anatomical structure;

О F - the middle layer of different thickness, consisting of semi-decomposed light brown litter, intertwined with fungal hyphae and plant roots;

Oh - the lower layer of highly decomposed litter, dark brown, almost black, smeared, with a noticeable admixture of mineral particles.

With the moder type, the forest floor usually consists of two layers. Under a layer of slightly decomposed litter, a well-decomposed humus layer with a thickness of about 1 cm is distinguished, gradually turning into a clearly defined humus horizon with a thickness of 7-10 cm. Earthworms play an important role in the decomposition of the litter. In the composition of the microflora, fungi predominate over bacteria. The organic matter of the humus layer is partially mixed with the mineral part of the soil. The reaction of the litter is slightly acidic. In forest soils with excessive moisture, the processes of decomposition of plant litter are inhibited and peat horizons are formed in them. The composition of initial plant residues influences the accumulation and rate of decomposition of organic matter in forest soils. The more lignin, resins, tannins in plant residues and the less nitrogen, the slower the decomposition process proceeds and the more organic residues accumulate in the litter.

Based on the determination of the composition of plants, from the litter of which the litter was formed, a classification of forest litter was proposed. According to N. N. Stepanov (1929), the following types of litter can be distinguished: coniferous, small-leaved, broad-leaved, lichen, green moss, moss, grass, moss, sphagnum, wet grass, grass marsh, and broad grass.

Soil humus status- this is a set of general reserves and properties of organic substances, created by the processes of their accumulation, transformation and migration in the soil profile and displayed in a set of external features. The system of indicators of the humus state includes the content and reserves of humus, its profile distribution, nitrogen enrichment, the degree of humification, and types of humic acids.

The levels of humus accumulation are in good agreement with the duration of the period of biological activity.

In the composition of organic carbon, a regular increase in the reserves of humic acids from north to south is traced.

The soils of the Arctic zone are characterized by low content and small reserves of organic matter. The process of humification takes place in extremely unfavorable conditions with low biochemical activity of soils. The soils of the northern taiga are characterized by a short period (about 60 days) and a low level of biological activity, as well as a poor species composition of microflora. The humification processes are slow. In the zonal soils of the northern taiga, a coarse-humus type of profile is formed. The humus-accumulative horizon in these soils is practically absent, the humus content under the litter is up to 1-2%.

In the subzone of soddy-podzolic soils of the southern taiga, the amount of solar radiation, moisture regime, vegetation cover, rich species composition of soil microflora and its higher biochemical activity over a fairly long period contribute to a deeper transformation of plant residues. One of the main features of the soils of the southern taiga subzone is the development of the soddy process. The thickness of the accumulative horizon is small and is due to the depth of penetration of the main mass of roots of herbaceous vegetation. The average content of humus in the AY horizon in forest soddy-podzolic soils ranges from 2.9 to 4.8%. The reserves of humus in these soils are small and, depending on the soil subtype and granulometric composition, range from 17 to 80 t/ha in a layer of 0-20 cm.

In the forest-steppe zone, humus reserves in the 0-20 cm layer range from 70 t/ha in gray soils to 129 t/ha in dark gray ones. Humus reserves in the chernozems of the forest-steppe zone in the 0-20 cm layer are up to 178 t/ha, and in the 0-100 cm layer - up to 488 t/ha. The content of humus in horizon A of chernozems reaches 7.2%, gradually decreasing with depth.

In the northern regions of the European part of Russia, a significant amount of organic matter is concentrated in peat soils. Bog landscapes are located mainly in the forest zone and tundra, where precipitation significantly exceeds evaporation. The peat content is especially high in the north of the taiga and in the forest-tundra. The oldest peat deposits, as a rule, occupy lake basins with sapropel deposits up to 12 thousand years old. The initial deposition of peat in such swamps occurred approximately 9-10 thousand years ago. The most active peat began to be deposited in the period about 8-9 thousand years ago. There are sometimes peat deposits about 11 thousand years old. The content of HA in peat ranges from 5 to 52%, increasing during the transition from high-moor peat to lowland peat.

The variety of ecological functions of the soil is associated with the content of humus. The humus layer forms a special energy shell of the planet, called humosphere. The energy accumulated in the humorosphere is the basis for the existence and evolution of life on Earth. Humosphere performs the following important functions: accumulative, transport, regulatory, protective, physiological.

Accumulative function characteristic of humic acids (HA). Its essence lies in the accumulation of the most important nutrients of living organisms in the composition of humic substances. In the form of amine substances, up to 90-99% of all nitrogen accumulates in soils, more than half of phosphorus and sulfur. In this form, potassium, calcium, magnesium, jelly - 30 and almost all the trace elements necessary for plants and microorganisms are accumulated and stored for a long time.

transport function due to the fact that humic substances can form stable, but soluble and capable of geochemical migration complex organomineral compounds with metal cations. Most microelements, a significant part of phosphorus and sulfur compounds migrate actively in this form.

Regulatory function due to the fact that humic substances are involved in the regulation of almost all the most important soil properties. They form the color of humus horizons and, on this basis, their thermal regime. Humic soils are generally much warmer than soils containing little humic substances. Humic substances play an important role in the formation of soil structure. They are involved in the regulation of mineral nutrition of plants. Soil organic matter is used by its inhabitants as the main source of food. Plants take about 50% of nitrogen from soil reserves.

Humic substances can dissolve many soil minerals, which leads to the mobilization of some mineral nutrition elements that are difficult for plants to access. The cation exchange capacity, ion-salt and acid-base buffer capacity of soils, and the redox regime depend on the number of properties of humic substances in soils. The physical, water-physical and physical-mechanical properties of soils are closely related to the content of humus by its group composition. Well-humused soils are better structured, their species composition of microflora is more diverse, and the number of invertebrates is higher. Such soils are more water-permeable, easier to mechanically work, better retain the elements of the nutritional regime of plants, have a high absorption capacity and buffering capacity, and the efficiency of mineral fertilizers is higher in them.

protective function due to the fact that soil humic substances protect or preserve soil biota, vegetation cover in the event of various kinds of adverse extreme situations. Humus soils are better resistant to drought or waterlogging, they are less susceptible to deflation erosion, and retain satisfactory properties longer when irrigated with high doses or mineralized waters.

Soils rich in humic substances withstand higher technogenic loads. Under equal conditions of soil contamination with heavy metals, their toxic effect on plants on chernozems is manifested to a lesser extent than on soddy podzolic soils. Humic substances quite strongly bind many radionuclides, pesticides, thereby preventing their entry into plants or other negative effects.

Physiological function is that humic acids and their salts can stimulate seed germination, activate plant respiration, and increase the productivity of cattle and poultry.

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