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Agrochemical indicators of soil fertility

Agrochemical indicators of soil fertility – a set of properties that characterize the ability of the soil to provide plants with elements of nutrition and the optimal nutrient regime.

Nutrient regime of soils

Nutrients come from the soil solution, which is in constant equilibrium with the soil solid phase. The rate of this process is very high and depends on the concentration of substances. As a consequence the composition of soil solution is highly dynamic.

Content of available forms of nutrients is influenced by their gross stock in soil. Soil microflora, especially living in the root zone (rhizosphere) has a significant impact on the translation of gross stocks into available forms.

Soil composition

Main article: Soil composition

Soil composition largely determines the agrochemical indicators of soil fertility. The composition is usually divided into three phases:

  • gas, or gaseous, phase;
  • liquid phase, or soil solution;
  • solid phase, which is divided into mineral part and organic part (soil organic matter).

Nitrogen content in soil and its availability

Sources of nitrogen intake and its transformation in the soil

Natural sources of nitrogen intake are: activity of nitrogen-fixing free-living and nodule bacteria and intake with atmospheric precipitation.

The process of nitrogen fixation is carried out by free-living anaerobic bacteria Clostridium pasterianum, aerobic Azotobacter croococcum and Rhizobium nodule bacteria living in symbiosis on the root system of legumes. Their activity and efficiency of nitrogen fixation are influenced by the provision of carbohydrates, phosphorus, calcium and other elements, the reaction of the soil environment, temperature, moisture. They accumulate 5-15 kg of nitrogen per 1 hectare during the year. Some algae and fungi in symbiosis with plants also have nitrogen fixation ability.

Bacteria of Azotobacter group thrive in aerated, cultured, well-warmed, neutral soils with phosphorus and calcium. Under favorable conditions, they accumulate up to 30 kg of nitrogen per hectare.

Bacterial strains and races of Rhizobium group are specific for each legume species. Nitrogen fixation efficiency depends on plant species, agrotechnics, soil and a number of other conditions. Under optimal conditions these bacteria can accumulate in symbiosis with: alfalfa – 250-300 (to 500) kg of nitrogen per ha, lupine – 160-170 (to 400), clover – 150-160 (to 250), soybeans – 100, vetch, peas, beans – 70-80 kg of nitrogen per 1 ha. Their activity is positively affected by the introduction of organic and phosphorus fertilizers and liming of the soil.

Introduction of leguminous crops into crop rotation contributes to the increase of nitrogen reserves in the soil.

With atmospheric precipitation annually in the form of ammonia and nitrates formed under the influence of lightning discharges, 2-11 kg of nitrogen per 1 ha is received.

Natural sources of nitrogen are of practical interest, but their quantity is much less than the amount of nitrogen taken out with a crop. Therefore, the reproduction of soil nitrogen reserves requires the application of organic and mineral fertilizers.

Humus reserves, which contain about 5% of nitrogen, play an important role in providing plants with nitrogen. Mineral forms of nitrogen account for about 1-3%. According to the data of I.V. Tyurin, the reserves of humus in a meter layer of soil per 1 ha are: gray soils – 50 tons, light chestnut – 100, dark chestnut and southern chernozems – 200-250, ordinary chernozems – 400-500, heavy chernozems – 800, leached chernozems – 500-600, gray forest-steppe – 150-300, sod-podzolic – 80-120 tons. Arable layer has the largest share of humus, which is enriched with microflora and from which the main part of mineralized nitrogen for plant nutrition comes.

Ammonification is a microbiological process of transforming organic matter nitrogen into ammonium compounds. Ammonium salts are oxidized by nitrifying bacteria (Nitrosomonas and Nitrobacter) into nitrates and nitrites. Normal life activity of these groups of bacteria requires optimal conditions: temperature 25-32 ° C, sufficient oxygen and water, soil acidity close to neutral. This is achieved by loosening the soil, applying organic fertilizers and liming acidic soils. Carrying out these methods allows to activate the processes of nitrogen transformation from organic matter and reduce its losses. Violation of these requirements leads to the opposite effect – the transition of nitrogen compounds in gaseous ammonia and nitrogen, that is, activates the process of denitrification.

Another method of regulating the balance of nitrogen in the soil is the use of bacterial preparations (rhizotorfin).

Nitrogen losses

Nitrogen content in mineral form is very dynamic and depends on the activity of soil microflora, moisture, and the phase of plant development.

Nitrogen losses consist of:

  • immobilization, which is the absorption of nitrogen by the soil microflora;
  • leaching, i.e. leaching of nitrogen, mainly in nitrate forms into the ground water;
  • volatilization in the form of ammonia into the atmosphere;
  • fixation of ammonium forms by soil or non-exchange absorption.

The process of immobilization proceeds especially intensively with the introduction of organic fertilizers with a wide ratio of carbon to nitrogen – 20-25:1. Microbial plasma contains much more nitrogen (10:1), due to which the consumption of nitrogen by microflora occurs at the expense of organic matter and mineral reserves of the soil. Which worsens the nitrogen nutrition of cultivated plants.

In order to compensate the effect of nitrogen immobilization by microorganisms, when plowing straw or other plant residues rich in cellulose before sowing subsequent crops additional about 1% of mineral nitrogen is added.

Nitrogen immobilization can have a positive effect on light soils with sufficient moisture by fixing the mobile forms of nitrogen in the conditions of their strong leachability. Further, during decomposition of microorganisms’ residues, a part of fixed nitrogen is bound by humus compounds, and another part goes to mineral forms.

Leaching of mobile forms of nitrogen, mainly nitrates, is especially relevant on light soils of granulometric composition with low levels of organic matter under conditions of sufficient, excessive moisture and irrigation. Continuous crops reduce this effect due to intensive nitrogen uptake, while in fallow fields the leaching effect is intensified.

Nitrogen loss in the form of gaseous matter occurs due to denitrification, i.e. the reduction of nitrate nitrogen to ammonia and gaseous nitrogen due to the activity of denitrifying microorganisms. Denitrifier activity is activated by anaerobic conditions, when microbes are forced to use oxygen in the nitrate form for respiration, reducing nitrogen to the free form. The denitrification process is stimulated by the creation of anaerobic conditions, alkaline reaction of the medium, excessive organic matter with high glucose and fiber content, and high soil moisture.

Another way of nitrogen loss in the form of gaseous forms (nitrogen dioxide and nitrogen monoxide) is the decomposition of nitrous acid at soil acidity 6 and below.

The total loss of nitrogen can reach 50%. The decomposition of 1 ton of humus produces 50 kg/ha of nitrogen, but part of it is lost to the atmosphere in the form of ammonia gas escaping into the atmosphere. It is especially important if the storage and application technologies of manure, slurry and other organic fertilizers are not complied with, with losses reaching 30-40%.

A significant portion of nitrogen is consumed by weed plants, and this amount can exceed the consumption of cultivated plants.

Nitrogen fixation by soil

Some nitrogen can be absorbed by some minerals of the hydromica group. When moistened, the crystalline lattice of these minerals exchangeably absorbs ammonium nitrogen, but when it dries, it binds it, making it inaccessible for plants and microflora.

According to A.V. Peterburgsky and V.N. Kudeyarov, the arable layer contains from 130 to 350 kg/ha of fixed nitrogen depending on the type and variety of soil. The top layer contains 2-7% of fixed ammonium of the total amount, in the subsoil its share increases to 30-35%. This is explained by the reduction of humus content in the deep layers and, consequently, nitrogen in the organic matter.

The ability of soils to bind ammonium without necessarily affecting the type of clay minerals, ambient temperatures, humus content, the reaction of the soil solution, microbiological activity, and moisture. Ammonium fixation increases with increasing temperature, pH (maximum on salts), humus content (chemical binding). Nitrogen fixation is influenced by the content of clay minerals with three-layer crystal lattice, primarily vermiculite.

Fixed ammonium can be displaced back into the soil under certain conditions, such as the introduction of calcium, magnesium, sodium cations into the crystal lattice, becoming available to plants.

Phosphorus content in soil and its availability

The content of phosphorus (P2O5) in soils ranges from 0.01% for poor sandy soils to 0.20% for thick highly humus soils. In the upper layers of the soil more P2O5 is concentrated, which is associated with its accumulation in the zone of the dying away of the main mass of the roots. The amount of P2O5 decreases with soil depth. Phosphorus is present in organic and mineral forms.

Organic phosphates are part of the humus, in the decomposition of which it becomes available to plants.

Some plants assimilate simple organophosphorus compounds, due to their decomposition by the enzyme phosphatase secreted by the root system. Such plants include peas, beans, corn and other crops.

Mineral forms are represented by calcium salts, predominant in neutral and alkaline soils, iron and aluminum oxide phosphates in acidic soils. Calcium phosphates are more soluble, and therefore more accessible to plants than aluminum and iron salts.

The main source of phosphorus for plant nutrition is the salts of orthophosphoric (H3PO4) and metaphosphoric (HPO3) acids. Phosphates of monovalent metals, due to their greatest solubility, are the most accessible. Single-substituted (dihydroorthophosphates) of calcium and magnesium are less soluble, but also well available for absorption. Metaphosphates are poorly soluble in water.

Two-substituted calcium and magnesium salts (hydroorthophosphates) are insoluble in water, but well soluble in solutions of weak acids, which makes them also accessible to plants, due to the creation of a weakly acidic reaction by the root system in the rhizosphere.

Orthophosphates of divalent and trivalent metals are insoluble in water, so they are inaccessible to most plants. Lupine, buckwheat, mustard, alfalfa and clover are the most adapted to the assimilation of hard-to-reach forms of phosphorus. Peas, melilot, sainfoin, hemp, rye, and corn show this property to a lesser extent (E. Rübenzam and K. Rauer, 1960).

In contrast to nitrogen, due to weak mobility, there are no natural ways of phosphorus loss as well as natural sources of replenishment.

The optimum level of phosphorus supply for most crops in mobile forms is considered to be 150-250 mg/kg of soil for gray forest and sod-podzolic soils (according to Kirsanov) and 45-60 mg/ha for chernozems (according to Machigin).

The regulation of phosphorus content in the soil is carried out mainly by applying organic and phosphorus fertilizers. To increase the content of phosphorus in the soil by 1 mg it is required depending on the granulometric composition and type of soil from 40 to 120 kg P2O5/ha.

Potassium content in soil and its availability

The gross potassium content often exceeds that of nitrogen and phosphorus, and is determined by the granulometric composition. Especially rich in potassium are clay and loamy soils, where the content reaches 2-3%. Sandy, sandy loam and peaty soils are poor in potassium, up to 0.1%.

However, gross potassium content in view of the peculiarities of exchange reactions, does not mean sufficient supply to plants, as only about 1% of its gross content is available to plants. Therefore, the characteristic feature of potassium availability is its amount in mobile forms.

In terms of availability to plants, all potassium compounds in soil are divided into five groups:

  1. Potassium, which is part of the soil minerals of aluminosilicates. Hardly available form of potassium. However, some minerals (muscovite, biotite and nepheline) can transform into accessible form some part of potassium under the influence of carbon dioxide and some organic acids produced by plant roots. The rate of transition of potassium from non-exchangeable to exchangeable forms depends on soil type. For sod-podzolic soils it is 15-30 kg/ha per year, for leached chernozems – about 60 kg/ha.
  2. Absorbed, or adsorption-bound by soil colloids, potassium is the main source of plant nutrition. The content in the soil can be from 50 to 300 mg per 1 kg of soil. During vegetation, plants use only part of the exchangeable potassium, which is determined by soil properties, biological features of plants, weather conditions, etc.
  3. Water-soluble forms of potassium are the most available form. They make up 10-20% (about 1% according to E. Rübenzam and K. Rauer) of exchangeable potassium. According to the Moscow Agricultural Academy in unfertilized sod-podzolic soil during the spring-summer period the content of water-soluble forms of potassium was 1.5-5 mg/kg of soil. It is formed as a result of chemical and biological effects on minerals. It partially passes into the water-soluble form from the exchange state as a result of displacement from the soil absorbing complex as well as from fertilizers.
  4. Biogenically bound potassium, that is included in the biomass of soil bacteria, plant residues and biota. Its proportion may reach, for example, in sod-podzolic soils 40 kg K2O per 1 ha. It passes into available form only after dying off and mineralization of residues.
  5. Potassium fixed by soil. Potassium can be fixed in the mineral part of the soil in a non-exchangeable state. The process proceeds most actively in conditions of alternating soil wetting and drying and prevails in soils of heavy granulometric composition containing clay minerals montmorillonite and hydromica, which are characterized by intracrystalline adsorption of cations, in contrast to kaolinite.

Potassium fixation into non-exchangeable form intensifies in alkaline environment and prevails in solonets. Black soils fix potassium to a greater extent than sod-podzolic soils.

Soils have a certain limit of potassium fixation from fertilizers: for sod-podzolic soils it rarely exceeds 200 kg/ha, for black earth can reach 300-700 kg K2O per 1 ha. The use of potassium fertilizers allows to achieve complete saturation of potassium fixation capacity.

The optimum content of exchangeable potassium in the soil, which observed the maximum crop yield is for sod-podzolic and gray forest soils – 170-225 kg/ha.

In the main subtypes of black earths optimal content of mobile potassium depending on the soil, culture and method of definition is Chirikov 130 to 200 mg/kg, by Machigin – up to 400 mg / kg.

Micronutrient content in soil

The removal of micronutrients with a crop from 1 hectare of soil ranges from fractions of grams (molybdenum) to several hundred grams (manganese, zinc).

As a rule, soils have a certain reserve of trace elements, which can be replenished by organic fertilizers, microfertilizers, and admixture with mineral fertilizers, in which they are in mobile form. Mobility of trace elements in manure is inferior to mineral fertilizers and is no more than 25%.

In order to obtain stable high yields, the plants’ need for micronutrients must be satisfied in full.

Reaction of soil environment

Main article: Acidity of soils

For most cultivated crops and microorganisms living in the soil, a slightly acidic or neutral soil reaction is optimal. Certain crops may exhibit special requirements for the pH interval. For example, alfalfa, sugar beets, and cotton cannot tolerate acidic soils, while others, on the contrary, prefer a slightly acidic reaction: lupine, potatoes, buckwheat, and flax.

Table. Optimal and permissible reaction of soil solution for the main crops (according to Shishkov and Mineev, 1987)[1]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Optimal pH
Permissible pH
Optimal pH
Permissible pH
Sugar beet

Indirectly, the reaction of the soil environment affects fertility due to the increased mobility of humus compounds and their leaching beyond the arable layer and the harmful effects of hydrogen ions H+ on the mineral part, which leads to the depletion of soil colloidal particles.

Soil acidification causes a lack of exchangeable calcium and magnesium, which sharply deteriorates the physical and physicochemical properties of the soil (structure, absorption capacity, buffering). Acidic environment increases the concentration of aluminum and manganese toxic for plants and reduces the mobility of some elements, such as molybdenum, creating violations in the nutrient regime of soils.

Increased acidity depresses soil biota, especially nitrifying and nitrogen-fixing bacteria, earthworms, mites, and cattails. In general, the biological activity of acidic soils is significantly lower than that of neutral soils.

The main factors of soil acidification are removal with the crop and washout of calcium and magnesium. Leaching of calcium and magnesium can reach tens to hundreds of kilograms per 1 ha. Cabbage, alfalfa, and clover, which are highly sensitive to soil acidity, have the highest removal rates.

Loss of calcium and magnesium from leaching is influenced by leaching water regime of soil depending on abundance of precipitation. For example, on acidic soils leaching can be 200-300 kg/ha, on carbonate soils – significantly more. Soils with light granulometric composition lose much more calcium and magnesium under washing water regime.

The main agronomic method of regulation of soil acidity is liming – the introduction of calcium carbonate CaCO3. In addition to normalizing the acidity, lime displaces hydrogen ions from the soil absorbing complex, neutralizes the organic acids. That leads to the elimination of exchange acidity and significantly reduces the hydrolytic acidity, increases the degree of saturation of the bases and increases the absorption capacity of the arable layer.

For alkaline saline soils the application of calcium sulfate CaSO4 (gypsum) is used. Introduction of gypsum eliminates the alkaline reaction, improves physical, chemical and biological properties of soil, aeration, facilitates tillage, increases microbiological activity and, in general, increases fertility. The effectiveness of gypsum depends on the depth of plowing, snow retention, fertilizer application, irrigation. The effect of gypsum persists for 8-10 years.

Degree of base saturation and soil buffering

Main article: Degree of base saturation and soil buffering

The degree of base saturation of soil is the sum of absorbed bases, expressed as a percentage of the cation exchange capacity (T). It is an indicator of the need for liming of soils. The lower it is, the higher the need for lime application.

Buffer capacity of the soil (soil buffering) is the ability to withstand changes in the reaction of the environment. Buffering capacity is characterized by the value of cationic absorption capacity (T), the composition of absorbed cations and the cationic-anionic composition of the soil solution. The indicator is used to calculate the optimal doses, forms, timing and methods of applying fertilizers and ameliorants to crops. The higher is the value of cationic absorption capacity, the higher is the soil buffering.

Absorption capacity of soil

Main article: Absorption capacity of soil

Absorption capacity is the ability of soil to absorb and retain solid, liquid and gaseous substances.

Absorption capacity of the soil keeps nutrients from leaching in forms that are accessible to plants. Soils with high absorption capacity accumulate moisture in the necessary amounts and keep it from seeping deep into the soil profile, being washed away on the surface and evaporating into the atmosphere, providing favorable water and air regimes. The high content of humus and texture of the soil provides a high moisture capacity.

Effect of fertilizers on soil properties

Main article: Effect of fertilizers on soil properties

Agrochemical methods and means have a complex effect on soil fertility and properties:

  • acidify or alkalize the soil solution;
  • change agrochemical properties;
  • influence on biological and enzymatic activity of soil;
  • strengthen or weaken physicochemical and chemical absorption;
  • affect mobilization or immobilization of toxic elements and radionuclides;
  • increase mineralization or synthesis of humus;
  • influence the intensity of nitrogen fixation from the atmosphere;
  • strengthen or weaken the effect of other soil nutrients and fertilizers;
  • influence the mobility of biogenic macro- and microelements in the soil;
  • cause antagonism or synergism of ions when absorbed by plants.

Agrochemical characteristics of soils in Russia

Main article: Agrochemical characteristics of soils in Russia

Reproduction of agrochemical indicators of soil fertility

The main method of reproduction of agrochemical indicators of soil fertility is the introduction of mineral and organic fertilizers.

On chemically poor soils extended fertility reproduction is necessary, i.e. introduction of nitrogen, phosphorus, potassium with crops should exceed their removal with crops. When optimal levels are reached, reproduction can be simple, i.e. aimed at maintaining optimal levels. Creation of deficit-free balance of nutrients is especially relevant on sod-podzolic soils characterized by low fertility potential.

In conditions of intensive farming, the use of fertilizers is indispensable. The effectiveness of their use is determined by a number of factors: doses of application, the coefficient of use by cultivated plants, agricultural techniques, forms of fertilizers, etc.


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