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Agrophysical indicators of soil fertility

Agrophysical indicators of soil fertility is a complex of soil properties characterizing granulometric, mineralogical composition, structure, density, porosity, air and moisture capacity, as well as agrotechnological parameters of soils.

Agrophysical indicators of fertility are the basis for creation of optimal conditions of water, air, thermal and nutrient regimes for plant life.

Agrophysical indicators, except for granulometric and mineralogical composition, are characterized by their dynamism during vegetation period, making their reproduction difficult.

Granulometric composition of soils

Solid phase of soil is a mixture of mechanical fractions: mineral, organic and organic-mineral. Mineral soils contain mainly mineral mechanical particles with different size, shape, chemical and mineralogical composition.

Granulometric composition is the relative content of the mechanical fractions in the soil. It is the factor of fertility of arable soils, which affects the productive capacity.

Particles of the mechanical fraction is subdivided into:

  • more than 1 mm in diameter – soil skeleton;
  • less than 1 mm – fine soil, subdivided also into:
  • particles more than 0.01 mm – physical sand;
  • particles less than 0.01 mm – physical clay.

Depending on the ratio of physical sand and clay, soils are divided into:

  • sandy;
  • sandy loam;
  • loamy (light, medium, heavy);
  • clayey (light, medium, heavy).

Depending on the resistance during tillage, soils are divided into:

  • light (sandy and sandy loam);
  • medium (light and medium loamy);
  • heavy (heavy loamy and clayey).

Chemical composition varies depending on the particle size distribution. With a decrease in particle dispersion the oxygen content increases sharply and the content of iron, calcium, magnesium, aluminum, potassium and sodium decreases. 

Granulometric composition affects on:

  • Absorption (sorption) properties: the more fine particles in the soil, and accordingly, the higher their specific surface area, the higher the absorption capacity, moisture capacity, hygroscopicity, plasticity, stickiness.
  • Density of soils: as the share of physical sand increases density decreases. Density of 1.0-1.3 g/cm3 is considered optimal for most crops.
  • Structuring: The fraction of particles smaller than 0.001 mm is characterized by high coagulation and absorption capacity, so it accumulates the greatest amount of humus and ash elements of nutrition, being the most valuable component of loose soils.
  • The onset of physical ripeness, that is, the ability of the soil to crumble into small clods at a certain moisture content. Soils of heavy granulometric composition mature later than light ones.
  • Plasticity is determined by the content of physical clay. As the proportion of physical clay increases, the plasticity limit expands.
  • Hardness. High hardness increases the resistance of the soil to the working bodies of tillage machines and hinders the growth of seedlings and plant roots.
  • Stickiness is a technological property of soil. It increases with high content of physical clay, worsening the quality of tillage.

The most favorable combination of agrophysical, agrochemical and biological indicators of fertility is noted in soils of medium granulometric composition. The influence of granulometric composition on fertility can vary greatly depending on other indicators. For example, for sod-podzolic soils formed in the zone of sufficient or excessive moisture, the optimal is a light granulometric composition, while the highest fertility of black earth, observed on soils with heavy granulometric composition.

The granulometric and mineralogical compositions do not undergo significant changes during long-term agricultural land use, which allows to build an effective fertility model based on a certain range of changes in soil properties. Granulometric composition does not require reproduction, except for protected soil and small areas where it is possible to change it by applying sand or clay.

The genetic properties of soils and their granulometric composition determine the potential yield of crops.

Table. Maximum possible yields of agricultural crops depending on the granulometric composition of the soil*, t/ha[1]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Crop
Soil
loamy
sandy loam and sandy, underlain by moraine
sandy loam, underlain by sands
sandy
Winter rye
5.0-5.5
4.5-5.0
4.0-4.5
3.0-3.5
Winter wheat
5.5-6.0
5.0-5.5
4.5-5.0
3.5-4.0
Barley
6.0-7.5
5.5-6.5
5.0-6.0
4.0-5.0
Oats
5.0-6.0
4.5-5.5
4.0-4.5
3.0-4.0
Potatoes
40.0-50.0
35.0-45.0
30.0-40.0
25.0-30.0
Sugar beet
50.0-60.0
45.0-50.0
40.0-45.0
30.0-35.0
Flax (fiber)
1.8-2.2
1.7-1.9
-
-

* Maximum possible is a term applied to certain varieties and accepted agricultural practices. In fact, the potential of plants, which is the maximum, is several times higher

Mineralogical composition of soils

Different in granulometric composition fractions of the mineral part of the soil differ in the content of different minerals. Quartz and feldspars prevail in sands and coarse dust, muscovite and other mica prevail in fine fractions. The fine-dispersed (<0.001 mm) silt and colloidal fractions mainly contain secondary aluminosilicate minerals – montmorillonite, nonthronite, halloysite, kaolinite, and illite.

Table. Approximate chemical composition of different granulometric fractions of soil, mass %[2]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

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 dusty soils, silica content is higher. Its content decreases with decreasing particle size, and the amount of aluminum, iron, potassium, magnesium and phosphorus increases. Highly dispersed fraction also includes organic matter of soil. Colloid 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 of cation-anion and molecular exchange, determine structure formation and buffering.

Mineralogical composition determines soil swellability – increase in soil volume due to binding of water by colloidal and clay particles in the form of film shells. Bound water reduces the bonding force of the particles. Soil swellability depends on the content of secondary minerals in the mobile crystal lattice.

Soil structure

Soil aggregates are the solid soil particles clumped together.

Soil structure is the ability to form soil aggregates.

Soil structure is a physical structure of solid phase and pore space of soil, which is conditioned by sizes, shape, quantitative ratio, character of interrelation and arrangement of mechanical elements and aggregates consisting of them. It is an important agro-physical indicator of fertility that determines water, air, physical-mechanical and technological properties as well as water-hydrological constants.

Classification

According to S.A. Zakharov’s classification, depending on the shape of the aggregates, the following types of structures are distinguished:

  • lumpy,
  • crumbly,
  • nutty,
  • granular,
  • columnar,
  • prismatic,
  • platy,
  • lamellar,
  • leafy,
  • scaly.

In the natural state black earths have granular structure, gray forests have nutty structure, well-cultivated sod-podzolic soils have crumbly structure, uncultivated podzols have platy and leafy structure.

Structural aggregates are classified according to their size into: lumpy structure – lumps of more than 10 mm, macrostructure – from 0.25 to 10 mm, microstructure – less than 0.25 mm.

Structurization

According to M.A. Kachinsky, the structural formation of soil aggregates is a process of mutual deposition (coagulation) and electrolytic coagulation of colloidal particles on the background of more general physical-mechanical, physical-chemical and biological factors.

Physico-mechanical factors of structure formation

The formation of individual clods occurs due to mechanical separation in natural conditions under the influence of the root system of plants, the vital activity of soil microflora, under the influence of wetting and drying, periodic freezing and thawing. On cultivated lands the impact of tillage implements is added to the above factors.

The influence of vegetation is due to the impact of the root system. Strong root system, for example, perennial grasses, has more influence on structure formation than less developed annual crops. Structure formation process under influence of vegetation consists of two stages: splitting of soil mass by root system to separate structural parts and aggregation (gluing) them by products of decomposition at the expense of root excretions and residues.

Structural formation manifested in periodic freezing and thawing under optimal soil moistening is caused by simultaneous freezing of non-capillary and capillary water in pores.

Soil cultivation in the state of physical ripeness makes a significant contribution to structure formation under the action of tillage implements.

Physico-chemical factors of structure formation

Physico-chemical factors of structure formation are conditioned by interaction of cations with soil colloids, interactions between colloids and their nature.

For example, the water resistant structure increases with irreversible coagulation of colloidal particles with cations of divalent and trivalent metals (Ca2+, Mg2+, Fe3+, Al3+), while monovalent metal cations (K+, Na+) due to reversible coagulation decrease the water resistant structure.

The strength of structural aggregates is affected by the nature of colloids formed by mineral and organic colloids. However, the water resistance of these aggregates varies greatly: soil aggregates cemented by organic colloids (humates of two- and three-valent metals) are characterized by the greatest water resistance. Mineral colloids formed from montmorillonite and hydromica are more water resistant than those formed from quartz, silicic acids and kaolinite.

Aluminum and iron hydroxides play an important role in formation of lateritic soil structure.

Change of reducing processes by oxidizing ones under temporary overwetting refers to chemical factor of structure formation. According to data of N.A.Kachinsky, soil structure formed under influence of chemical factors, as a rule, is not water resistant.

Biological factors of structure formation

Biological factors of structure formation are manifested under the influence of plants and soil biota.

The humus substances formed during decomposition of plant residues have high sorption and biological activity.

Around the root system of plants – rhizosphere – specific communities of soil biota are formed, as a result of the vital activity of which substances affecting soil structuring are formed.

The biological factor has a quantitative and qualitative influence on soil structure.

Relationship between soil structure and other agrophysical indicators of fertility

Soil structure is interconnected with other agro-physical indicators of fertility: structure of arable layer, density.

The structure of the arable layer is the ratio of the volumes of solid phase, capillary and non-capillary porosity in the soil with undisturbed structure. Capillary porosity of aggregates in structured soil is supplemented by non-capillary porosity due to interaggregate gaps, making total porosity.

Soil density is the ratio of the mass of undisturbed soil to its volume. Under action of compaction and loosening forces in natural conditions there comes equilibrium state between porosity and solid phase called equilibrium density. In structural soils the gap between optimal and equilibrium density is minimal, and in well-cultivated soils they may coincide, for example, in black earths.

Processes of destruction of soil structure

The processes of structure formation and fracture are in a dynamic state in cultivated soils. Factors of destruction of soil structure are manifested in weakening of forces, which glue soil particles into aggregates and lead aggregates to separate-particle state. Along with factors of structure formation, there are similar factors of destruction of soil aggregates.

Mechanical factors are destruction of structure by farm implements, wind, rain, grazing, etc.

Physical-chemical factors – destruction of structure as a result of cation-exchange reactions. For example, replacement of calcium and magnesium ions in the soil under conditions of leaching water regime, and their replacement by H+ and NH4+ ions contained in rainwater, leads to leaching of calcium and magnesium outside the arable layer.

Biological factors – destruction of the structure as a result of the vital activity of soil microorganisms that use organic matter as a source of nutrition and energy, which leads to its mineralization. Considering that the best bonding properties are shown by organic compounds, their mineralization leads to the destruction of aggregates.

Mineralization of organic matter is facilitated by liming of soils, application of mineral fertilizers, and mechanical treatment. 

Optimal soil structures for fertility

S.I. Dolgov and P.U. Bakhtin proposed the following parameters for evaluating soil structural condition:

  • excellent structure – the content of water resistant macroaggregates more than 70%;
  • good – 70-55%;
  • satisfactory – 55-40%;
  • unsatisfactory – 40-20%;
  • poor – less than 20%.

The fine-grained and granular structure with particle size of 0.25-10 mm is of the greatest agronomic interest. It should also have optimal porosity, mechanical elasticity and water resistance.For example, the illuvial horizon of sod-podzolic and black earth soils have a water resistance of less than 20%. For example, the illuvial horizon of sod-podzolic and black earth soils have a water-resistant structure, but with a low degree of porosity. This makes them agronomically unfavorable and uncharacteristic for fertile soil.

Microstructure of soils is also important as it has optimum water-air properties, which makes it necessary to consider this characteristic on a par with macrostructure.Microstructure is found, for example, in gray earth soils, whose absorbing soil complex contains many colloidal particles and is saturated with calcium.

Optimal sizes of macro- and microaggregates for arable soils are relative. In humid conditions they are 1-3 mm in diameter, in arid – 0,5-1 mm, in conditions of erosion danger – to 1-2 mm in diameter. Optimal soil structure allows to reduce moisture losses for evaporation.

Optimal ratio of solid phase volume and total porosity achieved in structured soil is about 1:1 (50:50%) for sod-podzolic soil, in black earth porosity is up to 60% and more of soil volume. Sustainable maintenance of agronomically most favorable structure of arable layer for a long time is possible on soils with high content of water resistant aggregates.

Reproduction of soil structure

To reproduce the structure of the soil is necessary:

  1. Replenishment of soil organic matter by applying organic fertilizers (manure, peat, compost, straw, siderates, sapropel, poultry manure) as a source of humus and energy for microorganisms, by sowing perennial grasses that leave large amounts of plant and root residues. The application of mineral fertilizers, indirectly also has a positive effect, increasing crop yields and thereby increasing the amount of plant and root residues in the soil.
  2. Replenishment of soil reserves of calcium and magnesium as the main elements involved in structure formation. This is done by lime treatment of acidic soils or gypsum treatment of saline soils.
  3. Reducing the number of passes of agricultural machinery on the fields, to prevent soil compaction. This is achieved through the use of resource-saving cultivation technologies and lightweight machinery.
  4. Prevention of water and wind erosion.
  5. Use of agromelioration techniques (drainage or irrigation) to create favorable conditions for the course of oxidation-reduction processes in soils with excessive or insufficient moisture.
  6. Application of surface artificial and environmentally safe structure-forming agents.

Arable layer thickness

The thickness of the arable layer is the depth of the cultivated soil layer, in which 70-90% of the root system of plants develops. Water, air and basic nutrients are concentrated in it, microbiological, oxidation-reduction processes, decomposition and mineralization of organic matter are most active. Arable layer is a mediator in soil-plant system, as organic and mineral fertilizers, ameliorants and artificial structural formers are introduced through it.

The arable layer accumulates irrigation or rainwater, storing it for a long time. Increase of water and nutrients storage is achieved by increasing the depth of the arable layer.

A strong (deep) arable layer allows:

  • create more favorable water, air, nutrient and thermal soil regimes;
  • activate the processes of humification and mineralization of organic matter;
  • increase the content of mobile forms of nitrogen, phosphorus and potassium;
  • reduce energy costs associated with maintaining a favorable soil structure in the arable layer by reducing the number and depth of tillage methods.

In general, a strong arable layer, allows you to create more favorable conditions for the development of the root system of plants, which leads to a steady increase in crop yields.

Crops respond differently to the deepening of the topsoil. The greatest effect in increasing the yield from a thick plowing layer is observed for row crops, especially for root and tuber crops, to a lesser extent – for perennial grasses and winter cereals, and insignificantly – for annual grasses and spring cereals.

The optimum 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, sod-podzolic soils with low natural fertility have a humus horizon up to 20 cm, so to create the optimal depth of the arable layer requires large expenditures of energy resources, labor and time.

Agrotechnical methods to create a strong topsoil include the use of liming, organic and mineral fertilizers, and gradual deepening of tillage.

Sources

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 с.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov, etc., ed. by V.G. Mineev. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.