What is organic matter in the soil. The composition of the organic part of the soil. Organic soil components
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 residues- it organic matter, tissues of plants and animals, partially retaining their original shape and structure. It should be noted that the different chemical composition of the various residues should be noted.
Humic substances are all organic matter of the soil, with the exception of living organisms and their remnants, which have not lost their tissue structure. It is generally accepted to subdivide them into specific, proper humic substances and nonspecific organic substances of an individual nature.
Non-specific 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, nonspecific 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 litters.
The humic substances themselves are a specific system of high-molecular nitrogen-containing organic compounds of cyclic structure and acidic nature. According to many researchers, the structure of the molecule of the humus compound is complex. It was found that the main components of the molecule are the core, side (peripheral) chains and functional groups.
The nucleus is believed to be aromatic and heterocyclic rings consisting of five- and six-membered compounds of the type:
benzene furan pyrrole naphthalene indole
Side chains extend from the nucleus 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), phenol-hydroxyl (-OH), methoxyl (-CH3O) and alcoholic hydroxyl. These functional groups define Chemical properties 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) humines, or, more precisely, a non-hydrolyzable residue.
Humic acids (HA)- 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, from HA solutions they are easily precipitated by the acids H +, Ca2 +, Fe3 +, A13 +. These are humic 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 substitution capacity depends on the nature of the cation, the pH of the medium, and other conditions. In a neutral reaction, only the hydrogen ions of the carboxyl groups are replaced. The absorption capacity due to this property of HA is from 250 to 560 mg-eq 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. When determined by various methods, the molecular weight of HA 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 in the soil in the form of gels, which are easily peptized under 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 readily soluble in water and can form colloidal and molecular solutions. The role of these compounds in soil is enormous. For example, humates Ca, Mg, Fe, and A1 are mostly slightly 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 in mineral acids. It is 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 readily soluble in water, acids and alkalis. The solutions are yellow or straw-yellow in color. Hence these compounds got their name: in Latin fulvus - yellow. Aqueous solutions of FA have a strongly acidic reaction of the medium (pH 2.5). The molecular weight of fulvic acids, determined by various methods, ranges from 100 to several hundred and even thousands of conventional mass units.
The fulvic acid molecule has a simpler structure in comparison with humic acids. The aromatic part of these compounds is less pronounced. Side (peripheral) chains predominate in the structure of the FA molecule. The active functional groups are carboxyl and phenol-hydroxyl groups, the hydrogen of which enters into exchange reactions. The exchange capacity of FA can reach 700-800 mEq 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 strongly acidic reaction and good solubility in water, actively destroy the mineral part of the soil. In this case, fulvic acid salts are formed, which are highly mobile 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) is a group of humic substances, which is a residue of alkali-insoluble organic soil compounds. This group consists of both the actual humic substances, for example, humins consist of humic acids, strongly bound to minerals, and from strongly bound individual substances and organic residues of varying degrees of decomposition with the mineral part of the soil.
Chapter 4. SOIL ORGANIC SUBSTANCE AND ITS COMPOSITION
§one. Sources of organic matter and its composition
The most important constituent part of the soil is organic matter, which is a complex combination of plant and animal residues at various stages of decomposition and specific soil organic matter called humus.
All components of the biocenosis that fall on or into the soil (dying microorganisms, mosses, lichens, animals, etc.) are considered a potential source of organic matter, but the main source of humus accumulation in soils is green plants, which are left annually in and on the soil. the surface has 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 matter entering the soil is very diverse and largely depends on the type of dead plants. Most of their mass is water (75 - 90%). The dry matter contains carbohydrates, proteins, fats, waxes, resins, lipids, tannins and other compounds. The overwhelming majority of these compounds are high molecular weight substances. The bulk of plant residues consists mainly of cellulose, hemicellulose, lignin and tannins, while the richest in them are tree species. Protein is found most of all in bacteria and legumes, and the smallest amount is found in wood.
In addition, organic residues always contain some ash elements. The bulk of ash is calcium, magnesium, silicon, potassium, sodium, phosphorus, sulfur, iron, aluminum, manganese, which form organomineral complexonates in humus. The content of silica (SiO 2) ranges from 10 to 70%, phosphorus - from 2 to 10% of the ash mass. The name of the ash elements is associated with 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 very small quantities in ash there are trace elements - boron, zinc, iodine, fluorine, molybdenum, cobalt, nickel, copper, etc. The highest ash content is in algae, cereals and legumes, the least ash is found in coniferous wood. The composition of organic matter can be represented as follows (Fig. 6).
§2. The transformation of organic matter in the 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, oxygen of air and water. In this process, the main and decisive role belongs to microorganisms that participate in all stages of humus formation, which is facilitated by the huge population of soils by microflora. Animals inhabiting the soil are also actively involved in the transformation of organic residues into humus. Insects and their larvae, earthworms grind and grind plant residues, mix them with the soil, swallow, process and throw the unused part in the form of excrement into the soil.
Dying off, all plant and animal organisms undergo decomposition processes to simpler compounds, the final stage of which is complete mineralization organic matter. The formed inorganic substances are used by plants as food elements. 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 latter die off, it is again subjected to the decomposition process. 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 high-molecular substances - humic substances. The set of complex biochemical and physicochemical processes of the 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 occur in the soil at the same time 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 ondensation theory developed by M.M. Kononova. In accordance with this theory, humus is formed 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 her, the leading role in the process of humification are reactions of slow biochemical oxidation of decomposition products, as a result of which a system of high molecular weight humic acids of variable elemental composition is formed. 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 splits 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: water-air and thermal regimes of the soil, its granulometric composition and physicochemical properties, the composition and nature of the intake of plant residues, the species composition and the intensity of the vital activity of microorganisms.
The transformation of residues occurs under aerobic or anaerobic conditions, depending on the water-air regime. V aerobic Under conditions with a sufficient amount of moisture in the soil, a favorable temperature and free access of O 2, the 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 full moisture capacity of the soil. But under the same conditions, the mineralization of both intermediate decomposition products and humic substances rapidly proceeds, therefore, relatively little humus accumulates in the soil, but many elements of ash and nitrogen nutrition of plants (in gray soils and other soils of the subtropics).
Under anaerobic conditions (with a constant excess of moisture, as well as at low temperatures, lack of O 2), humus formation processes proceed slowly with the participation of mainly 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 dies out, and organic remains turn into peat - a mass of weakly decomposed and undecomposed plant remains, partially retaining the anatomical structure. The most favorable for the accumulation of humus is a combination of aerobic and anaerobic conditions in the soil with alternating periods of drying and moisture. 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. As a result of specific hydrothermal conditions, northern podzolic soils are characterized by the lowest content of microorganisms with low species diversity and low activity. The consequence of this is the slow decomposition of plant residues and the accumulation of poorly decomposed peat. In the humid subtropics and tropics, there is an intensive development of microbiological activity and, in connection with this, an active mineralization of the remains. Comparison of humus reserves in different soils with different numbers of microorganisms in them indicates that both very weak and high soil biogenicity does not contribute to the accumulation of humus. The largest amount of humus accumulates in soils with an average content of microorganisms (chernozems).
The granulometric composition and physicochemical properties of the soil have an equally significant influence. In sandy and sandy loam, well-heated and aerated soils, decomposition of organic residues proceeds quickly, a significant part of them is mineralized, humic substances are few and they are poorly fixed on the surface of sand particles. In clay 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 required for microorganisms, the reaction of the environment in which humus is formed, and the conditions for the fixation of humic substances in the soil. So, 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 the soil is 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 influencing 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% of humus, gray dark forest - 4 - 9%, soddy-podzolic - 2 - 3%, dark chestnut, yellow soils - 4 - 5%, brown and gray-brown semi-desert - 1 - 2%. The reserves of organic matter in natural zones are also different. The largest reserves, according to I.V. Tyurin, have various subtypes of chernozems, peat bogs, gray forest, medium - dark chestnut, red soils, low - podzolic, sod-podzolic, typical sierozem. The arable soils of the Republic of Belarus contain humus: clayey- 65 t / ha, in loamy- 52 t / ha, in sandy loam - 47 t / ha, in sandy- 35 t / ha. The soils of the Republic of Belarus, depending on the humus content 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
Grouping of soils of the Republic of Belarus by humus content
|
Soil groups |
% organic matter (based on soil weight) |
|
|
very low |
||
|
increased |
||
|
very high |
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 acidic nature. It is 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. The humates of monovalent metals are readily soluble in water and are washed out from the soil, while humates of divalent and trivalent metals do not dissolve in water and are well fixed in soils. Average molecular mass 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. The functional groups are represented by several carboxyl (–COOH) and phenol-hydroxyl 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 constitute the most valuable part of humus, they increase the absorption capacity of the soil, contribute to the accumulation of elements of soil fertility and the formation of a water-resistant structure.
Fulvic acids Is a group of humic acids that remain in solution after precipitation of humic acids. They are also high molecular weight organic nitrogen-containing acids, which, unlike humic acids, contain less carbon, but more oxygen and hydrogen. They are light in color (yellow, orange), readily soluble in water. Salts (fulvates) are also water soluble 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 the soil subzogenic process.
The ratio between humic acids and fulvic acids is not the same in different soils. Depending on this indicator (C HA: C FK), the following types of humus are distinguished: humate(> 1,5), humate-fulvate (1,5 – 1), fulvate-humate (1 – 0,5), fulvate (< 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.
Humin- this is a part of humic substances that do not dissolve in any solvent, 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. This is the inert part of the soil humus.
The specificity and composition of humus complexes serves as the basis for the classification of humus types. R.E. Muller proposed a classification of forest forms of humus as biological system interactions of organic matter, microbiota and vegetation. Three types of humus are distinguished among these complexes.
Soft humus - mul formed 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 litters and soils, has a slightly acidic reaction, evenly permeates the mineral part of the soil and is easily mineralized. In mule soils, litter hardly accumulates, since the incoming litter is vigorously decomposed by the microbiota. Humic acids prevail in the humus composition.
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 litter, a lack of bases and a high content of silica in the soil, has an acid reaction, is resistant to microorganisms, mineralizes slowly with the participation of fungi. As a result of the slow development of humification and mineralization processes in the 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 - modern develops in conditions of rather rapid mineralization of plant residues, where the functional activity of soil animals, crushing plant residues, plays a significant role, which greatly facilitates their subsequent decomposition by soil microflora.
§4. The value and balance of soil humus
The accumulation of humus is the result of the soil-forming process, at the same time the humic substances themselves have a great influence on the further direction of the process of soil formation and the properties of the soil. The functions of humus in the soil are very diverse:
1) the formation of a specific soil profile (with horizon A), the formation of a soil structure, an improvement in the water-physical properties of the soil, an increase in the absorption capacity and buffering 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 CO2 in the surface layer of the atmosphere and biologically active compounds in the soil, which directly stimulates the growth and plant development, mobilizes nutrients, affects the biological activity of the soil;
3) performs sanitary-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 gaining momentum. Like any balance, the humus balance includes items of input (input of organic residues and their humification) and consumption (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 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 (about 50 kg of humus is formed from 1 ton of manure), peat compost, sowing of perennial grasses, the use of green fertilizers (siderates), liming of acidic soils and gypsum of alkaline soils.
The humus state of soils is an important indicator of fertility and is determined by a system of indicators that include the level of content and reserves of organic matter, its profile distribution, enrichment in 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.
Soil organic matter 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 different 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 by its origin, character and function: organic residues and humus. The term "humus" is sometimes used as a synonym for the term "humus".
Organic residues represented mainly by ground and root litter of higher plants, which has not lost its anatomical structure. The chemical composition of plant residues of different cenoses varies widely. Common to them is the predominance of carbohydrates (cellulose, hemicellulose, pectin substances), lignin, proteins and lipids. All this complex complex of substances, after the death of living organisms, enters the soil and is transformed into mineral and humic substances, and is partially carried out from the soil with groundwater, possibly to the oil-bearing horizons.
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 that have many functional groups. The main source of enzymes are; plants. Fulfilling the role of catalysts in the soil, enzymes accelerate the processes of decomposition and synthesis of organic substances by a factor of millions.
Humus is a collection of all organic compounds found in the soil, except for those that make up living organisms and organic residues that have preserved the anatomical structure.
The humus contains nonspecific organic compounds and specific - humic substances.
Non-specific called a group of organic matter of a known nature and individual structure. They enter the soil from decaying plant and animal debris and from root secretions. Nonspecific 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: redox potential, the content of mobile forms of nutrients, the number 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- are a heterogeneous (heterogeneous) polydisperse system of high molecular weight nitrogen-containing aromatic compounds of acidic nature. Humic substances are formed as a result of a complex biophysicochemical process of transformation (humification) of decomposition products of organic residues entering the soil.
Depending on the chemical composition of plant residues, the factors of their decomposition (temperature, humidity, composition of microorganisms), there are two main types of humification: 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). Fulvic acids contain 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 fractions of a percent) and cations of various metals.
Fractions are isolated as part of the HA and FK groups. The fractional composition of humus characterizes the set and content of various substances included in the HA and FA groups, according to the forms of their compounds with the mineral components of the soil. Highest value for soil formation have the following fractions: 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 (Cg) to the carbon content of fulvic acids (Cfc). By the value of this ratio (C gc / C fc), 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 regularly and consistently changes in the zonal-genetic series of soils. In podzolic and soddy-podzolic soils, humic acids are almost not formed and little of them accumulate. The Cg / Cfc ratio is usually less than 1 and most often amounts to 0.3-0.6. In gray soils and chernozems, the absolute content and proportion of humic acids is much higher. The C g / C fc ratio in chernozems can reach 2.0-2.5. In soils located to the south of chernozems, the proportion of fulvic acids gradually increases again.
Excessive moisture, carbonate content of the rock, salinization leave an imprint on the group composition of humus. Additional hydration usually promotes the accumulation of humic acids. An increased humate content is also characteristic of soils that form 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
soils are subject to one general pattern: with depth, the proportion of humic acids decreases, the proportion of fulvic acids increases, the C g / C fc ratio decreases to 0.1-0.3.
The depth of humification, or the degree of transformation of plant residues into humic substances, as well as the Cg / Cfc ratio, depend on the rate (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 converted more easily, lignin, polysaccharides are more difficult) ...
In humus horizons of soils of a temperate climate, the type of humus and the depth of humification, expressed by the C h / C ph 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 supply of productive moisture is at least 1-2%. In the zonal series of soils, the Cg / Cfc 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 the 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, soddy-podzolic soils), regardless of the period of biological activity, have fulvate humus.
The humic substances of the soil are highly reactive and actively interact with the mineral matrix. Under the influence of organic matter, 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 condition of the soil.
Fulvic acids destroy soil minerals most actively. By interacting with sesquioxides (Fe 2 O 3 and Al 2 O 3), FAs form mobile aluminum and iron-humus complexes (fulvates of iron and aluminum). These complexes are associated with the formation of humus-illuvial soil horizons, in which they are deposited. Fulvates of alkaline and alkaline earth bases are readily soluble in water and easily migrate down the profile. An important feature of FCs is their inability to fix calcium. Therefore, liming of acidic soils has to be carried out regularly, after 3-4 years.
Humic acids, in contrast to FA, form poorly soluble organomineral compounds with calcium (calcium humates). Due to this, humus-accumulative horizons are formed in the soils. Soil humic substances bind ions of many potentially toxic metals - Al, Pb, Cd, Ni, Co, which reduces dangerous influence chemical pollution of soils.
The processes of humus formation in forest soils have their own characteristics. The overwhelming part of the plant litter in the forest goes to the soil surface, where special conditions are created for the decomposition of organic residues. On the one hand, this is the free access of oxygen and the outflow of moisture, on the other - a humid and cool climate, a high content of difficult-to-decompose compounds in the litter, a rapid loss due to the leaching of bases released during litter mineralization. Such conditions affect the life of soil animals and microflora, which plays an important role in the 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 organic matter of forest soils are formed: mull, moder, pestilence. The form of organic matter in forest soils is understood as the entire totality of organic matter contained both in the forest litter and in the humus horizon.
With the transition from moraine to moder and mull, 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 forest litter. In mull-type soil, the litter contains no more than 10% of the total supply of organic matter, and in the case of the pestilence type, the litter accounts for up to 40% of its total supply.
During the formation of organic matter such as pestilence, a thick three-layer litter is formed, which is well separated from the underlying mineral horizon (usually horizons E, EI, AY). Mostly fungal microflora takes part in the decomposition of the litter. Earthworms are absent, the reaction is highly acidic. Forest litter has the following structure:
O L - the top layer, about 1 cm thick, consisting of litter with preserved anatomical structure;
О F - middle layer of various thickness, consisting of semi-decomposed litter of light brown color, intertwined with fungal hyphae and plant roots;
Oh - the lower layer of strongly decomposed litter, dark brown almost black in color, smeared, with a noticeable admixture of mineral particles.
With the moder type, the forest litter usually consists of two layers. Under the layer of poorly decomposed litter, a well-decomposed humus layer with a thickness of about 1 cm is distinguished, gradually turning into a clearly pronounced humus horizon with a thickness of 7-10 cm. Insects, earthworms, play an important role in the decomposition of the litter. As part 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 litter reaction is weakly acidic. In forest soils with excessive moisture, the processes of decomposition of plant litter are inhibited and peat horizons are formed in them. The accumulation and rate of decomposition of organic matter in forest soils is influenced by the composition of the original plant residues. The more lignin, resins, tannins in the plant residues and the less nitrogen, the slower the decomposition process and the more organic residues accumulate in the litter.
Based on the determination of the composition of the plants, from the litter of which the litter was formed, a classification of forest litters 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 and marsh and wide grass.
Humus state of soils 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, 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, there is a regular increase in the reserves of humic acids from north to south.
The soils of the Arctic zone are characterized by a low content and small reserves of organic matter. The humification process takes place under extremely unfavorable conditions with low biochemical soil activity. The northern taiga soils are characterized by a short period (about 60 days) and a low level of biological activity, as well as a poor species composition microflora. 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 for a rather 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 sod process. The thickness of the accumulative horizon is small and is due to the depth of penetration of the bulk of the roots of herbaceous vegetation. The average humus content in the AY horizon in forest soddy-podzolic soils ranges from 2.9 to 4.8%. The humus reserves 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 soils. The 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 humus content 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. Swamp landscapes are located mainly in the forest zone and tundra, where precipitation significantly exceeds evaporation. Peat is especially high in the north of the taiga and in the forest-tundra. The most ancient peat deposits, as a rule, occupy lacustrine basins with sapropel deposits up to 12 thousand years old. The initial deposition of peat in such bogs took place about 9-10 thousand years ago. The most active peat began to be deposited in the period about 8-9 thousand years ago. Sometimes there are peat deposits about 11 thousand years old. The content of HA in peat ranges from 5 to 52%, increasing with the transition from high-moor peat to low-moor peat.
The variety of ecological functions of the soil is associated with the humus content. The humus layer forms a special energy shell of the planet, called humosphere... The energy accumulated in the humosphere is the basis for the existence and evolution of life on Earth. The 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 in soils accumulates up to 90-99% of all nitrogen, more than half of phosphorus and sulfur. In this form, potassium, calcium, magnesium, jelly - 30 and practically all microelements necessary for plants and microorganisms are accumulated and stored for a long time.
Transport function connected with the fact that humic substances can form with metal cations stable, but soluble and capable of geochemical migration complex organomineral compounds. Most of the trace elements, a significant part of the phosphorus and sulfur compounds, actively migrate 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 the humus horizons and, on this basis, their thermal regime. Humus soils are all and much warmer than soils containing little humic substances. Humic substances play an important role in the formation of the soil structure. They are involved in the regulation of the mineral nutrition of plants. Soil organic matter is used by its inhabitants as the main food source. Plants take about 50% of nitrogen from soil reserves.
Humic substances can dissolve many soil minerals, which leads to the mobilization of some mineral nutrients that are difficult for plants to access. The amount of properties of humic substances in soils determines the capacity of cation exchange, ion-salt and acid-base buffering of soils, redox regime. The physical, water-physical and physical-mechanical properties of soils are closely related to the content of humus by its group composition. Well-humified soils are better structured, the species composition of the microflora is more varied, and the number of invertebrates is higher. Such soils are more permeable to water, more easily amenable to mechanical processing, better retain elements of the food regime of plants, have a high absorption capacity and buffering capacity, and the efficiency of mineral fertilizers is higher in them.
Protective function is associated with the fact that humic substances of the soil protect or preserve the soil biota, vegetation cover in case of different kinds unfavorable extreme situations. Humified soils better resist drought or waterlogging, they are less susceptible to deflation erosion, and retain satisfactory properties longer when irrigated with increased doses or saline 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 consists in the fact 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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The soil is a complex system consisting of mineral and organic components. It serves as a substrate for plant development. For successful agriculture, it is necessary to know the characteristics and ways of soil formation - this helps to increase its fertility, that is, it is of great economic importance.
To the composition of the soil there are 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.
Soil mineral matter
By Chva consists of mineral components of different sizes: stones, crushed stone and "fine earth". It is customary to subdivide the latter into clay, silt, and sand in the order of particle enlargement. 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 affects drainage, nutrient content and soil temperature, in other words, soil structure from an agronomic point of view. Medium to fine textured soils such as clays, loams and silts are generally more suitable for plant growth because they contain sufficient nutrients and are better able to retain water and 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 clayey. The presence of stones, i.e. particles with a diameter greater than 2 mm, is important in terms of wear and tear on agricultural implements and impact on drainage. Typically, as the stone content of the soil increases, its ability to retain water decreases.
Soil organic matter
Organic matter, as a rule, constitutes 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 promotes the formation of soil aggregates, i.e., a fine crumbly 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 classified into detritus, or dead organic matter (MOB), and biota.
Humus(humus) is an organic material produced by 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 makes up the so-called absorbing 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 detritus feeders, together with the MOB, ingest large amounts of mineral particles. Moving between different layers of soil, worms constantly mix it. In addition, they leave passages that facilitate its aeration and drainage, thereby improving its structure and related properties. Earthworms feel best in a neutral and slightly acidic environment, rarely found at a pH below 4.5.
What is soil made of? It would seem like a simple question. We all know what it is. Every day we walk on it, plant plants in it that give us a harvest. We fertilize the earth, we dig it up. Sometimes you can hear that the land is barren. But what do we really know about soil? In most cases, only that it is the topmost layer earth surface... And this is not so much. Let's figure out what components the earth consists of, what it can be and how it is formed.
Soil composition
So, the soil is the top fertile. It consists of various components. In addition to solid particles, it includes water and air, and even living organisms. Actually, the latter play an important role in its formation. The degree of its fertility also depends on microorganisms. In general, the soil consists of phases: solid, liquid, gaseous and "living". Let's analyze what components form them.
Solids include various minerals and chemical elements. In includes almost the entire periodic table, but in different concentrations. The degree of soil fertility depends on the component of solid particles. Liquid components are also called soil solution. This is water in which chemical elements dissolve. There is liquid even in desert soils, but there are scanty amounts of it.
So what is soil made of besides these basic ingredients? The space between the solid particles is filled with gaseous components. Soil air consists of oxygen, nitrogen, carbon dioxide and thanks to it, various processes occur in the earth, for example, respiration of plant roots and decay. Living organisms - fungi, bacteria, invertebrates and algae - actively participate in the process of soil formation and significantly change its composition by introducing chemical elements.
Mechanical soil structure
What the soil consists of is now clear. But is its structure uniform? It's no secret that the soil is different. It can be sandy and clayey or stony. So, the soil consists of particles of different sizes. Its structure can include huge boulders and tiny grains of sand. Usually, the particles entering the soil are divided into several groups: clay, silt, sand, gravel. This is essential for agriculture. It is the structure of the soil that determines the degree of effort that must be made to cultivate it. It also depends on how well the earth will absorb moisture. Good soil contains equal percentages of sand and clay. This land is called loamy. If there is a little more sand, then the soil is crumbly and easy to process. But at the same time, such soil retains water and minerals worse. The clayey ground is damp and sticky. It drains poorly. But at the same time, it is in it that the most nutrients are contained.
The role of microorganisms in soil formation
The properties of the soil depend on what components the soil consists of. But not only this determines its qualities. From the dead remains of animals and plants, organic matter gets into the soil. This is due to microorganisms - saprophytes. They play an essential role in the decomposition processes. Due to their vigorous activity, the so-called humus accumulates in the soil. It is a dark brown substance. The humus contains fatty acid esters, phenolic compounds and carboxylic acids. In soil, particles of this substance stick together with clay. It turns out a single complex. Humus improves the quality of the earth. Its ability to retain moisture and minerals is increased. In a swampy area, the formation of a humus mass proceeds very slowly. Organic residues are gradually compressed into peat.
The process of soil formation
The soil forms very slowly. For a complete renewal of its mineral part to a depth of approximately 1 meter, it takes at least 10 thousand years. What the soil consists of is the products of the constant work of wind and water. So where does the soil come from?
First of all, these are particles of rocks. They serve as the basis of the soil. Under the influence of climatic factors, they are destroyed and crushed, settling on the ground. Gradually, this mineral part of the soil is colonized by microorganisms, which, processing organic remains, form humus in it. Invertebrates, constantly breaking through passages in it, loosen it, contributing to good aeration.
Over time, the structure of the soil changes, it becomes more fertile. Plants also influence this process. Growing up, they contribute to changing its microclimate. Human activities also affect the formation of soil. He cultivates and cultivates the land. And if the soil consists of infertile components, then a person fertilizes it, introducing both mineral and organic fertilizing.
by composition
In general, there is currently no generally accepted classification of soils. But nevertheless, it is customary to divide them into several groups according to their mechanical composition. This division is especially relevant in agriculture. So, the classification is based on how much the soil is made of clay:
Loose sandy (less than 5%);
Connected sandy (5-10%);
Sandy loam (11-20%);
Light loamy (21-30%);
Medium loamy (31-45%);
Heavy loamy (46-60%);
Clay (more than 60%).
What does the term "fertile" soil mean?
The part of the soil affects the degree of its fertility. But what makes the earth such? The composition of the soil directly depends on many factors. This is the climate, and the abundance of plants, and the presence of living organisms that live in it. All this affects the chemical. It depends on what components are contained in the soil, and the degree of its fertility depends. Mineral components such as calcium, nitrogen, copper, potassium, magnesium, phosphorus are considered very useful for high yields. These substances enter the ground during the decomposition of organic residues. If the soil is rich in mineral compounds, then it is fertile. Plants will bloom violently on it. This soil is ideal for growing vegetables and fruit crops.
