Soil Organic Matter

Last updated: June 21, 2025

Soil organic matter — the totality of organic substances present in soil in the form of humus, remains of animals and plants, representing a complex of complex chemical organic substances of biogenic origin.

Biological indicators of fertility of soil — quantity, composition and properties of organic matter in soil.

The reserve of soil organic matter is a key indicator of fertility.

Importance of Organic Matter

Soil organic matter accumulates reserves of carbon, nitrogen, potassium, phosphorus, microelements, promotes creation of optimal soil regimes and structure, prevents erosion processes, weakens effect of toxic substances. Organic substances regulate consumption of nutrients, preventing unproductive losses from leaching, formation of gaseous products and sparingly soluble mineral compounds, increase efficiency of mineral fertilizers. It is the only source of energy for vital activity of soil microflora and biota, participates in self-regulation of decomposition, promoting inactivation by clay minerals of enzymes released by microorganisms. Soils with high humus content are biologically more active: they have greater numbers of microorganisms, more diverse species composition, more intensively formed carbon dioxide CO₂, increased enzymatic activity.

Soil organic matter participates in soil formation, due to its ability to bind with mineral part of soil. The organo-mineral compounds formed thereby represent the soil absorption complex — the most important property of any soil. Forms of organo-mineral compounds can be complexes with metal cations, hydroxides, anions, silicates, etc. Soil mineral-organic complex compounds are an example of such forms.

A mandatory condition for stable agriculture is reproduction of organic matter, which means simultaneous reproduction of biological, agrophysical and agrochemical factors of fertility.

Humus is an excellent water absorber — 1 g of humus can absorb from 4 to 20 g of water available to plants. This property depends on presence of humic acids.

Humus content is an indicator of potential soil fertility and activity of all biological processes. Humus constitutes 85–90% of total soil organic matter. Temperature, air, nutrient regimes, water-physical properties, absorption capacity, soil buffering capacity, total and mobile reserves of nutrients and fertilizers, transformations and movement of nutrients depend on humus content, composition and properties. Mobile forms of humus, due to slow decomposition, participate in plant nutrition to a lesser extent than non-humified substances, but create a favorable environment for this process.

Organic matter content in arable layer of different soil types varies greatly — from very low (less than 1.0%) to high (more than 10%). Enrichment of soil with organic matter reduces losses of fertilizer nutrients as a result of migration processes, thereby also reducing negative impact on environment. Cyclic processes of synthesis and transformation of organic substances in soil are the basis of biogeochemical cycles of biophilic elements and play an important role in reproduction of soil fertility.

Soil organic matter serves as a source of plant nutrition. It contains 98–99% of nitrogen, 30–40% of phosphorus, 90% of sulfur of their total content in soil.
Humic acids, fulvic acids and other humic substances, as well as carbonic acid, gradually destroy silicates and aluminosilicates, transform calcium and magnesium carbonates, phosphates and other salts into a form available to plants.
Organic substances serve as a nutrient medium for microorganisms.
Humic acids in highly dispersed state, some organic acids, enzymes, antibiotics, vitamins are stimulators of plant growth, including under water and sand culture conditions.
Soil organic matter increases absorption capacity and buffering, improves agrophysical properties;
Soil organic matter is able to regulate water-physical properties of soils, transform excess amounts of mineral fertilizers, inactivate heavy metals, pesticides and their metabolites, delaying their entry into plant products, surface and subsurface waters.

Depletion of soils in organic matter leads to deterioration of water-physical, chemical and biological properties. Due to high absorption capacity, humus retains cations from migration along soil profile, enhances biological activity, absorbs toxic substances and heavy metals, preventing their entry into groundwater and plants.

Composition of Soil Organic Matter

Soil organic matter, or humus, is divided into two groups:

humic, or humus, substances of specific nature, resistant to decomposition, free and bound fulvic acids, humic acids and humin;
non-humified, or nonspecific, substances of plant and animal origin and intermediate products of their decomposition, for example, fiber, cellulose, proteins, peptides, starch and other polysaccharides, organic acids and amino acids, fats, waxes, resins, aldehydes, polyuronic acids, polyphenols, lignin, chitin, tannins, etc. This part of organic matter accounts for 10–15% of its total reserve in soil, decomposes easily and is a source of nutrients for plants.

Nonspecific substances also include substances that are part of plant and animal tissues, products of vital activity of organisms and plants. Nonspecific substances can be present in free and bound with mineral components of soil state. Many of these substances are a nutrient medium for microorganisms that transform them into soil humus.

The share of specific humic compounds is 85–90% of soil organic matter. They are dark-colored high-molecular compounds with complex chemical structure.

Humic compounds are divided into humic acids (humic and fulvic acids), prohumic substances — intermediate compounds of humin-like substances and humins. Given the complex structure of these substances, this division is very conditional.

Soil types differ in humus content, in amount and ratio of humic acids and fulvic acids. Thus, in sod-podzolic soils this ratio is 0.4–0.6, in chernozems — 1.0–1.5 and more. These differences determine higher mobility of organic matter and, consequently, nitrogen in sod-podzolic soils.

Humic Substances

Humus is divided into three groups:

humic acids;
fulvic acids;
humins.

Humic substances of soil can be in the form of calcium, magnesium, sodium humates; humates and mixed salts with aluminum and iron hydroxides; complex organomineral compounds with aluminum, iron, phosphorus and silicon. These compounds can be absorbed by clay minerals, especially strongly by minerals of montmorillonite type. With kaolinite or feldspars the bond is less strong. Formation of organomineral compounds in soil promotes fixation of humus.

Humic Acids

Humic acids — a fraction of high-molecular organic compounds extracted from soil by alkaline solutions. Dark-colored, contain nitrogen, do not dissolve in acids, soluble in solutions of pyrophosphates, oxalates, fluorides and ammonia with formation of soluble salts — humates. Acidification of alkaline solution of humic acids leads to precipitation in the form of humates.

Depending on concentration and soil type, humates can have color from cherry-brown to black.

Humic acids include: carbon — 46–62%, hydrogen — 2.8–5.8, oxygen — 31–39, nitrogen — 1.7–6%. In small amounts they contain sulfur (from 0.1 to 1.2%), phosphorus (less than 1%), aluminum, iron, silicon, some metals. The ratio of elements depends on soil type, chemical composition of decomposing residues and humification conditions. The highest carbon content in humic acids is noted in chernozems. Agricultural production practically does not affect their elemental composition.

The humic acid molecule is a core consisting of aromatic and heterocyclic rings (benzene, furan, pyridine, naphthalene, anthracene, indole, quinoline and others), connected to each other, and peripheral aliphatic chains, the ends of which often terminate in carboxyl (for this reason they are called acids) and hydroxyl/phenolic groups. The core of the molecule exhibits hydrophobic (water-repellent) properties, while peripheral chains are hydrophilic.

Humic acids include the following functional groups:

3–6 phenolic hydroxyl groups (-OH);
3–4 carboxyl (-COOH);
primary and secondary alcohol (-OH);
methoxyl (-O-CH₃);
carbonyl (-C=O).

These functional groups determine properties of humic acids and character of interaction with soil. Thus, carboxyl groups determine acidic and exchange properties.

Fulvic Acids

Fulvic acids — high-molecular nitrogen-containing organic compounds of acidic character. Soluble in water, acids, weak alkali solutions, sodium pyrophosphate, in aqueous ammonia solution, forming soluble salts — fulvates, as well as in many organic solvents. Colored in shades from straw-yellow to orange and brown. Total elemental composition of fulvic acids: carbon 36–52%, oxygen 42–52%, hydrogen 3–6%, nitrogen 2–6%. Contain smaller amounts of sulfur, phosphorus and some metals. Mobile and relatively easily move along soil profile.

Fulvic acids, having strongly acidic reaction, have a destructive effect on mineral part of soil, which depends on content of humic acids: the less humic acids, the stronger the effect of fulvic acids.

They have functional groups capable of exchange absorption of cations with formation of salts — fulvates.

Nitrogen of fulvic acids is more easily subjected to acid hydrolysis. Humic acids account for 15–30%, and fulvic acids — 20–40% of soil nitrogen.

Humins

Humins — organic compounds of humic nature, insoluble in water, acid solutions, alkalis or organic solvents. They are a complex of esters of humic and fulvic acids, chemically bound with clay minerals and mineral part, which determines high resistance to chemical and microbiological decomposition.

Humins cannot be sources of nutrients for plants, but due to complex structure including functional groups, they serve as a buffer for retaining nutrients in a form assimilable by plants. Nitrogen of humins constitutes 20–30% of total soil nitrogen. They influence capacity, buffering of soils, migration and transformation of nutrients.

Non-Humified Substances

Non-humified substances constitute 10–20% of total reserve of organic substances in soil and serve as a source of nutrients for plants and biota. Some of them stimulate or inhibit growth and development of living organisms, influence transformation of soil nutrients and fertilizers from forms unavailable to plants.

From 10 to 30% of non-humified substances participate in new formation of humus. Plant residues account for from 3–5 to 12–15 t/ha or for sod-podzolic soils — up to 10%, in chernozems — 2–3% of total reserves of organic matter. Mass of microorganisms in 20-cm soil layer is from 0.7 to 2.7 t/ha, reaching 5–7 t/ha, which is 1–3% of total reserves of organic matter.

Deficit of non-humified substances in soils affects nutrient regime of all organisms.

Properties of Humus

The most important property of humus is colloidal nature. Colloidal, surface-active properties of humus are due to cation-anion micelles, thanks to which high activity is manifested even at small thickness of adsorption layers. These properties, despite the small share of humus in solid phase of soil, except peat soils, are important in plant nutrition, transformation of fertilizers and soil fertility.

Sources of Organic Matter Input into Soil

Soil organic matter is formed under natural conditions during dying off of plants, microorganisms, soil animals and decomposition of products of their vital activity. Under arable land conditions, the main sources of organic matter are plant residues and organic fertilizers. The advantage of using plant residues is reduced costs for applying organic fertilizers, which, unlike plant residues entering annually, may be irregular and unevenly distributed.

Plant residues can be divided into:

stubble — plant residues remaining after harvesting cereal crops, including parts of stems, leaves;
leaf-stem — plant residues remaining after harvesting potato, perennial grasses, vegetables, root crops, including rhizomes, root collars, stems, etc.;
root — remains of plant roots remaining after harvest.

Input of organic matter with plant residues occurs throughout the growing season, since the process of dying off of individual plant parts occurs constantly, and especially after flowering and beginning of fruiting. Annually, due to stubble and root residues under cereal crops it is 0.3–0.5 t/ha, under row crops — 0.15–0.25 t/ha.

Depth of root systems of different crops and also their mass differently affect replenishment of soil with organic matter. By amount of organic matter entering with plant residues, main crops can also be divided into three groups:

The first group includes perennial leguminous and cereal grasses. They leave the greatest amount of root and stubble residues. Additionally, perennial leguminous crops, due to the property of nitrogen fixation, replenish soil nitrogen reserves.
The second group includes annual cereal and grain legume crops of continuous sowing. They leave significantly fewer plant residues. However, the amount of organic matter left within the group can vary greatly. For example, ryegrass and its mixtures with annual leguminous crops leave approximately the same amount of plant residues as perennial grasses. Winter crops surpass spring cereals and grain legumes in this indicator. After harvesting annual cereal and grain legume crops, 1.5–3.0 t/ha of organic matter remains.
Plant residues of root systems of plants of the second group contain more nitrogen and phosphorus, and in stubble — potassium. Such ratio of carbon and nitrogen makes it a more valuable source of organic matter than stem residues. Application of fertilizers promotes greater accumulation in plant residues of nitrogen and potassium, phosphorus (to a lesser extent).

The third group includes row crops, leaving behind the smallest amount of plant residues. Input of organic matter into soil is determined by amount of plant residues, and its quality and rate of transformation — by their chemical composition.

Moisture and nutrient supply determine power of development of root system and aboveground part. Under favorable conditions, aboveground part grows more than roots. As a result, ratio of masses of root system and aboveground part shifts toward the latter.

Content of nutrients is higher in perennial grasses than in other crops: nitrogen content in root residues of perennial leguminous grasses is 2.25–2.60%, phosphorus — 0.34–0.80, in aftercut residues — 1.82–2.65 and 0.30–0.71%, respectively; in legume-cereal grass mixtures it depends on composition of mixture and is 0.91–2.37% nitrogen and 0.25–1.06% phosphorus, in aftercut residues — 1.60–2.18 and 0.17–0.54%, respectively. Content of nitrogen and phosphorus of bluegrass grasses and their aftercut residues is significantly lower.

Chemical composition is influenced by — age of roots of perennial grasses: the older the plants, the lower the content of nitrogen and ash elements; as well as fertilization and soil-climatic conditions.

Plant residues of annual crops (except legumes) and cereals are significantly poorer in nutrients compared to perennial grasses.

Soil organic matter is formed under action of vital activity of plants, microorganisms and soil fauna. With sufficient air content in soil and optimal moisture, a rapid aerobic decomposition process occurs. Under conditions of insufficient air and excess moisture, anaerobic microbiological decomposition occurs. Optimal conditions for decomposition of organic substances develop in structural, loose, cultivated soils.

A positive balance is achieved with application of organic fertilizers and cultivation of perennial grasses. 1 t of straw accumulates up to 170–180 kg of humus, or about 100 kg of carbon. Perennial grasses at hay yield of 4–5 t/ha annually accumulate 800–900 kg/ha of humus, or 500–600 kg of bound carbon.

Reserve of Organic Matter in Soil

The greatest effect of positive influence of organic matter on crop yield is achieved by creating an optimal reserve of organic matter for each soil type. Optimal can be considered such amount of organic matter, composition and quality that allows obtaining planned yield with effective use of fertilizers and soil tillage technology.

For sod-podzolic soils, humus content at level of 2.5–4% can be considered optimal. Lower organic matter content leads to decrease in crop productivity, higher — does not create a cost-effective yield increase.

Humus reserve is one of the most important indicators of humus state of soil. Humus reserve depends on thickness of arable layer and its density. In soils of light particle-size composition, humus reserve is, as a rule, lower than in analogous heavier soils. However, due to thicker arable layer, reserve in light soils may exceed humus reserve in heavy soils.

According to studies of L. A. Grishina and D. S. Orlov, humus reserve in 20-cm arable layer is assessed as: very low — at reserve less than 50 t of humus per 1 ha, low — at 50–100, medium — at 100–150, high — at 150–200, very high — at more than 200 t/ha.

Amount of humus in soil varies and depends on a number of factors, soil type, natural-climatic conditions, crop rotation, farming system. The highest humus content is in upper soil layers; down the profile content of organic substances decreases.

Table. Humus content in main soil types (according to I. V. Tyurin).

Soil	Humus content in arable layer, %	Humus reserves, t/ha
Soil	Humus content in arable layer, %	layer 0–20 cm (average)	layer 0–120 cm
Sod-podzolic	2–4	53	80–120
Gray forest podzolized	4–6	109	150–300
Chernozems:	col	col	col
- leached	7–8	192	500–600
- thick	10–12	224	650–800
- ordinary	6–8	137	400–500
- southern	4–5	—	300–350
Dark chestnut	3–4	99	200–250
Chestnut and light chestnut	1.5–3	—	100–200
Serozem	1–2	37	50
Krasnozem	5–7	153	150–300

Transformation of Organic Matter in Soil

Soil organic matter undergoes transformation in soil, which is divided into several stages. At the first stage, individual chemical compounds of the dead plant interact with each other. For example, protein compounds of plant cells can interact with aromatic substances of cell walls. This process can be accelerated by biological and mineral catalysts.

At the second stage, absorption and mechanical mixing by soil fauna of plant residues with soil occurs. Probably, at this stage biochemical reactions of transformation of primary organic matter also occur.

At the third stage, mineralization of organic matter occurs under action of microbial processes. First of all, water-soluble compounds, starch, pectin and protein substances are mineralized. The rate of mineralization of cellulose and lignin formed from it is significantly lower.

Final products of transformation of organic matter are carbon dioxide, water, nitrates, phosphates, etc. Products are also low-molecular organic acids: formic, acetic, oxalic, etc. Mineralization processes proceed with release of heat: upon decomposition of 1 g of dry matter, 17–21 J of energy is released.

Part of decomposition products of organic matter is used by new generations of plants, as well as by heterotrophic microorganisms for synthesis of proteins, fats, carbohydrates. Another part is transformed into humic substances.

Humification — the process of transformation of soil organic matter into humic compounds. Humification begins at the first stage of transformation of soil organic matter.

M. M. Kononova presented the following generally accepted mechanism of formation of specific humic substances:

The initial substances for synthesis of humic substances are any components of decomposition of plant tissues and products of metabolism of microorganisms.
Initial substances condense (combine) by oxidation of phenols under action of phenol oxidases to quinones through the stage of formation of intermediate compounds semiquinones. Quinones react with amino acids and peptides.
Polycondensation (polymerization) of compounds formed at the second stage leads to formation of humic substances.

This mechanism is not the only one. L. N. Aleksandrova, developing the assumption of academician I. V. Tyurin, proposes another mechanism of humification, consisting in slow biochemical oxidation of high-molecular cyclic compounds with their subsequent polycondensation.

The rate and character of transformation are influenced by external environmental conditions: moisture, soil acidity, temperature, availability of air and nutrients, as well as chemical composition of plant residues. The ratio of carbon and nitrogen in plant residues influences decomposition rates. Rapid decomposition is ensured by a narrow C : N ratio, which is characteristic, for example, of plant residues of clover; this process proceeds less intensively with plant residues of cereal crops and vetch-oat mixture. Organic and mineral fertilizers accelerate decomposition of residues. In sod-podzolic soils, up to 30–60% of plant residues decompose within one year.

On the surface of structural aggregates (crumbs) of soil, an aerobic process of decomposition of organic matter occurs, and inside aggregates, where air deficit develops due to saturation of capillaries with water — an anaerobic process. The completeness and character of decomposition are influenced by temperature, soil reaction, presence of organic matter and presence of mineral substances for normal vital activity of microorganisms.

Simple sugars and proteins decompose relatively quickly in soil. Resins and waxes are resistant to decomposition by microorganisms, lignin is most resistant, from which a dark-colored complex complex substance is formed, which is the core of humus molecules.

During decomposition of forest litter by fungal flora under aerobic conditions, soluble colorless fulvic acids are formed. During bacterial decomposition of residues of herbaceous plants — poorly soluble, dark-colored humic acids.

Change in composition of plant residues due to different rates and completeness of decomposition of components and activity of microorganisms leads to new formation of specific humus substances.

Processes of decomposition of soil organic matter are determined by microorganisms. Their composition and number depend on soil type and degree of cultivation. The higher the cultivation, the more beneficial microbes are contained. Mass of microbes per 1 ha is 5–7 t. Microorganisms are the most energetic and mobile part of soil. Their role in soil processes and plant nutrition is associated with enzymatic action on dead soil substrate and enormous active surface on which various biochemical reactions occur.

Total surface of microbial flora per 1 ha is about 500–600 ha. Transformations of applied fertilizers are also associated with vital activity of soil biota.

Losses of Organic Matter

The greatest losses of soil organic matter are observed during cultivation of row crops and introduction of bare fallows, which is explained by multiple tillage and high soil aeration, creating favorable conditions for mineralization.

Losses of organic matter under cereal crops at low intensity of soil tillage were 0.4–1.0 t of humus per hectare during the year, under row crops — 1.5–3 times higher.

Arable crops are the main factor of reproduction of organic matter in arable soils. If under natural conditions all plant mass enters soil and accumulates carbon, nitrogen and other nutrients in the upper layer, then in agrocenoses with plant mass and harvest a significant amount of nutrients is removed, which creates a deficit-free balance.

In natural phytocenoses, represented mainly by perennial plants, removal of nutrients is significantly lower than in agrophytocenoses with annual crops, which consume more nutrients per unit of green mass. As a result, in arable soils with insufficient fertilizer application, depletion of reserves of organic matter and ash elements occurs. Soil depletion can be reduced by applying directed agronomic practices that reduce the negative effect of tillage on the balance.

Individual crops cultivated in agrophytocenoses are capable of exerting a positive influence on humus reserves and individual soil properties, but even in this case, without application of fertilizers, the organic matter balance remains negative.

Despite large differences in quantitative and qualitative composition of humus of different soils, the dynamics of organic matter as a whole is the same. For example, in a long-term experiment of MAA, cultivation of annual plants in crop rotation and continuously without fertilizers on sod-podzolic soil led to a gradual reduction of humus reserves. Losses of soil organic matter depend on cultivated crop, particle-size composition, intensity of soil tillage and other factors.

Decomposition of organic matter proceeds more intensively on soils of light particle-size composition and under irrigation, other conditions being equal.

Mechanical tillage affects loss of organic matter first of all. Loosening, activating soil processes, promotes mineralization of humus, and under conditions of leaching regime, also washing out of formed nutrients beyond the arable layer or reduction to gaseous nitrogen. This problem is most relevant for virgin lands. Reducing the number and depth of tillage also reduces rates of organic matter loss.

Losses of humus of typical chernozems were over 80 years of use in Voronezh Oblast — 2.5–3.0%; over 25 years in Krasnodar Krai — 1.3%. Humus content in chestnut soils of Altai Krai in 1896–1899 was 3.7–5.5%, and in 1973–1975 it decreased to 1.1–2.1%, on southern chernozems of Altai 5.0–6.5 in 1896–1899 and 2.9–4.1% in 1973–1975, on leached chernozems — 8.3–8.9 and 4.2–6.3%, respectively.

Over 30–50 years of intensive land use, humus content in soils can decrease by 20–25%, sometimes reaching 50%. Annually in arable layer of sod-podzolic soils, 0.6–0.7 t/ha of organic matter is mineralized, in chernozems — about 1 t/ha, which is about 1% and 0.4–0.5%, respectively. The highest rates of mineralization are observed in bare fallows — 1–2 t/ha due to absence of return of organic matter to soil and 60–120 kg/ha of nitrate nitrogen can accumulate.

Main causes of humus losses by soils:

decrease in mass of plant residues entering soil when natural biocenosis is replaced by agrocenosis;
increased mineralization as a result of intensive tillage and increased soil aeration;
decomposition and biodegradation of humus under action of physiologically acidic fertilizers and activation of microflora by applied fertilizers;
increased mineralization of humus on irrigated lands in first years of irrigation (in subsequent years humus maintenance stabilizes and even increases);
mineralization of humus during drainage reclamation of waterlogged soils;
water and wind erosion.

Reproduction of Humus

Application of mineral fertilizers is often a key factor in rapid increase of crop yields. But yield is not an absolute indicator of fertility. High efficiency of applying high doses of mineral fertilizers can be ensured by reproduction of soil organic matter. Two opposite processes occur simultaneously in soil: formation and decomposition of organic matter. The intensity of both processes determines the balance of organic matter in soil and depends on cultivated crop and technology of its cultivation.

Under natural conditions, the balance of organic matter is regulated by natural exchange between soil and plant community, which is in equilibrium characteristic of each zone. In modern agriculture, the problem of negative balance of organic matter remains. The main causes of this problem are:

imbalance in structure of crops by mass of plant residues remaining in soil;
increased mineralization of organic matter due to multiple tillage and increased soil aeration;
influence of physiologically acidic fertilizers, leading to decomposition and biodegradation of humus;
mineralization caused by drainage or irrigation reclamation;
soil erosion.

Systematic application of mineral and organic fertilizers in crop rotations influences transformation of soil organic matter, but their impact is fundamentally different. Organic fertilizers exert direct influence on balance due to humification of fertilizer components and indirect influence. While mineral fertilizers influence the organic matter balance only indirectly due to greater accumulation by crops of plant mass and, consequently, plant residues, as well as inhibitory effect of some mineral fertilizers on microbiological activity of soil, preventing humification.

Cultivation of perennial grasses gives a positive result due to accumulation of a large amount of plant residues and slowing down of humus mineralization, provided there is no soil tillage for several years.

Under perennial grasses on gray forest and sod-podzolic soils, mineralization of organic matter is about 0.2–0.35 t/ha, and annual replenishment is 0.6–0.9 t/ha. Therefore, the higher the percentage of perennial grasses in structure of sown area, the more humus is formed in soil.

In modern agriculture with intensive crop cultivation technologies, it becomes impossible to create a deficit-free balance without application of organic fertilizers.

In the Non-Chernozem zone, for a deficit-free balance, annual application of at least 5–12 t of manure per 1 ha of arable land is required, in the Ural region — at least 12 t/ha, in the Chernozem zone — at least 6–8 t/ha.

Application of organic fertilizers should be accompanied by a complex of agronomic measures, for example, liming or gypsuming of soils, rational use of mineral fertilizers, adjustment of structure of sown areas, etc.

Approaches aimed at creating a positive balance of organic matter in soil:

increasing the share of perennial leguminous, cereal and grain legume crops in crop rotations in the zone of sufficient moisture;
introduction into crop rotation of catch crops leaving a large mass of plant residues in soil;
improvement of technologies of cultivation of agricultural crops;
reduction of number and depth of soil tillage;
use of modern lightweight machinery;
erosion control measures.

These techniques should be built and planned within the framework of an integrated system of agricultural production adapted to specific soil-climatic conditions of the area.

The main ways to compensate for humus losses are:

use of all types of organic fertilizers in combination with mineral fertilizers;
plowing in of green manures, stubble-root residues;
introduction into crop rotation of leguminous grasses and legume-cereal grass mixtures.

Modeling the Balance of Organic Matter in Crop Rotation

Calculation of Organic Matter Balance

Modeling the balance of organic matter — forecasting changes in humus balance within a crop rotation, based on accounting for input and losses of organic matter of the arable layer. The expenditure part of the organic matter balance includes its mineralization under various crops under conditions of a specific soil tillage technology and production; removal of decomposition products by plants from soil and their leaching as a result of vertical and surface runoff. The input part of the balance includes input of organic matter with plant residues, with organic fertilizers, seeds and planting material; binding of atmospheric carbon dioxide by some blue-green algae.

Calculation of organic matter balance is usually carried out by nitrogen, since it is concentrated mainly in organic matter in carbon to nitrogen ratio equal to 10 : 1, and is directly absorbed by the plant from soil, unlike carbon. Therefore, amounts of nitrogen removed and entering soil allow judging the mineralized amounts of humus to cover the deficit.

Nitrogen removal with crop harvest is determined from reference data, and the degree of its use by plants from fertilizers and plant residues — from normative data. When calculating the organic matter balance, correction coefficients should be taken into account that characterize efficiency of use of humus nitrogen, which depends on soil texture and cultivated crops.

According to A. M. Lykov, correction coefficients of nitrogen use for soils different in particle-size composition and crops are as follows:

heavy loamy — 0.8, medium — 1.0, light — 1.2, sandy loam — 1.4, sandy — 1.8;
perennial grasses — 1.0, row crops — 1.6, cereals and other annual crops of continuous sowing — 1.2.

The coefficient of nitrogen use of mineral fertilizers in recommended doses, manure and plant residues is 50%. Nitrogen supply of clover from nitrogen fixation in variants without fertilizers is 80%, with application of fertilizers — 70; for vetch-oat mixture this indicator is respectively 20 and 10%.

The amount of plant residues can be calculated by their ratio to yield or by linear regression equation.

Humification coefficients (isohumic coefficients) of organic matter of manure and plant residues, based on studies of the Department of Agriculture and Experimental Methodology of MAA, are calculated by carbon and are:

for plant residues of cereal crops, grain legumes, flax and perennial grasses — 0.20–0.25;
maize and other silage crops — 0.1–0.15;
vegetables, potato and root crops — 0.05–0.08;
manure — 0.22–0.30;
peat — 0.30–0.35;
straw — 0.20–0.25;
green fertilizers — 0.04–0.06.

Under conditions of water or wind erosion, when calculating humus balance, soil losses from erosion processes are taken into account.

Humus balance is calculated for the purpose of forecasting and calculating the need of arable soils for organic fertilizers necessary for obtaining planned yield and reproduction of fertility. Humus balance is determined by the difference between the input item (due to input and humification of stubble-root residues, organic matter of fertilizers) and expenditure (mineralization of humus during cultivation of agricultural crops and fallowing of fields).

Balance calculation can be carried out by crop rotation, department, farm, district, oblast, etc. For calculation of balance the following data are needed:

placement of crops in crop rotation;
planned yield;
area occupied by crops;
doses of mineral fertilizers;
types and doses of organic fertilizers;
type, subtype and variety of soil.

Humus expenditure due to mineralization depends on a whole range of factors:

soil-climatic conditions;
intensity of soil tillage;
structure of sown areas;
crop yield;
level of chemicalization, etc.

The greatest mineralization of humus is noted in bare fallows, then in decreasing order follow row crops, continuous sowing crops (cereals, grain legumes, annual grasses), perennial grasses, meadows and pastures.

For example, on light sod-podzolic soils, annually mineralized under bare fallow — 4.5%, potato, root crops and vegetables — 3.4–3.9%, maize for silage — 3.3–3.9%, silage crops — 1.9–2.2%, cereals — 1.9–2.1%, annual grasses — 1.7–1.8%, perennial grasses and lupine — 0.6–0.9% of total humus reserves in arable layer (Popov et al., 1986).

The level of humus mineralization also depends on particle-size composition of soils. On light soils it increases.

For calculating humus mineralization taking into account factors influencing this process and intensity of soil tillage, A. M. Lykov (1976) proposed correction coefficients.

For heavy loamy soils coefficient is 0.8; medium loamy — 1.0; light loamy — 1.2; sandy loam — 1.4; sandy — 1.8.
For perennial grasses coefficient is 1.0; for cereals and other continuous sowing crops — 1.2; for row crops — 1.6.

Method of calculating humus balance by nitrogen removal by plants. The method is based on studies of I. V. Tyurin (1957) that humus on average contains 5–6% nitrogen. Therefore, use by plants of 50 kg of nitrogen is accompanied by mineralization of about 1 ton of humus. Studies conducted with ¹⁵N isotope showed that even with application of high doses of fertilizers, on average 50–60% of nitrogen removal by plants is soil nitrogen. Therefore, knowing the total nitrogen removal, one can determine the amount of humus that can be mineralized during cultivation of crops taking into account intensity of agriculture.

In practical conditions, humus mineralization can be determined by the formula:

H_m = (Y_o K_N + Y_o K_r K_{N_r}) \cdot 0.6 \cdot 20,

where

H_m: — amount of mineralized humus, t/ha;
Y_o: — yield of main product, t/ha;
K_N: — nitrogen removal per 1 t of main and by-product, kg;
K_r: — coefficient of plant residue output relative to main product;
K_Nr: — nitrogen removal by 1 t of plant residues, kg;
0.6: — averaged coefficient of soil nitrogen removal relative to total removal;
20: — conversion factor of nitrogen to humus.

All necessary initial data for use in the given formula are contained in reference literature. Annual replenishment of humus from root and stubble residues on average is: for cereal crops — 0.4–0.6 t/ha, row crops — 0.2–0.3 t/ha, perennial grasses — 0.5–1.0 t/ha.

The proposed method is convenient for calculations, but is conditional, since it is based on averaged indicators. Another method of calculating humus balance is based on use of data on humus content and coefficients of mineralization or humification of organic residues.

According to the method, the need for organic matter can be determined by the formula:

$$A = K (H_o + A),$$

where

A: — annual application of organic substances, t/ha;
H_o: — level of humus content in equilibrium state, t/ha;
K: — average coefficient of mineralization of organic fertilizers, which is equal to 0.02–0.08 depending on intensity of agriculture.

For calculation of the input item of the balance, the organic mass entering soil with fertilizers, root and stubble residues, and coefficients of their humification are taken into account. In this case, isohumic coefficients are used, that is, the amount of humus formed from various organic materials, in % of dry matter. For calculating expected humus reserves in soil for a link or full rotation of crop rotation, N. F. Ganzhara (1978) proposed the formula:

S_t = (S_o + K_rAt) \cdot (1 - K_m),

where

S_t: — humus reserve, t/ha after t years;
S_o: — initial humus reserves, t/ha;
K_r: — humification coefficient of fresh organic substances in fractions of unity (A is taken as unity);
A: — amount of fresh organic substances entering soil, t/ha;
t: — time for which humus balance is calculated, years;
K_m: — humus mineralization coefficient in fractions of unity (the value S_o + K_rAt is taken as unity).

For determining the total volume of accumulation of organic fertilizers at an agricultural enterprise, conversion coefficients to standard manure can be used:

litter manure with moisture content 75–77% — 1.0;
solid fraction of litterless manure — 1.0;
semi-liquid manure with moisture content up to 92% — 0.5;
liquid manure with moisture content up to 97% — 0.25;
compost based on manure with moisture content up to 84% — 1.0;
poultry manure semi-liquid — 1.4;
straw — 2.0;
sapropels — 0.8;
defecation lime — 0.8;
green manure of leguminous crops — 0.7;
green manure of cruciferous crops — 0.8.

VNIIA named after D. N. Pryanishnikov in cooperation with agricultural institutes developed normative data for calculating humus balance, based on generalization of available data of field experiments on input and expenditure items for different soil types under cultivation of various crops, which allow assessing humus balance for different types of crop rotations.

Regulation of Organic Matter Balance

Critical, or minimum, level of organic matter content — organic matter content below which rapid soil degradation develops, accompanied by decrease in its productivity and efficiency of agronomic practices.

Optimal level of organic matter content — humus content at which high efficiency of applied techniques and methods of intensification of agriculture is ensured.

For sod-podzolic soils, according to observation results, the optimal level of organic matter content is 2.5–4%, increased from 1.0–1.7%. As a result of such increase on this soil type, physical and physical-mechanical properties improve, which allows reducing tillage costs by 20–25% and shortening field work periods. Yield of maize and cereal crops from application of mineral fertilizers increases twofold. Further increase in organic matter content does not give proportional yield growth. Therefore, the optimal level of organic matter content in sod-podzolic soils is considered to be 4.0%.

To ensure a deficit-free balance of organic matter in a 40-cm deep arable layer of sod-podzolic soils of the Non-Chernozem zone at a level of about 2%, annual application of 5–5.5 t of organic fertilizers in terms of dry matter per 1 ha of arable land is required. To ensure a level of 4%, application of 10–11 t/ha is required, respectively. Application of such an amount of organic fertilizers is possible with the maximum possible increase in farms of output of manure and composts, use of straw and catch crops, high saturation of crop rotations with perennial grasses, which increases economic costs of reproduction of soil organic matter. Therefore, for each specific soil of a crop rotation, the optimal humus content is calculated, corresponding to maximum agronomic efficiency at acceptable economic costs.

Reduction of losses of organic matter, especially of sod-podzolic and other soils of the Non-Chernozem zone, whose humus content is already at levels close to critical, is an important agronomic task in modern agriculture. For this, a number of solutions are proposed:

replacement of annual fodder crops with perennial grasses;
use of straw;
introduction of green manure crops as catch crops;
replacement of bare fallows with occupied fallows.

For example, mineralization of humus in bare fallow averages 3 t/ha of humus, for reproduction of which more than 40 t/ha of manure is required. This problem is especially acute on soils under continuous fallowing. For example, humus losses over 20 years of typical chernozems of Voronezh Oblast under continuous fallowing were 3%.

Regardless of size and forms of ownership of agricultural production, the balance of organic matter should at least be deficit-free, and on humus-poor soils — positive. A deficit-free balance of organic matter at optimal level of humus content can be achieved by a combination of the following conditions:

rational structure of use of sown areas with inclusion of perennial grasses;
effective use of plant residues;
application of organic fertilizers;
liming and gypsuming of soils;
application of mineral fertilizers;
regulation of water regime of reclaimed soils;
application of soil-protective measures;
optimization of soil tillage.

Quantitative parameters of these methods depend on specific economic and soil-climatic conditions.

References

Niklyaev V. S. (2000). Fundamentals of Technology of Agricultural Production. Agriculture and Crop Production. Moscow: “Bylina”.
Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. (2000). Agriculture. Textbook for Universities. Moscow: “Kolos” Publishing House.
Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. Mineev V. G. Moscow: VNIIA named after D. N. Pryanishnikov.