The role of organic matter and humus in the process of soil formation

How parent material becomes soil: the role of organic matter and its composition

The main characteristic distinguishing fertile soil from barren parent rock is the presence of organic matter. The initial process of its accumulation begins with the vital activity of lower organisms that colonize the parent material. Under the influence of physical, chemical, and biological weathering, the rock gradually becomes loose and acquires the ability to retain moisture. As a result, favorable conditions are created for the growth and development of crops.

With the appearance of higher plants, the process of organic matter accumulation accelerates sharply. After the decay of roots and above-ground biomass, their remains are decomposed by microorganisms and synthesized into new compounds. This is how soil humus is formed — a complex system consisting of non-humified residues of plant and animal origin and actual humic substances.

• Specific part of humus — 85–90 %
• Non-specific compounds — 10–15 %
• Elements of biological accumulation — nitrogen, phosphorus, sulfur, calcium

Individual organic compounds within the composition of humus are represented by products of deep decomposition and microbial activity. These include proteins, amino acids, carbohydrates and their derivatives, fats, waxes, resins, tannins, and lignins. The bulk of humus, however, accounts for its specific part. These are high-molecular-weight compounds of an acidic nature that bind firmly to the mineral part of the soil and reflect specific conditions of soil formation.

How humus is formed: mechanisms and pathways of synthesis

Scientific perspectives on the nature of humus have undergone a long evolution. According to the classical biological theory, soil organic matter is created during the vital activity of plants, animals, and soil microflora. In this process, easily assimilable organic acids serve as an indirect source of humus, first being converted into microbial plasma. Complex aromatic compounds, such as lignin and tannins, act as a direct source of humic substances.

There are other views on the process of humification. One considers humus as a lignin-protein complex formed during the chemical condensation of protein with lignin. Another approach treats humus formation as a continuous cycle where the synthesis of organic matter in the living tissues of higher plants alternates with its decomposition by microorganisms after death.

Modern agronomic science unites these theories. Humification is viewed as a combination of opposing processes: the decomposition of organic residues and the synthesis of new stable compounds. This entire cycle proceeds in several successive stages.

1. Decomposition of plant and animal residues under the influence of microorganisms, soil fauna, and abiotic environmental factors.
2. Formation of cyclic decomposition products and assimilation of residues by microorganisms to form microbial plasma.
3. Oxidation and polymerization of decay and microbial metabolism products with the participation of catalysts.
4. Formation of stable humic compounds that become fixed in the soil profile.

In the process of humification, plant residues pass through a stage of processing by microorganisms, and their metabolic products serve as the primary material for creating humus. However, this pathway, while very important, is not the only one in the soil. For an agronomist, this means the necessity of maintaining the nutrient balance of soil biota to stimulate the natural synthesis of humic substances.

F. Duchaufour defines the process of humification as the aggregate of processes leading to the formation of a colloidal complex, biochemically transformed from plant residues and bound to mineral colloids. According to Duchaufour, the process of humification has two phases. The first—biological—is relatively short, where microorganisms and their enzymes predominate, culminating in the formation of "young" humus with weak bonds to the mineral part of the soil. The second—the maturation phase—is climatic, longer in duration, and conditioned by seasonal climate contrasts; it concludes with the formation of mature humus and the multiplication of bonds with the mineral part of the soil.

The most complete and comprehensive scheme of humus formation was developed by L. N. Alexandrova (Fig. 57; Alexandrova L. N., 1980). It includes the processes of decay of organic residues, microbial synthesis, humification, interaction with the mineral part of the soil, and, on the other hand, the processes of mineralization and the involvement of mineral components in the biological cycle. According to this scheme, the products of microbial synthesis are also sources of humic acids. Conversely, semi-decomposition products can interact with the mineral part of the soil and be leached out of the soil profile.

Fig. 57. Scheme of the humus formation process in soil

Thus, the main process characteristic primarily of soils and leading to the formation of specific organic matter—humus—is humification. As defined by L. N. Alexandrova, humification is a complex biophysicochemical process of transforming intermediate high-molecular-weight products of organic residue decomposition into a specific class of organic compounds—humic acids. In turn, it is divided into several stages. The initial stage is oxidative acid formation, i.e., the formation of humic acids, which are subsequently subjected to long and complex transformation processes. Simultaneously, the nitrogenous part of the humic acid molecule is formed. In the first stage, fractionation of the resulting humic acids by solubility into a group of humic and fulvic acids also occurs. As a result of interaction with the mineral part of the soil, organo-mineral compounds are formed. The second stage is expressed by a gradual increase in aromatization due to partial destruction of aliphatic chains via hydrolysis and oxidation processes involving oxidative enzymes. The stage of mineralization of humic substances completes the process of their transformation.

The rate and nature of humification depend on a multitude of factors. These include the quantity and nature of incoming plant residues, their chemical composition, the moisture and aeration regime, the soil reaction and redox conditions, the intensity of microbiological activity and the group composition of microorganisms, as well as the granulometric, mineralogical, and chemical composition of the mineral part of the soil.

The most favorable organic substrate for humus formation is the residue of herbaceous vegetation, especially legumes. They are rich in easily decomposable compounds such as proteins, carbohydrates, organic acids, as well as ash elements. When they decompose in soils with a high content of bases, primarily calcium, "soft" or mull humus is formed, which uniformly permeates the mineral part of the soil. Mull humus forms in soils under deciduous and mixed forests with active soil fauna activity. The latter, in turn, facilitates the process of humification by mixing leaf litter with the mineral part of the soil.

Residues of woody vegetation (needles, wood) are characterized by a low content of proteins and ash elements, but are enriched with lignin, cellulose, and hemicelluloses, which are difficult to decompose, and are also distinguished by the presence of resins and tannins that slow down humification. They enter the soil primarily in the form of surface litter, the decomposition of which occurs under conditions of leaching by precipitation.

Litter decomposes with the participation of fungi, forming large amounts of organic acids, the neutralization of which is difficult due to the intensive leaching of bases. The acidic reaction suppresses humification, and "coarse" humus (moder humus) forms on the soil surface, containing many semi-decomposed residues.

Depending on the water-air regime, humus formation can proceed under aerobic or anaerobic conditions. The most typical combinations of temperature and water-air regimes are as follows:

Factors affecting humus accumulation in soil

Humus accumulation directly depends on the hydrothermal regime of the field. Under conditions of excessive moisture and high temperatures, organic matter mineralizes too quickly. Soils in such regions are poor in humus, even though they are well-supplied with nitrogen and ash elements. With a constant moisture deficit, the vegetation cover becomes sparse, and the decomposition processes of organic residues are severely hindered.

• Humidity for active mineralization — 60–80 % of full water capacity
• Temperature for intensive mineralization — 25–30 °C
• Share of specific compounds in humus — 85–90 %
• Reserves in the meter-thick layer of soil — from 50 to 650–800 t/ha

In cold climates with excessive moisture, anaerobic conditions are created. The activity of anaerobic bacteria leads to the accumulation of low-molecular organic acids, methane (CH₄), and hydrogen sulfide (H₂S). These products inhibit microflora, slow down humification, and lead to the formation of peat. Optimal conditions for humus accumulation are periodic moderate moisture and drying at moderate temperatures, which is characteristic of chernozems.

The intensity of humus formation is closely linked to the activity of soil microorganisms. In northern podzolic soils, their numbers are low, but as one moves south, the species composition of the microflora expands. However, excessive soil biogenicity, just like weak biogenicity, hinders humus accumulation. Maximum reserves of organic matter are formed at an average number of microorganisms per unit of nitrogen.

Granulometric composition of the soil also dictates its conditions for humus formation. In clay and loamy soils, decomposition processes are slowed down, which allows humus substances to firmly anchor themselves to highly dispersed mineral particles.

Reserves and qualitative composition of soil humus

Humus is a dynamic self-regulating system of an open type. Its reserves and percentage content in the soil profile strongly depend on the type of soil formation. In the meter-deep layer of soil, humus reserves can vary within very wide limits.

Soil type	Humus content
Serozems	1–2 %
Deep chernozem	10–12 %

The qualitative composition of humus is evaluated by the quantitative ratio of the groups and fractions of its constituent parts. Group composition refers to a set of specific and nonspecific compounds, where each group combines substances related in structure and properties. The first three groups of specific compounds play the most significant role in soil formation and the development of soil fertility.

The most important groups of humus substances:

• Humic acids (HA);
• Fulvic acids (FA);
• Humin;
• Various groups of nonspecific compounds.

Humic and fulvic acids: the basis of soil fertility and buffering

Humic and fulvic acids (collectively referred to as humic acids) are the active part of soil organic matter, which directly affects the soil's adsorption capacity and buffering. They are dark brown nitrogen-containing high-molecular-weight compounds. For an agronomist, these acids are important because they determine the cation exchange capacity, help retain moisture and nutrients in the root zone, and prevent them from leaching into lower horizons.

Humic acids (HA) play a conservative role, ensuring the stability of soil properties for many years. They have a complex aromatic structure: a core of benzene rings surrounded by peripheral chains that firmly hold mineral elements — iron, aluminum, calcium, and phosphates. In an acidic environment with a pH level of 1 to 2, humic acids precipitate, which limits their mobility in highly acidic soils.

• Precipitation of humic acids — pH 1–2
• Sulfur content in HA — up to 1.2%
• Carbon in humic acids — 46–62%
• Oxygen in fulvic acids — 45–50%

Fulvic acids (FA) are much more mobile, as they are completely soluble in water, alkalis, and acids. Their structure is dominated by lateral carbon chains, while the aromatic core is less pronounced. The structural fragment of FA includes two aromatic rings, six carboxyl groups (COOH), two ketone groups (C=O), and two phenolic and three alcoholic hydroxyls. The carbon-to-hydrogen ratio in fulvic acids is always narrower than in humic acids.

Element	Content in humic acids, %	Content in fulvic acids, %
Carbon (C)	46–62	36–44
Oxygen (O)	32–38	45–50
Hydrogen (H)	3–5	3–5
Nitrogen (N)	3–6	3.0–4.5

In addition to the main elements, humic acids contain from tenths of a percent to 1.2% sulfur. It is part of the amino acids methionine and cystine, and is also present in the form of adsorbed sulfates. Phosphorus is represented by hundredths and tenths of a percent in the form of residues of nucleoproteins, phospholipids, and chemisorbed phosphates.

Humin: the insoluble reserve of the soil

Humin is the non-hydrolyzable residue of organic matter that cannot be extracted from the soil by ordinary alkaline solvents. It is the most stable and firmly bound part of soil organic matter. In field conditions, humin performs a structural function, helping to bind soil microaggregates and protecting them from erosion.

Humin consists of two main groups of compounds:

• The plant residues most resistant to decomposition, such as lignin and cellulose.
• Specific humic acids that have become tightly fixed to clay minerals of the soil adsorption complex or have lost the ability to dissolve.

According to the accepted classification of soil scientists, humin includes all specific and non-specific organic substances, as well as their organo-mineral derivatives, which remain in the soil after the leaching of all mobile fractions. This is a long-term carbon pool that provides the base structure of the field.

How organic matter binds with minerals: three forms of humic substances

The fractional composition of humus shows exactly how organic compounds are distributed according to their forms of binding with the mineral part of the soil. This depends directly on the salt content, mineralogical composition, and soil acidity. In practical agronomy, three main forms of humic substances are distinguished: free humic acids, their simple salts (heteropolar), and complex organo-mineral compounds (complex-heteropolar salts).

• Groups of humic substances — 3
• Forms of binding with minerals — 3
• Charge of iron ions in complexes — Fe2+, Fe3+
• Charge of aluminum ions in complexes — Al3+

A separate place is occupied by pro-humic substances — "young" humin-like products that are formed in culture media and during enzymatic synthesis. They are very close to the intermediate products of the decomposition of plant residues, so the boundaries between them are conditional. Although they are not determined in standard analyses, their presence proves that an active process of humification is currently taking place in the soil.

• Free humic acids — humic and fulvic acids not bound to minerals.
• Heteropolar salts — compounds of humic acids with ammonium, alkali, and alkaline-earth metals (humates and fulvates).
• Complex-heteropolar salts — complex alumino- and iron-humic compounds in which metals are firmly blocked within the molecules.

Solubility of humus salts and their effect on soil structure

Physical properties of simple humus salts (humates and fulvates) in the field depend on which cations they have bound with. Sodium, potassium, and ammonium humates are easily soluble in water and leach from the soil, although over time they can become fixed on the surface of mineral particles. Fulvates of any metals (including calcium, magnesium, ammonium, and alkali metals) are soluble at any soil acidity and are easily leached from the root zone, taking nutrients with them. All heteropolar salts of humus possess a high capacity for exchange reactions with any other cations in the soil solution.

Complex (complex-heteropolar) salts are formed through the interaction of humic acids with non-silicate forms of iron and aluminum. Metals bind a portion of the functional groups of humic acids, causing them to lose their ability to absorb other elements. However, some groups remain free, preserving exchange activity. The mobility of such complexes is inconsistent: aluminum and iron fulvates are highly mobile; they form films on structural particles and can precipitate upon the accumulation of iron or aluminum. Aluminum and iron humate salts are more stable and typically accumulate at the site of their formation.

How humus builds the soil profile and retains nutrients

Humus is not merely an indicator of soil fertility, but the primary architect of the soil. During the initial stages of soil formation, organic decomposition products act as agents of biological weathering. Organic acids, phenols, and amino acids gradually break down rocks, converting minerals into a form available to crops. Fulvic acids participate particularly actively in this process, as their aqueous solutions have a highly acidic reaction.

The structure of the soil profile depends directly on the composition of humic substances. In soils where humic acids predominate, a thick humus horizon with high absorption capacity is formed. If, however, fulvic acids predominate in the soil, which is characteristic of conditions with excessive humidity, calcium, magnesium, and potassium are leached into lower horizons. This occurs due to the formation of easily soluble salts, resulting in soil acidification and the destruction of its primary and secondary minerals.

• The share of humus-derived nitrogen in crop nutrition — up to 60%
• Thickness of the humus horizon with humic acid accumulation — from 5–20 to 50–70 cm
• HA/FA ratio during the formation of a pronounced humus horizon — more than 1
• HA/FA ratio in acidic, waterlogged soils — less than 1

The most important task of humus in arable land is the accumulation and gradual release of nutrients. Unlike fertilizers, which can be quickly leached from the root zone, organic matter reliably retains carbon, nitrogen, phosphorus, and microelements. Humic acids also perform a transport function: they bind to metals and clay minerals in mobile complexes, through which nutrients migrate through the profile and are absorbed by plants.

Regulatory properties of humus and its preservation during tillage

The influence of humus on yield consists of several key factors. It binds soil particles into an agronomically valuable structure, improving the water-physical properties of the field. The dark color of humus helps the soil warm up faster in the spring, improving its heat capacity and thermal conductivity. Humic substances also regulate the acid-base balance and optimize the nutritional regime, increasing the availability of mineral compounds for roots.

In addition to nutrition, humus acts as a natural protector for plants. Humic acids and humin strongly bind heavy metals, radionuclides, and residual amounts of pesticides, turning them into low-mobility compounds. Thanks to this, the toxic effect of plant protection products and heavy metals is reduced in humified soils, and the negative effect of excessive mineral fertilizer application is mitigated. At the physiological level, humic compounds stimulate the respiration and photosynthesis of crops, positively affecting the function of chloroplasts and mitochondria.

Why arable soils lose humus

Intensive agriculture leads to the gradual depletion of arable land. In major soil types, humus losses already range from 33 to 50% of their initial stocks. The rate of organic matter mineralization depends on the soil type and the structure of the crop rotation.

Soil or cultivation conditions	Annual humus mineralization in the arable layer, t
Sod-podzolic soils	0.6–0.7
Typical chernozem	0.5–1.0
Areas under row crops	up to 1.5

• Losses from initial humus reserves — 33–50%
• Annual loss of humus reserves in intensive agriculture — about 1.5%
• Organic fertilizer application rate for a non-deficit balance — 6.5 t/ha

The decline in organic matter content in arable soils occurs for several main reasons. The primary one is the replacement of the natural biocenosis with an agrocenosis, resulting in significantly fewer plant residues entering the soil. An additional factor in humus degradation is the systematic application of physiologically acidic fertilizers, which activate soil microflora.

Intensive tillage increases soil aeration, which accelerates the mineralization of organic matter. This same process is activated when draining waterlogged lands. Erosion processes pose a particular danger: as long as erosion is not stopped, the humus content will continue to fall, and the volume of its losses may exceed all other causes of arable land degradation combined.

How to stop degradation and manage humus reserves

To effectively manage the humus status of soils, it is necessary to solve three main tasks. The first is overcoming the organic matter deficit by introducing new types of fertilizers. Livestock farming is unable to fully meet the fields' demand for bedding manure, and the share of non-traditional sources in the fertilizer structure still does not exceed a few percent.

To create a non-deficit humus balance, it is necessary to use additional sources of organic matter of natural origin:

• peat and sapropel;
• straw;
• household and municipal organic waste;
• large-tonnage wood waste (bark, branches, sawdust) and hydrolyzed lignin after composting or special treatment;
• plant residues of industrial crops (e.g., cotton).

The second task is the development of agrotechnical practices for preserving and increasing humus reserves. Based on the kinetic theory of humification, to achieve humus quality close to the reference chernozem type, three rules must be followed:

1. Replenish organic matter reserves in amounts exceeding its losses from mineralization.
2. Reduce unproductive humus losses from erosion and excessive aeration.
3. Accelerate the humification process and increase the completeness of the conversion of plant residues into humic substances.

In practice, these rules are implemented by minimizing tillage, introducing perennial grasses into crop rotation, regulating acidity through liming or acidification, and optimizing mineral plant nutrition. Maximum efficiency is achieved through the simultaneous, complex application of these measures.

The third task is optimizing application methods and incorporation of organic fertilizers. It is necessary to create conditions most favorable for their humification directly in the soil. A set of measures must always be selected individually for each natural-climatic zone based on a detailed study of the conditions of humus formation.