Within the soil profile, a variety of mineral and organic substances are present. The soil in its composition has both living biomass, and all kinds of chemical compounds. Without knowledge of their properties, an agronomist cannot effectively manage fertility and increase productivity. Therefore, the study of soil organic and mineral substances has long attracted the attention of agricultural experts. In recent years, interest in the study of plant and animal residues in the composition of the upper layer of land has especially increased. Our article will also discuss this.
Definition and sources
So, the organic matter of the soil is the totality of all living biomass in the form of humus and the remains of plants and animals. It plays a key role in the formation of soil properties, which are associated with phytosanitary functions and the development of fertility.
The main sources of organic matter in the soil are dead plants in the form of root and aboveground masses. In a smaller amount, the remains of the fauna come. Their ratio depends on local conditions and the composition of zonal vegetation. Thus, a small fraction of organic residues is characteristic of tundra soils. Then it grows to the taiga, forests and forest-steppes. Upon transition to the steppe zone, the litter decreases due to the dry climate, but the share of the root fall falls. In desert regions, the proportion of organic residues is minimal, and in the forests of the tropics and subtropics it again increases sharply.
The nature of the flow of litter into the soil
Organic residues penetrate into the soil profile differently: in forests, their main quantity enters the layer surface, and in grassy areas directly inside in the form of dead roots. The further processes of its transformation depend on the nature of the fall of soil into the soil. The chemical composition of solids contains proteins, carbohydrates, waxes, resins, lignin and other substances. It also contains ash elements: silicon, potassium, magnesium, calcium, sulfur, phosphorus, iron and several others.
The fastest humification and mineralization is subjected to litter, which is rich in bases (magnesium, calcium) and substances readily available to microorganisms (amino acids, proteins, soluble carbohydrates). Plant residues containing many resins, lignin and tannins decompose slowly. As for the litter of cultivated plants, among them, the remnants of legumes are the fastest to be transformed, and the straw of cereal grasses is the slowest.
Gumification
Coming into the soil, organic residues undergo various transformations, including biochemical changes under the influence of microorganisms and grinding of soil fauna. The main direction of such transformations is humification. Currently, there are three options for its process.
- The condensation (polymerization) concept distinguishes three stages of humification. First, from the dead microorganisms and decay products of plants, structural units are formed that serve to form humic substances. Then, through the oxidation of phenols to quinones, structural units are condensed. Humification ends with a polycondensation step.
- The concept of biochemical oxidation considers the process of conversion of organic residues as a reaction of slow enzymatic oxidation, which results in the formation of high molecular weight humic acids, which are then subjected to aromatization.
- The biological concept characterizes humic substances as a product of the synthesis of various microorganisms and suggests that the humification process includes both polymerization and condensation reactions and biochemical oxidation reactions. The composition of the already formed humus is constantly updated by the inclusion of individual fragments of organic compounds in its molecules.
Composition Characteristics
The formation of soil organic matter occurs by combining the organic residues of dead organisms and their humification products. The first group includes parts of plants and animals visible to the naked eye, as well as a small fraction of substances of certain classes of organic compounds (carbohydrates, amino acids, proteins, tannins, sugars, enzymes).
The main organic substance of the soil is humus - a mixture of nitrogen-containing high molecular weight organic compounds of different properties and composition. According to extractability from the soil layer and solubility, humic substances are divided into fulvic acids, humine and humic acids.
Fulvic acids are the most soluble and less complex group. They have lower molecular weights and high migration ability. This is the brightest part of humus, prevailing in the podzolic, red earth, and gray earth soils of the tropics. Humin is a substance that is not extracted from the soil layer with alkalis and acids. It is most strongly associated with clay minerals. Humic acids are an insoluble part of humus, characterized by a more complex structure, high molecular weights and an increased carbon content. They prevail in chestnut, turf, chernozem and gray forest soils.
Labile and stable group
In addition to the above characteristics of the composition of the organic matter of the soil, there is its division into labile and stable parts. The first is composed of mobile forms of humus (water-soluble and poorly fixed by minerals), pre-humus fraction and plant residues. The labile group acts as the main source of food and energy for soil biota. It was also found that the remains of plants improve the physical and mechanical properties of the soil layer.
A stable group consists of humic substances, which are firmly fixed by compounds of minerals (humic-clay complexes, calcium humates, etc.). This is a slowly mineralizing, stable part of the soil organic matter. For it to be completely renewed, millennia are needed. Stable humus is a potential reserve of various nutrients, but its greatest agronomic importance is the formation of favorable physical, mechanical and water-air properties of the soil, as well as the fulfillment of its sanitary-protective functions.
The role of organic matter in the soil
A large number of elementary soil processes occur with the participation of humic substances. These are eluvial, biogenic-accumulative, metamorphic and other EPPs. Reserves, composition and content of organic matter in the soil are the main indicators of fertility and affect all its properties and regimes.
Living biomass is a source of ash elements and nitrogen necessary for plant nutrition. Some of these substances are absorbed by the flora during ion-exchange reactions or are in an absorbed state. Another part becomes available to plants after the release and mineralization of organic substances.
In soil, humus acts as an optimizer of physicochemical properties. More humus layers have a high buffering capacity for oxidation-reduction, acid-base effects and toxicants. Cations absorbed by organo-mineral colloids become accessible to plants and intensively nourish them.
Organic matter also affects the structure, mechanical and physical properties of soils. The higher the humus content, the lower the density, the better the structure and water resistance of structural aggregates, the more optimized are hardness, ductility, stickiness and resistivity. Due to humus, the soil gets a dark color, which contributes to the absorption of heat.
Fertility effects
The organic matter of the soil plays a leading role in its biological regime, contributes to the preservation of microorganisms in it and creates comfortable conditions for their functioning. The high biological activity of the soil layer leads to a decrease in pathogenic microorganisms and accelerate the microbiological degradation of pesticides.
The main role in the formation of fertility is played by humic substances - the final products of humification. The litter of the aerial parts of dying plants creates a litter layer on the soil surface. Its annual amount is not the same in different zones and types of vegetation.
Plant litter processing
The litter stock depends on the decomposition rate. If the litter is rich in tannins and is strongly lignified, it decomposes much more slowly than the remnants of hardwood. Many animal organisms that absorb litter as food are involved in litter decomposition. In broad-leaved forests, earthworms come into play, and on acidic soils of coniferous forests, plant litter is mainly processed by mushrooms.
No less important in the formation of humus are dying roots. According to their mass, meadow steppes and broad-leaved forests come first, followed by subtropical and humid tropical forests and, finally, deserts. A high supply of dying roots under the steppe grass vegetation is due to the predominance of thin and easily decomposing plants. This humus provides high fertility of chernozem steppe soils.
Factors affecting humus formation
The content of organic matter in the soil is largely dependent on temperature. This explains the lack of humus in tropical areas, where at high humidity and high temperatures, saprotophores powerfully process residues. In the tundra, on the contrary, the activity of heterotrophic organisms is very small, and plant residues practically do not decompose.
Where the mineralization of organic matter occurs quickly, mineral elements are released in a short time and again become available to green plants. This leads to the formation of large phytomass, but also an increased risk of leaching of mineral substances from the soil.
Biological cycle
Fertility largely depends on how quickly the elements taken away from it return to the soil. Some substances are lost, leaving through a drain with draining water or entering the atmosphere. But processes such as nitrogen fixation, dust deposition, ongoing weathering, partially restore the lost elements.
In general, green plants give more to the soil than take from it. They remove relatively few dissolved compounds, but return a significant amount of organic substances: lignin, fats, cellulose, sugars, starch, proteins and so on. Thanks to this, many animals and organisms that feed on these animals get the opportunity to develop in the soil.
Finally
So, the organic matter of the soil, in fact, is a complex of living biomass, which is part of it. The presence of organic compounds distinguishes soil from parent rocks. Living biomass is formed as a result of decomposition of animal and plant materials and is the most important link in the metabolism of inanimate and living nature. The organic matter of the soil largely determines its biological, chemical and physical properties, as well as fertility.