Classification of soil by particle-size distribution and management of soil fertility

Clay or sandy: how the mechanical composition of the soil determines fertilizer management and moisture

The composition of any soil includes solid particles (mineral and organic substances), moisture, gases, and living organisms. The solid phase is formed during the weathering of rocks and consists of quartz, silicates, clay aluminosilicates, and secondary minerals. Based on the size of these particles, soils are classified into clay, heavy, medium, and light loamy, sandy loamy, and sandy.

Clay and loamy soils are natural storage reservoirs for nutrients. The finest silt particles of clay are rich in potassium, calcium, phosphorus, and iron. They have a high absorption capacity, so applied fertilizers are firmly fixed in the root-inhabiting layer and are not leached out. However, working with such soils is difficult: in a wet state, they are viscous and sticky, while when dry, they become dense, hard, and require increased traction effort during tillage.

Absorption capacity is directly related to the humus content and particle-size distribution. The heavier the soil according to its mechanical composition, the higher its natural potential for soil fertility and nitrogen reserves. According to field research data on sod-podzolic soils in Belarus, nitrogen reserves are distributed strictly in accordance with their mechanical composition.

Medium reaction and soil biota: how to manage absorption capacity

Agrochemical indicators of soil fertility are not only the reserve of elements but also the reaction of the soil solution (pH), as well as the composition of adsorbed cations. Hydrogen ions, which interact with the solid phase of the soil, actively participate in exchange reactions. In well-cultivated soils, calcium (Ca²⁺) and magnesium (Mg²⁺) predominate among the adsorbed cations, while a high proportion of hydrogen (H⁺) and aluminum (Al³⁺) indicates acidification.

• Acidic soils — H⁺ and Al³⁺ cations
• Neutral soils — Ca²⁺ ions
• Alkaline soils — K⁺ and Na⁺ ions

The reaction of the soil solution can and should be managed to create a cultivated arable layer. The acidity of acidic soils is corrected by liming, and excessive alkalinity by gypsum application. This optimizes conditions for plant nutrition and activates soil microflora, which converts organic residues into an available form.

Decomposition of organic matter in the soil is carried out by three main groups of microorganisms: anaerobic bacteria, aerobic bacteria, and fungi. Anaerobes obtain oxygen from oxidized chemical compounds, whereas aerobes and fungi require access to free air. Bacteria are sensitive to environmental conditions — they develop poorly in acidic areas and are suppressed by tannins in leaf litter. Fungi and actinomycetes, on the contrary, easily tolerate an acidic environment and effectively decompose woody organic matter.

Under aerobic conditions, the decomposition of organic matter in the soil occurs quickly, and under anaerobic conditions, it is slow; in the latter case, compounds that are harmful (unoxidized) to cultivated plants may also form. To improve living conditions for plants, i.e., to weaken anaerobic and strengthen aerobic processes, appropriate agricultural techniques and special methods are used (drainage (on waterlogged soils), etc.). However, the anaerobic process, by slowing down the decomposition of organic matter in the soil, thereby contributes to its preservation and more economical use.

The composition of humus varies in different soils. Examples are the soils of the northern forest and steppe natural zones.

In the soils of northern regions under forest vegetation, dead residues (needles, leaves, and others) concentrate on the soil surface. Moisture in its surface horizons throughout the year is significant, and the decomposition of organic litter rich in tannins proceeds mainly with the participation of fungi rather than bacteria. Under these conditions, light and acidic humus is formed, containing few humic substances and a lot of crenic acid, which is easily soluble in water and quickly leached from the soil. The binding capacity of such humus is weak.

In the steppe soils of southern regions, herbaceous vegetation provides more organic matter throughout the soil profile, with a higher nitrogen content and ash content; given the significant amounts of calcium in a warm climate, surface organic matter is decomposed by bacteria in the presence of air. The humus of steppe soils contains many stable, oxygen-rich neutral humic substances that are weakly susceptible to water and are firmly retained in the soil. Such humus gives the soil a dark color (containing melanins and phenolic substances — products of lignin decomposition) and glues its particles into crumbs.

Humus is a product that lacks a precise chemical formula but is characterized by the following properties: its color ranges from dark brown to black; it is practically insoluble in water, dissolves well in weak alkalis, and produces a precipitate upon neutralization; it contains more carbon than plants, animals, and microorganisms, as well as a high amount of nitrogen; it possesses high absorptive and exchange capacity; it serves as a source of energy and food for microorganisms; and it is a constant source of carbon dioxide in the soil.

Soil humus is of great importance for agriculture as a primary source of plant nutrients; carbonic and other acids formed during the decomposition and synthesis of organic matter dissolve the salts of the mineral part of the soil, making them available for plant nutrition. The mobile fraction of humus forms a number of compounds that are easily mineralized and serve as a source of nitrogen nutrition for plants. Humus also improves the physical properties of the soil, and its role in soil structuring is especially significant — its ability to form individual crumbs of various shapes, sizes, and durability. The stronger the structural aggregates of the soil, the more fertile it is. 82 plant productivity and harvest formation

The soil mass always contains secretions and accumulations of various substances of chemical and biological origin that have arisen during the process of soil formation, known as neoformations. Neoformations of biological origin include the passages and tunnels of worms, granular glomeruli of worm excrement, burrows of moles, gophers, and other burrowing animals, etc. The soil structure is also influenced by the root systems of plants and physical factors such as fluctuations in soil temperature and humidity, freezing and thawing, etc.

Soil contains a certain amount of moisture, mainly due to precipitation. In addition, soil wetting occurs through the upward movement of water via capillaries if the groundwater level is not far from the soil surface, as well as through vapor condensation (from the air and deep layers of the earth). Water vapor, moving from warm layers of the soil to colder ones, cools and condenses into tiny droplets, forming intra-soil dew.

Soil moisture dissolves various salts, including those necessary for plant nutrition, and interacts with soil air, microorganisms, plant roots, etc., thus turning into a soil solution. The pH reaction of the soil solution (acidic, alkaline, neutral), its ionic composition, and many other properties depend on the quantity (concentration) and composition of the substances dissolved in the soil moisture.

Air in the soil (the gaseous phase of the soil) is located in its pores and is partially absorbed by the surface of soil particles. The amount of air in the soil is in an inverse ratio to its moisture content. Soil air contains components necessary for plants: oxygen, carbon dioxide, and nitrogen. Plants are in great need of a high oxygen content in the soil: high concentrations of carbon dioxide inhibit the respiratory and absorptive functions of the roots.

Atmospheric nitrogen, as a rule, is inaccessible to plants; only nitrogen-fixing microorganisms living in the soil can supply it to the roots. composition, structure, types 83

Soil air differs in composition from atmospheric air. It contains significantly more carbon dioxide and less oxygen. Soils rich in organic matter contain especially high amounts of carbon dioxide.

Water-air regime: how soil structure saves roots from stress

In structureless and waterlogged soils, water completely displaces air from the pores. Without oxygen, plant roots and beneficial aerobic bacteria begin to suffocate, and an excess of carbon dioxide accumulates in the soil. When drying out, such land turns into a dense monolith where all voids are filled with air, causing seedlings to suffer from dehydration. Only a strong crumbly-porous soil structure helps to resolve this conflict between water and air.

In structured soil, water is held by capillary forces within soil aggregates, while air circulates freely in the spaces between them. This balance is maintained even during periods of maximum humidity. The reference point here is the field capacity — the state where soil particles retain the maximum amount of moisture after excess gravitational water has drained away (this is taken as 100%).

When capillaries rupture, a root hair quickly drinks water from the isolated area accessible to it and dies off. Under normal conditions, a single root hair functions for 10 to 15 days. With a moisture deficit, this period is reduced to 3–4 days or even a few hours. The plant has to expend photosynthesis resources on constantly growing new roots in search of water instead of forming a harvest. The suction system of a single plant is colossal: the total length of root hairs reaches an average of 3–4 km.

• Lower limit of optimal humidity — 60%
• Average root hair length — 1 mm
• Root hair length in Poaceae — 1.5 mm
• Number of root hairs in corn per 1 mm² — 1900 units
• Total length of root hairs in pumpkin — 25 km
• Suction surface area of wheat roots per 1 ha — 100,000 m²

Genetic horizons: what the profile section tells us

Any soil in a cross-section is clearly divided vertically into genetic horizons. To quickly assess the potential of a field, an agronomist must analyze the morphological characteristics of these layers. Key indicators for assessment under field conditions include:

• layer color;
• soil structure;
• fabric (density);
• presence of neoformations;
• foreign inclusions;
• total depth and structure of genetic horizons.

At the top is the humus-accumulative horizon A. If there is sod or undecomposed residues on the surface, they are designated as layer A0. Horizon A has a dark color, its thickness ranges from a few centimeters to 1.5 m or more. Since precipitation constantly leaches part of the substances from it downwards, it is also called humus-eluvial. In zones where leaching prevails over accumulation, this horizon becomes purely eluvial.

Below lies the illuvial horizon B — the zone of illuviation. It is always denser, often cracks, and has a coarse nut-like or columnar-prismatic structure. Here, all elements leached from above accumulate. Beneath it lies horizon C — the parent material, which is practically unaffected by the soil formation process.

The color of each layer indicates its chemical composition. Humus provides dark and gray tones; iron oxides color the earth red, orange, and yellow. Carbonate lime, aluminum oxide hydrates, and kaolin lighten the soil, while ferrous iron compounds give it grayish-blue and bluish tints. The ratio of these components determines the final soil type — from marshy and podzolic to steppe and lateritic.

When considering the soil types of the globe, it should be kept in mind that all of them are merely stages, steps in the development of a single soil formation process. In its natural development, each soil goes through these stages associated with the factors of soil formation. Externally and most clearly, this connection manifests itself in a certain dependence Fig. 11. Schematic meridional profile of the main soil types of the CIS: a: dependence of nitrogen content on humus reserves; b: 1 — tundra gley soil; 2 — sod-podzolic; 3 — gray forest;

4 — typical chernozem; 5 — chestnut; 6 — brown; 7 — sierozem; 8 — krasnozem 86 productivity of plants and harvest formation of soils and their vegetation cover on the climate. The subtropical climate creates one type of soil, the moderately warm climate — another, and the cold climate — a third. Accordingly, as a result of the impact of climate and other soil-formation factors, vegetation is formed on one soil or another. The factor of time or the age of the soils plays an important role here. Thus, ultimately, for a given historical period, under the influence of soil-forming factors, the zonality of soil types is formed, with features inherent to each of them.

In Belarus, more than half of agricultural land is represented by light soils. In the composition of arable land, they occupy 56%, including 42.4% — sandy loam and 13.6% — sandy. Their specific weight is higher in the Brest (77.4%) and Gomel (70.7%) regions and lower in the Minsk (47.8%) and Vitebsk (32.1%) regions.

These soils warm up faster in the spring than others, which is why they are called warm soils. They are easy to till, have good aeration, possess well-defined water permeability, and processes of decomposition of organic residues take place vigorously in them, with the release of nitrogen and ash plant nutrients.

Light soils are also characterized by a number of negative properties. They are poor in humus and contain few colloids and nutrients. They have a spontaneous water regime, which depends on the pattern of precipitation. Due to increased water permeability and low water-holding capacity of the soil, atmospheric precipitation does not linger in the root zone but filters into the underlying horizons. Therefore, in the absence of precipitation, plants suffer more from a lack of soil moisture.

The rapid decomposition of organic residues and fertilizers and the low absorption capacity of light soils lead to significant losses of nutrients. The process of decomposition of organic matter, which is well-pronounced in such soils, excludes intensive humification processes and the accumulation of humus in them. Methods for improving soil fertility of these soils should primarily be aimed at providing them with organic matter. Organic fertilizers play the most important role in the cultivation of such soils.

Heavy soils account for 32% of cultivated land in Belarus, of which more than 25% are medium-loamy and light-loamy soils, and about 5% are clay and heavy-loamy soils.

Medium- and light-loamy soils are located mainly to the north of the Orsha — Minsk — Grodno line and, to a lesser extent, in the Brest and Gomel regions. These soils are weakly to moderately podzolized.

Loamy and clay soils offer great resistance during tillage. They are often weakly permeable and are capable of holding a lot of moisture for a long time. In such soils, more humus and nutrients accumulate compared to light soils. composition, structure, types 87

The most important measure ensuring high yields on light and heavy soils is the development and adherence to scientifically grounded crop rotation. When developing them, it is necessary to take into account the specific characteristics of these soils.

Peat soils of Belarus are located in 7 thousand plots. Their area is about 2.7 million hectares. These soils are unevenly distributed throughout the territory of the republic. Large massifs (over 67%) are concentrated in the Vitebsk, Minsk, and Brest regions, while smaller ones (about 17%) are found in the Grodno and Mogilev regions.

In Belarus, approximately 40% of the total area of peat bogs has been drained. Unlike mineral soils, on which crops have been cultivated for tens of thousands of years, peat soils have been used for these purposes relatively recently. This is due to the more complex conditions for developing such lands. Peat soils differ significantly from sod-podzolic soils. They have a shorter frost-free period, which limits the possibility of cultivating thermophilic crops. The conditions of the water regime are also peculiar. During rainy periods on drained lands, groundwater levels rise into the root zone, which limits or completely excludes normal harvests, for example, of winter cereal crops.

The abundance of nitrogen available to plants in the first years after drainage and the poverty in potassium, phosphorus, and trace elements, as well as the difficulty in balancing them, lead to the rapid development of vegetative mass to the detriment of the productive one, which limits the possibilities for, for example, cereal crops. On the other hand, this favors the development of forage crops, such as perennial grasses. The exploitation of drained peat-bog soils often leads to their degradation.

At present, when the area of used peat soils in Belarus has reached 1.5 million hectares and a certain scientific and practical experience has been accumulated, the need has arisen and conditions have been created for the development of theoretical foundations and practical methods of farming systems on these soils.

Rules for working on peat soils: crop rotation and water level regulation

Peat soils require a special approach to crop rotation planning. If perennial grasses are constantly grown on them, a thick sod forms in the soil. Organic matter accumulates in huge quantities but is only partially mineralized. To prevent these processes from stagnating, grassland farming must be periodically interrupted by the cultivation of annual plants.

The most suitable for this are grain-forage crops:

• barley;
• oats.

For cereal crops on arable land, a deeper lowering of the bog water level is required. Perennial grasses on cultivated grassland, on the contrary, develop better with higher moisture levels. The optimal parameters for both methods of use are given in the table.

Special caution should be exercised with row crops, as the environmental situation on drained peatlands is unfavorable for them. Potatoes on such lands can produce high but unstable annual yields due to frequent frosts. Moreover, the tubers turn out to be practically unsuitable for food purposes, and the crop itself triggers intense mineralization of organic matter. Among the row crop group, only vegetable crops are justified on peat soils, as they provide a guaranteed harvest and are in demand on the market.