Agrophysical Indicators of Soil Fertility
Agrophysical indicators of soil fertility — a complex of soil properties characterizing particle-size distribution, mineralogical composition, structure, density, porosity, air and water capacity, as well as agrotechnological parameters of soils. Agrophysical indicators directly or indirectly affect all factors of plant life and soil tillage conditions.
Soil Particle-Size Distribution
Main page: Soil particle-size distribution
Mineralogical Composition of Soils
Fractions of the mineral part of soil that differ in particle-size distribution differ in their content of various minerals. Quartz and feldspars predominate in sands and coarse silt, while muscovite and other micas predominate in fine fractions. In the finely dispersed (< 0.001 mm) silt and colloidal fractions, secondary aluminosilicate minerals are mainly present — montmorillonite, nontronite, halloysite, kaolinite, and illite.
Table. Approximate chemical composition of different particle-size fractions of soil, wt. %.
| Fractions | Silicon | Aluminum | Iron | Calcium | Magnesium | Potassium | Phosphorus |
|---|---|---|---|---|---|---|---|
| 1.0–0.2 | 43.4 | 0.8 | 0.8 | 0.3 | 0.3 | 0.7 | 0.02 |
| 0.2–0.04 | 43.8 | 1.1 | 0.8 | 0.4 | 0.1 | 1.2 | 0.04 |
| 0.04–0.01 | 41.6 | 2.7 | 1.0 | 0.6 | 0.2 | 1.9 | 0.09 |
| 0.01–0.002 | 34.6 | 7.0 | 3.6 | 1.1 | 0.2 | 3.5 | 0.04 |
| < 0.002 | 24.8 | 11.6 | 9.2 | 1.1 | 0.6 | 4.1 | 0.18 |
In sandy and silty soils, the silicon content is higher. As particle size decreases, its content decreases, while the amounts of aluminum, iron, potassium, magnesium, and phosphorus increase. The composition of the highly dispersed fraction also includes soil organic matter. The colloidal and silt fractions of soils are the main source of nutrients for plants and the most active part of the soil in forming the capacity for cation-anion and molecular exchange; they determine structure formation and buffering.
Mineralogical composition determines soil swelling — an increase in soil volume due to the binding of water by colloidal and clay particles in the form of film coatings. Bound water reduces the cohesion of particles. Soil swelling depends on the content of secondary minerals with a mobile crystal lattice.
Management
Mineralogical composition does not undergo significant changes during long-term agricultural use of land, which makes it possible to build an effective fertility model based on a certain range of changes in soil properties.
Soil Structure
Main page: Soil structure
Thickness of the Arable Layer
Thickness of the arable layer — the depth of the cultivated (arable) soil layer.
Importance
In the cultivated soil layer, 70–90% of the root system of most plants develops. It contains reserves of air and nutrients; microbiological and redox processes, decomposition, and mineralization of organic matter occur most actively in it. The arable layer accumulates irrigation or rainwater.
The arable layer is an intermediary in the soil–plant system, since organic and mineral fertilizers, ameliorants, and artificial structure formers are applied through it.
A thick arable layer makes it possible to:
- create optimal water, air, nutrient, and thermal regimes of the soil;
- provide favorable conditions for the development of the plant root system;
- activate the processes of humification and mineralization of organic matter;
- increase the reserves of water and mobile forms of nutrients;
- reduce energy costs for maintaining optimal soil structure by reducing the number and depth of tillage operations.
The greatest yield increase from increasing the thickness of the arable layer is observed in row crops, especially root and tuber crops, to a lesser extent in perennial grasses and winter cereals, and insignificantly in annual grasses and spring cereals.
Optimal Thickness of the Arable Layer
The optimal thickness of the arable layer is 27–30 cm for most soils. However, it may be limited by the depth of the humus horizon. For example, soddy-podzolic soils with low natural fertility have a humus horizon up to 20 cm, so creating the optimal depth of the arable layer requires high inputs of energy, labor, and time.
Management
Agrotechnical practices for creating a thick arable layer include: liming, application of organic and mineral fertilizers, and gradual deepening of soil tillage.
Constitution of the Arable Layer
Soil constitution, also soil fabric — the ratio of volumes of the solid phase, capillary and non-capillary porosity. Conditions of soil aeration and air exchange with the atmosphere that are optimal for plant growth and development are formed in a soddy-podzolic medium loamy soil at total porosity of 46–56%, non-capillary porosity 18–25%, capillary porosity 28–31%, and solid phase share 44–54% of soil volume. Soil constitution depends on the content of water-stable aggregates, affects its resistance to erosion and compaction, soil regimes, and determines crop productivity.
For chernozem soils, optimal conditions are formed at total porosity of 51–62% and aeration porosity of 15–25%. The optimal ratio of solid phase volume to total porosity is 50:50 for soddy-podzolic soil and 40:60 for chernozems. The limiting porosity for stable aeration, at which a decrease in the yield of cereal crops is observed, is 13–15% of soil volume. At the same time, the oxygen content in moistened soil is at least 20%, and CO2 is no more than 0.2–0.5%.
Creating an optimal fertility model of the arable layer makes it possible to ensure the most favorable soil regimes, which helps increase crop yields. Modeling of the homogeneous and heterogeneous state of the arable layer of soddy-podzolic soil with different thicknesses of the arable horizon showed that potato, maize, and other field crops respond positively to a heterogeneous constitution of the soil profile, in which more optimal agrophysical and agrochemical properties are created in the upper 20-cm layer through fertilizers and lime.
The yield increase of field crops with a heterogeneous constitution of the arable layer and application of high fertilizer rates to a depth of up to 20 cm over 15 years increased from 3.8 to 9.7 thousand feed units per 1 ha compared with the unfertilized background. Under conditions of homogeneous constitution, it increased from 3.4 to 8.9 thousand feed units per 1 ha. Application of fertilizers to a depth of up to 40 cm reduced yield in feed units by 10.8%, which indicates mixing of the arable layer with soil of the eluvial horizon, which is characterized by low natural fertility and does not allow fertility to be restored to the initial level even over a 15-year period.
Table. Yield of field crops depending on the constitution of the 0–40 cm layer of soddy-podzolic soil, t/ha.
| Crop | Years | Without fertilizers | Manure+NPK | ||
|---|---|---|---|---|---|
| Heterogeneous constitution | Homogeneous constitution | Heterogeneous constitution | Homogeneous constitution | ||
| Potato | 1975–1989 | 12.18 | 12.10 | 29.39 | 27.21 |
| Vetch-oat mixture (hay) | 1976–1978 | 3.22 | 2.96 | 6.08 | 5.66 |
| Maize for silage | 1980–1988 | 24.38 | 21.30 | 71.38 | 62.80 |
| Average over 15 years, thousand feed units per 1 ha | 3.8 | 3.4 | 9.7 | 8.9 | |
Soil Density
Main page: Soil density
Soil Stoniness
Soil stoniness — the content of rocky inclusions (stones) in the soil. Rocky inclusions include particles larger than 3 mm.
Classification of Soils by Stoniness
According to the mass fraction of stones in soils, they are divided into:
- non-stony (stones less than 0.5%);
- slightly stony (0.5…5%);
- moderately stony (5…10%);
- strongly stony (more than 10%).
Properties of Stony Soils
The presence of large stones (larger than 100 mm) in the soil poses a danger to agricultural machines, especially tillage and harvesting machines, so such stones are recommended to be removed from the soil with special machines. An increase in stones in the soil increases wear of the working parts of tillage machines. For example, when plowing sandy strongly stony soils, abrasive wear of plowshares is 100–450 g/ha.
Large stones (more than 100 mm in diameter) are removed before tillage.
Physico-Mechanical Properties of Soil
Main page: Physico-mechanical properties of soil
References
1. Klenin N. I., Kiselev S. N., Levshin A. G. Agricultural Machines. Moscow: KolosS. 2008.
2. Khalansky V. M., Gorbachev I. V. Agricultural Machines. Moscow: KolosS. 2004.
3. Bazdyrev G. I., Loshakov V. G., Puponin A. I., et al. Agriculture. Textbook for universities. Moscow: Kolos Publishing House. 2000.
4. Mineev V. G., Sychev V. G., Gamzikov G. P., et al. Agrochemistry. Textbook. / Ed. Mineev V. G. Moscow: Publishing House of the D. N. Pryanishnikov All-Russian Research Institute of Agrochemistry (VNIIA). 2017.
5. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. Agrochemistry. / Ed. B. A. Yagodin. Moscow: Kolos. 2002.
