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Organic matter of the soil

Organic matter of the soil is a set of organic substances in the form of humus, animal and plant residues in the soil, representing a complex of complex chemical organic substances of biogenic origin.

Biological indicators of soil fertility – the amount, composition and properties of organic matter in the soil.

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


The 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 the effect of toxic substances. Organic matter regulates the flow of nutrients, preventing unproductive losses from leaching, the formation of gaseous products and hard-soluble mineral compounds, increases the effectiveness of mineral fertilizers. 

It is the only source of energy for vital activity of soil microflora and biota, participates in self-regulation of decomposition, contributing to the inactivation by clay minerals of enzymes released by microorganisms. Soils with a high content of humus are biologically more active: they have a greater number of microorganisms, a greater diversity of their species composition, more intense formation of carbon dioxide CO2, increased enzymatic activity. 

Organic matter participates in soil formation due to its ability to bind with mineral part of soil. Organomineral compounds formed in this process constitute soil absorption complex which is the most important property of any soil. Forms of organic-mineral compounds can be complexes with metal cations, hydroxides, anions, silicates etc. Soil mineral-organic complex compounds are an example of such forms.

Obligatory condition of stable farming is the reproduction of organic matter, which means the simultaneous reproduction of biological, agrophysical and agrochemical factors of fertility.

Humus is an excellent water absorber – 1 g of humus is able to absorb 4 to 20 g of water, which is available to plants. This property depends on the presence of humic acids.

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

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

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

Depletion of soils by organic matter leads to deterioration of water-physical, chemical and biological properties. Due to high absorption capacity, humus keeps cations from migrating along the 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:

Humus, or mold, substances of a 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, such as 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 the total stock of it in the soil, is easily broken down and is a source of nutrients for plants.

Non-specific substances can also include substances included in the composition of plant and animal tissues, products of vital activity of organisms and plants. Nonspecific substances may be present in free and bound to mineral components of the soil. Many of these substances are a breeding ground for microorganisms that transform them into soil humus.

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

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

Soil types differ in the content of humus and in the amount and ratio of humic acids and fulvic acids. For example, in sod-podzolic soils, this ratio is 0.4-0.6, in the chernozems – 1.0-1.5 and more. These differences cause 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.

Soil humus substances can be in the form of humates of calcium, magnesium, sodium; humates and mixed salts with aluminum and iron hydroxide; complex organomineral compounds with aluminum, iron, phosphorus and silicon. These compounds can be absorbed by clay minerals, especially firmly with minerals like montmorillonite. With kaolinite or feldspars the bond is less strong. The formation of organomineral compounds in the soil contributes to the consolidation of humus.

Humic acids

Humic acids are a fraction of high molecular weight organic compounds extracted from soil by alkaline solutions. They are dark colored, contain nitrogen, are insoluble in acids, soluble in solutions of pyrophosphates, oxalates, fluorides and ammonia with formation of soluble salts – humates. Acidification of an alkaline solution of humic acids leads to precipitation in the form of humates.

Depending on the concentration and type of soil, humates may have a color from cherry-brown to black.

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

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

Humic acids contain the following functional groups:

  • 3-6 phenolic hydroxide groups (-ON),
  • 3-4 carboxyl (-COH),
  • primary and secondary alcoholic (-ON),
  • methoxyl (-O-CH3),
  • carbonyl (-C=O).

These functional groups determine the properties of humic acids and the nature of interaction with soil. Thus, carboxyl groups determine the acid and exchange properties.

Fulvic acids

Fulvic acids are high molecular weight nitrogen-containing organic compounds of acidic nature. They are soluble in water, acids, weak alkali solutions, sodium pyrophosphate, in aqueous ammonia solution, forming soluble salts – fulvates, as well as many organic solvents. They are colored in shades from straw-yellow to orange and brown. Gross elemental composition of fulvic acids: carbon 36-52%, oxygen 42-52%, hydrogen 3-6%, nitrogen 2-6%. They contain, in smaller amounts, sulfur, phosphorus and some metals. They are mobile and relatively easy to move along the soil profile.

Fulvic acids, having a strongly acidic reaction, have a destructive effect on the mineral part of the soil, which depends on the 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 the formation of salts – fulvates.

The nitrogen of fulvic acids is more easily subjected to acid hydrolysis. Humic acids account for 15-30% of soil nitrogen and fulvic acids for 20-40%.


Humines are organic compounds of humus nature, insoluble in water, acid solutions, alkalis or organic solvents. It is a complex of complex esters of humic and fulvic acids, chemically bound to clay minerals and mineral part, which causes high resistance to chemical and microbiological decomposition.

Humines cannot be a source of nutrients for plants, but due to their complex structure, which includes functional groups, they serve as a buffer for the retention of nutrients in an accessible form for plants. Humic nitrogen makes up 20-30% of total soil nitrogen. They influence the capacity, buffering of soils, migration and transformation of nutrients.

Non-humified substances

Non-humified substances account for 10-20% of the total stock of soil organic matter and serve as a source of nutrients for plants and biota. Some of them stimulate or inhibit the growth and development of living organisms, influence the transformation of soil nutrients and fertilizers from forms inaccessible to plants.

Between 10 and 30% of non-humified substances are involved in the new formation of humus. Plant residues account for 3-5 to 12-15 t/ha or for sod-podzolic soils – up to 10%, in chernozems – 2-3% of the total reserves of organic matter. The mass of microorganisms in the 20-centimeter layer of soil is from 0.7 to 2.7 t/ha, reaching 5-7 t/ha, which is 1-3% of the total reserves of organic matter.

Deficiency of non-humic substances in soils affects the nutrient regime of all organisms.

Properties of humus

The most important property of humus is colloidity. The colloidal, surface-active properties of humus are caused by cation-anion micelles, due to which high activity is manifested even at a small thickness of the adsorption layers. These properties, despite the small proportion of humus in the solid phase of the soil, with the exception of peat soils, are important in plant nutrition, fertilizer conversion and soil fertility.

Sources of organic matter in the soil

Soil organic matter is formed in natural conditions by the death of plants, microorganisms, soil animals and the decomposition of the products of their vital activity. 

Under arable conditions the main sources of organic matter are plant residues and organic fertilizers. The advantage of using crop residues is to reduce the cost of applying organic fertilizers, which, unlike crop residues that come in annually, may not be regular and their even distribution. 

Plant residues can be divided into:

  • stubble – plant residues left after harvesting crops, including parts of stems, leaves;
  • leafy – plant remains after harvesting potatoes, perennial grasses, vegetables, root crops, including rhizomes, root necks, stems, etc;
  • root – the remains of plant roots remaining after harvesting.

Receipt of organic matter with plant residues occurs during the entire growing season, since the process of dying off of individual parts of the plant is constant, and especially after flowering and the beginning of fruiting. The annual input of crop residues under cereal crops is 0.3-0.5 t/ha, under row crops – 0.15-0.25 t/ha.

The depth of the root systems of different crops and also their mass, differently affects the replenishment of organic matter in the soil. The main crops can also be divided into three groups according to the amount of organic matter that comes with crop residues:

  1. The first group includes perennial legumes and grasses. They leave the greatest amount of root and crop residues. In addition, leguminous perennial crops replenish the soil’s nitrogen reserves through their nitrogen fixation properties.
  2. The second group includes annual grain and legume crops of continuous sowing. They leave much less crop residues. However, the amount of organic matter left within the group can vary greatly. For example, ryegrass and its mixtures with legume annual crops leave about the same amount of crop residues as perennial grasses. Winter crops surpass spring cereals and grain legumes in this indicator. After harvesting of annual cereals and leguminous 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. This ratio of carbon and nitrogen makes it a more valuable source of organic matter than stubble residues. Application of fertilizers contributes to a greater accumulation of nitrogen and potassium in crop residues, phosphorus (to a lesser extent).
  3. The third group includes row crops, which leave the least amount of plant residues.

The amount of organic matter in the soil is determined by the amount of plant residues, and its quality and rate of transformation – by their chemical composition.

Moisture and nutrient supply determine the power of development of the root system and the above-ground part. Under favorable conditions, the above-ground part increases more than the roots. As a result, the ratio of masses of root system and above-ground part shifts in favor of the latter.

The content of nutrients is higher in perennial grasses than in other crops: Nitrogen content in root residues of perennial legume grasses is 2.25-2.60%, phosphorus – 0.34-0.80, in post-owing residues – 1.82-2.65 and 0.30-0.71% respectively, in legume-grass grass mixtures it depends on the mixture composition and is 0.91-2.37% nitrogen and 0.25-1.06% phosphorus, in single-cut (after mowing the grass) residues – 1.60-2.18 and 0.17-0.54% respectively. The content of nitrogen and phosphorus of bluegrasses and their post-harvest residues is significantly lower.

The chemical composition is influenced by the 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 and climatic conditions.

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

Soil organic matter is formed under the action of vital activity of plants, microorganisms and soil fauna. With sufficient air in the soil and optimum humidity a rapid aerobic decomposition process takes place. Under conditions of lack of air and excessive moisture, anaerobic microbial decomposition occurs. Optimal conditions for decomposition of organic matter are formed in structural, loose, cultivated soils.

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

Stock of organic matter in the soil

The greatest effect of the positive effect of organic matter on crop yields is achieved by creating an optimal stock of organic matter for each type of soil. The optimal amount of organic matter of a certain composition and quality can be considered as such, which allows to obtain the planned yield with the effective use of fertilizers and tillage technology.

For sod-podzolic soils can be considered optimal humus content of 2.5-4%. A lower content of organic matter leads to a decrease in crop productivity, while a higher content does not create a recoupable yield increase.

Humus stock is one of the most important indicators of the humus state of the soil. Stock of humus depends on the thickness of the arable layer and its density. In soils with light granulometric composition humus stock is usually lower than in similar heavier soils. However, due to a thicker topsoil the stock in the light soils may exceed the stock of humus in heavy soils.

According to the research of L.A. Grishina and D.S. Orlov, the humus reserve in 20 cm of arable layer is estimated as very low – with a reserve of less than 50 tons of humus per 1 ha, low – with 50-100, average – with 100-150, high – with 150-200, very high – with more than 200 tons/ha.

The amount of humus in the soil is different and depends on a number of factors, soil type, climatic conditions, crop rotation, farming system. The highest content of humus is in the upper layers of the soil, while down the profile the content of organic matter decreases.

Table. Humus content in the main types of soils (by I.V. Tyurin)

Humus content in the arable layer, %
Humus reserves, t/ha
0-20 cm layer (average)
layer 0-120 cm
Gray forest podzoled
Black Earths
- leached
- strong
- common
- south
Dark chestnut
Chestnut and light chestnut
Gray Earth
Red Earth

Transformation of organic matter in the soil

Organic matter undergoes a transformation in the soil, which is divided into several stages.

In the first stage, the 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 membranes. This process can be accelerated by biological and mineral catalysts.

The second stage is the absorption and mechanical mixing of plant residues with the soil by the soil fauna. Probably, biochemical reactions of transformation of primary organic matter take place at this stage as well.

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

The end products of organic matter transformation are carbon dioxide, water, nitrates, phosphates, etc. The products are also low-molecular-weight organic acids: formic, acetic, oxalic, and others. Mineralization processes proceed with the emission of heat: the decomposition of 1 g of dry matter releases 17-21 J of energy.

Some of the products of organic matter decomposition are used by new generations of plants and by heterotrophic microorganisms to synthesize proteins, fats, and carbohydrates. Another part is transformed into humus substances.

Humification is the process of transformation of soil organic matter into humus compounds. Humification begins at the first stage of transformation of soil organic matter.

M.M. Kononova presents the following generally recognized mechanism of formation of specific humus substances:

  1. Initial substances for synthesis of humus substances are any components of decomposition of plant tissues and products of metabolism of microorganisms.
  2. Initial substances are condensed (combined) by oxidation of phenols under the action of phenoxidases to quinones through the stage of formation of intermediate compounds semiquinones. Quinones react with amino acids and peptides.
  3. The polycondensation (polymerization) of compounds formed at the second stage leads to the formation of humus substances.

This mechanism is not the only one. L.N. Aleksandrova, developing the assumption of Academician I.V. Tyurin, suggests a different mechanism of humification, which consists in slow biochemical oxidation of high-molecular-weight cyclic compounds with their subsequent polycondensation.

The rate and nature of transformation are influenced by external environmental conditions: moisture, soil acidity, temperature, air and nutrient availability), and the chemical composition of plant residues.

The ratio of carbon to nitrogen in plant residues affects the rate of decomposition. Fast decomposition is provided by a narrow C : N ratio, which is characteristic, for example, of plant residues of clover; this process is less intensive with crop residues of grain crops and vetch-grass mixture. Organic and mineral fertilizers accelerate the decomposition of residues. In sod-podzolic soils up to 30-60% of plant residues decompose during one year.

On the surface of the structural aggregates (lumps) of soil goes aerobic process of decomposition of organic matter, and inside the aggregates where there is a deficit of air due to saturation of capillaries of water – anaerobic process. Completeness and character of decomposition are affected by temperature, soil reaction, presence of organic matter and availability of minerals for normal life of microorganisms.

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

The decomposition of forest litter by fungal flora under aerobic conditions produces soluble, colorless fulvic acids. Bacterial decomposition of herbaceous plant residues produces insoluble, dark-colored humic acids.

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

Decomposition processes of soil organic matter are determined by microorganisms. Their composition and number depends on soil type and degree of cultivation. The higher the degree of cultivation, the more useful microbes are contained. The mass of microbes per 1 hectare is 5-7 tons. Microorganisms are the most vigorous and mobile part of the soil. Their role in soil processes and plant nutrition is connected with enzymatic action on dead soil substrate and huge active surface where different biochemical reactions take place.

The total surface area of microbial flora per 1 ha is about 500-600 ha. Transformations of applied fertilizers are also connected with vital activity of soil biota.

Loss of organic matter

The greatest losses of soil organic matter are observed in the cultivation of row crops and the presence of clean fallows in the rotation, which is explained by repeated tillage and high aeration of the soil, creating favorable conditions for mineralization.

Losses of organic matter under grain crops at low intensity of tillage were 0.4-1.0 tons of humus per hectare during the year, under row crops – 1.5-3 times higher.

Tillage crops are the main factor of reproduction of organic matter in arable soils. If in natural conditions all plant mass enters the soil and thus contributes to the accumulation of carbon, nitrogen and other nutrients in the upper layer, in agrocenoses with plant mass and harvest a significant amount of nutrients is alienated from the fields, which creates a deficit balance.

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

Certain crops cultivated in agrophytocenoses can have a positive effect on humus reserves and certain soil properties, but even in this case without the use of fertilizers the balance of organic matter remains negative. 

Despite the large differences in the quantitative and qualitative composition of humus in different soils, the dynamics of organic matter is generally the same. For example, in a long-term experiment of the Moscow Agricultural Academy, cultivation of annual plants in rotation and continuously without fertilizers on sod-podzolic soil led to a gradual reduction of humus reserves. Loss of soil organic matter depends on the cultivated crop, granulometric composition, intensity of tillage and other factors.

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

Mechanical tillage affects the loss of organic matter in the first place. Loosening, activating soil processes contributes to the mineralization of humus, and under washing conditions and washout of formed nutrients outside the arable layer or reduction to gaseous nitrogen. This problem is most urgent for virgin lands. Reducing the amount and depth of tillage also reduces the rate of loss of organic matter.

Losses of humus of typical chernozems for 80 years of use in Voronezh region amounted to 2.5-3.0%; for 25 years in Krasnodar region – 1.3%. The humus content of chestnut soils in Altai Krai in 1896-1899 was 3.7-5.5%, and in 1973-1975, it decreased to 1.1-2.1%; in the southern chernozems of Altai 5.0-6.5 in 1896-1899 and 2.9-4.1% in 1973-1975; in 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 up to 50%. Annually in the arable layer of sod-podzolic soils 600-700 kg/ha of organic matter is mineralized, in chernozems – about 1000 kg/ha, which is about 1% and 0,4-0,5% respectively. The highest rates of mineralization are observed in clean fallows – 1-2 t/ha due to the lack of return of organic matter into the soil and can accumulate 60-120 kg/ha of nitrate nitrogen.

The main causes of humus losses by soils:

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

Humus reproduction

Mineral fertilizer application is often a key factor in rapidly increasing crop yields. But the yield is not an absolute indicator of fertility. High efficiency of application of large doses of mineral fertilizers can be provided only by the reproduction of soil organic matter.

In soil, two opposing processes occur simultaneously: the formation and decomposition of organic matter. The intensity of both processes determines the balance of organic matter in the soil and depends on the cultivated crop and its cultivation technology.

Under natural conditions, the balance of organic matter is regulated by the natural exchange between the soil and the plant community, which is in equilibrium characteristic of each climatic zone.

In modern agriculture remains a problem of negative balance of organic matter. The main causes of this problem are:

  • imbalance in the structure of crops by the mass of plant residues remaining in the soil;
  • increased mineralization of organic matter due to repeated tillage and increased aeration of soils;
  • the impact 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 affects the transformation of soil organic matter, but their impact is fundamentally different. Organic fertilizers have a direct impact on the balance by humification of the fertilizer components, as well as indirectly. Whereas mineral fertilizers have only an indirect effect on the balance of organic matter through greater accumulation of plant matter and, therefore, plant residues, as well as the inhibiting effect of some mineral fertilizers on soil microbiological activity, which prevents the humification.

Cultivation of perennial grasses gives a positive result due to the accumulation of large amounts of plant residues and slows down the mineralization of humus, 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 input is 0.6-0.9 t/ha. Therefore, the higher the percentage of perennial grasses in the cropping pattern, the more humus is formed in the soil.

In modern agriculture with intensive technologies of growing crops, it becomes impossible to create a deficit-free balance without the introduction of organic fertilizers.

In the Nonchernozem zone for deficit-free balance requires annual application of at least 5-12 tons of manure per hectare of arable land, in the Ural region – at least 12 tons / ha, in the Black Earth zone – at least 6-8 tons / ha.

Application of organic fertilizers should be accompanied by a set of agronomic measures, such as liming or application of gypsum, rational use of mineral fertilizers, adjusting the structure of cultivated areas, etc.

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

  • increasing the share of perennial legumes, grain and grain legumes in crop rotations in the zone of sufficient moisture;
  • introduction of intermediate crops into crop rotation, leaving a large mass of plant residues in the soil;
  • improvement of crop cultivation technologies;
  • reducing the number and depth of tillage;
  • use of modern lightweight equipment;
  • anti-erosion measures.

These methods should be built and planned as part of an integrated system of agricultural production, adapted to the specific soil and climatic conditions of the area.

The main ways to compensate for the loss of humus are:

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

Modeling the balance of organic matter in the crop rotation

Calculation of the balance of organic matter

Modeling the balance of organic matter – predicting changes in the balance of humus within a crop rotation, based on the account of inputs and losses of organic matter of the arable layer.

The expenditure part of the balance of organic matter includes its mineralization under different crops under conditions of a particular technology of tillage and production, removal of products of decomposition by plants from the soil and their leaching as a result of vertical and surface runoff.

The incoming part of the balance includes the entry of organic matter with crop residues, with organic fertilizers, seeds and planting material; binding of atmospheric carbon dioxide by some blue-green algae.

Calculation of the balance of organic matter is usually performed on nitrogen, as it is mainly concentrated in the organic matter in the ratio of carbon to nitrogen equal to 10 : 1, and is directly absorbed by the plant from the soil, unlike carbon. Therefore, the amounts of nitrogen carried out and entering the soil allows us to judge about the mineralizable amounts of humus to cover the deficit.

Nitrogen removal from the soil with the crop is determined by reference data, and the extent of its use by plants from fertilizers and crop residues – by regulatory data. When calculating the balance of organic matter one should take into account correction factors that characterize the efficiency of the use of humus nitrogen, which depends on the granulometric composition of the soil and crops under cultivation.

According to A.M. Lykov, correction factors of nitrogen use for different granulometric composition of soils 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, grains and other annual crops of continuous sowing – 1.2.

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

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

Humification coefficients (isohumus coefficients) of organic matter of manure and plant residues, based on the research of Department of Agriculture and Experimental Methods of the Moscow Agricultural Academy, are calculated by carbon and are

  • for plant residues of grain crops, grain legumes, flax and perennial grasses – 0,20-0,25;
  • corn and other silage crops – 0,1-0,15;
  • vegetables, potatoes 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.

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

The balance of humus is calculated in order to forecast and calculate the needs of arable soils in organic fertilizers necessary to obtain the planned yield and reproduction of fertility.

The balance of humus is determined by the difference between the input (due to receipt and humification of crop residues, organic matter of fertilizers) and output (mineralization of humus during cultivation of crops and fallowing of fields).

A balance calculation can be performed for a crop rotation, department, farm, district, region, etc.

To calculate the balance, the following data are needed:

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

The consumption of humus by mineralization depends on a number of factors:

  • soil and climatic conditions;
  • intensity of soil tillage;
  • the structure of cultivated areas;
  • crop yields;
  • level of chemicalization, etc.

The highest humus mineralization is observed in clean fallows, followed in descending order by row crops, continuous crops (cereals, legumes, annual grasses), perennial grasses, meadows and pastures.

For example, on light sod-podzolic soils annual mineralization under clean fallow is 4.5%, potatoes, 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 gross humus stocks in arable layer (Popov et al. , 1986).

The level of humus mineralization also depends on the granulometric composition of soils. It increases on light soils.

To calculate the mineralization of humus, taking into account the factors affecting this process and the intensity of tillage, A.M. Lykov (1976) proposed correction factors.

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

The method of calculating the balance of humus by nitrogen removal by plants. The method is based on the research of I.V. Tyurin (1957) that humus contains on average 5-6% of nitrogen. Therefore, the use of 50 kg of nitrogen by plants is accompanied by mineralization of about 1 ton of humus. Studies carried out with the isotope 15N showed that even with the application of high doses of fertilizers, on average 50-60% of the nitrogen removal by plants is soil nitrogen. Therefore, knowing the value of total nitrogen removal, it is possible to determine the amount of humus that can be mineralized under cultivation of crops, taking into account the intensity of farming.

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

Hm = (Ym х RN + Ym х Kr х KNr) x 0,6 x 20,

where Hm is the amount of mineralized humus, t/ha; Ym is the yield of the main product, t/ha; RN is the nitrogen removal in recalculation per 1 t of the main and by-products, kg; Kr is the coefficient of plant residues output in relation to the main product; KNr is the nitrogen removal with 1 t of plant residues, kg; 0,6 is the average coefficient of soil nitrogen removal in relation to the entire removal; 20 is the coefficient of conversion of nitrogen into humus.

All necessary input data for use in the above formula are contained in the reference literature. Annual humus replenishment from root and stubble residues averages 0.4-0.6 t/ha for grain crops, 0.2-0.3 t/ha for row crops, and 0.5-1.0 t/ha for perennial grasses.

The proposed method is convenient for calculations, but is conditional, since it is based on averaged indicators.

Another method of calculating the balance of humus is based on the use of data on humus content and coefficients of mineralization or humification of organic residues.

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

A = K (H0 + A),

where A – annual application of organic matter, t / ha; H0 – the level of humus in the equilibrium state, t / ha; K – the average coefficient of mineralization of organic fertilizers, which is 0.02-0.08, depending on the intensity of farming.

To calculate the input item of the balance take into account the organic matter entering the soil with fertilizers, root and stubble residues, and coefficients of their humification. This uses isohumus coefficients, that is the amount of humus formed from various organic materials, in % of dry matter.

To calculate the expected reserves of humus in the soil for a link or a complete rotation of crop rotation N.F. Ganzhara (1978) proposed the formula:

St = (So + Kr x A x t) x (1 – Km),

where St – reserve of humus, t/ha in t years; So – initial reserves of humus, t/ha; Kr – coefficient of humification of fresh organic matter in fractions of one (A is taken as a unit); A – amount of fresh organic matter entering the soil, t/ha; t – time for which the balance of humus is calculated, years; Km – coefficient of humus mineralization in fractions of one (So + Kr х A х t value is taken as a unit).

To determine the total amount of accumulation of organic fertilizers at an agricultural enterprise we can use conversion coefficients for standard manure:

  • bedding manure with moisture content of 75-77% – 1.0;
  • solid fraction of litter-free manure – 1.0;
  • semi-liquid manure with moisture content up to 92% – 0.5;
  • liquid manure with moisture up to 97% – 0.25;
  • manure-based compost with moisture up to 84% – 1.0;
  • semi-liquid poultry manure – 1.4;
  • straw – 2,0;
  • sapropel – 0,8;
  • defecate – 0,8;
  • legume crop siderat – 0,7;
  • cruciferous crop siderat – 0.8.

VNIIA named after D.N. Pryanishnikov in cooperation with agricultural institutes developed normative data for calculating the balance of humus based on the generalization of available data of field experiments on the items of input and output for different types of soils during the cultivation of various crops, which allow to estimate the balance of humus for different types of crop rotations.

Regulating the balance of organic matter

Critical, or minimum, level of organic matter content is the content of organic matter, below which rapid degradation of soils develops, accompanied by a decrease in its productivity and effectiveness of agronomic methods.

The optimum level of organic matter content is the content of humus, which provides high efficiency of applied methods and methods of intensification of agriculture.

For sod-podzolic soils according to the results of observations the optimal level of organic matter content is 2.5-4%, which was increased from 1.0-1.7%. As a result of such an increase on this type of soil physical and physical-mechanical properties improve, which allows to reduce the cost of processing by 20-25% and reduce the terms of field works. Yield of corn and grain crops from making mineral fertilizers increases twofold. A further increase in the content of organic matter does not give a proportional increase in yield. Therefore, the optimal level of organic matter content in sod-podzolic soils is considered to be 4.0%.

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

Reducing losses of organic matter, especially sod-podzolic and other soils of the Non-Chernozem zone, whose humus content is already close to critical levels, is an important agronomic task in modern farming. A number of solutions are proposed for this purpose:

  • replacement of annual fodder crops with perennial grasses;
  • using straw;
  • introduction of green crops as intercrops in crop rotation;
  • replacement of pure fallows with occupied ones.

For example, humus mineralization in clean fallow is on average 3 t/ha of humus, which requires more than 40 t/ha of manure to reproduce.

This problem is particularly acute on soils with permanent clean cultivation (fallowing). For example, the loss of humus for 20 years of typical chernozems of Voronezh region in a permanent fallow was 3%.

Regardless of the size and form of ownership of agricultural production balance of organic matter should be in the extreme case deficit-free, and on humus-poor soils – positive.

The deficit-free balance of organic matter at the optimal level of humus content can be achieved by a combination of the following conditions:

  • rational structure of the use of sown areas with the inclusion of perennial grasses;
  • effective use of crop residues;
  • Introduction of organic fertilizers;
  • liming and gypsumization of soils;
  • application of mineral fertilizers;
  • regulation of water regime of reclaimed soils;
  • application of soil protective measures;
  • optimization of soil treatment.

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


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