Phosphorus cycle and balance in agriculture

Phosphorus content in soils

The phosphorus content (P₂O₅) in soils ranges from 0.01% for poor sandy soils to 0.20% for deep, highly humus-rich soils. The upper soil layers contain a greater amount of P₂O₅, which is associated with its accumulation in the zone where the bulk of roots die off. With soil depth, the amount of P₂O₅ decreases.

Phosphorus occurs in organic and mineral forms. Organic phosphates are part of humus; upon decomposition, phosphorus becomes available to plants. In sod-podzolic soils, one third of phosphorus is bound in organic compounds; in chernozems, about half. Plants assimilate phosphorus in mineral form; therefore, phosphorus bound by organic compounds becomes available to plants only after mineralization.

Mineral forms of phosphorus are represented by sparingly soluble and insoluble phosphates of iron, calcium, aluminum, magnesium, potassium, etc. The amount of plant-available phosphorus is considerably less than its total content in the soil. For example, in gray forest and sod-podzolic soils, the total phosphorus content (P₂O₅) is 1.2–3.6 t/ha, while only 100–200 kg/ha of it is in a plant-available form.

Sources of phosphorus nutrition for plants

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

Mineral forms are represented by calcium salts, which predominate in neutral and alkaline soils, and by phosphates of iron and aluminum oxides in acidic soils. Calcium phosphates are more soluble and therefore more available to plants than aluminum and iron salts.

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

Disubstituted calcium and magnesium salts (hydroorthophosphates) are poorly soluble in water but readily soluble in solutions of weak acids, which also makes them available to plants, owing to the creation by the root system of a slightly acidic reaction in the rhizosphere.

Orthophosphates of divalent and trivalent metals are insoluble in water and therefore unavailable to most plants. The plants most adapted to assimilating hard-to-access phosphorus forms include lupine, buckwheat, mustard, alfalfa and clover. This property is manifested to a lesser extent by peas, sweet clover, sainfoin, hemp, rye and maize (E. Rübenzam and K. Raue, 1960).

Unlike nitrogen, because of its low mobility, there are no natural pathways of phosphorus loss, nor natural sources of replenishment.

The optimal level of supply with mobile forms of phosphorus for most crops is considered to be: for gray forest and sod-podzolic soils (by Kirsanov) — 150–250 mg/kg soil; for chernozems (by Machigin) — 45–60 mg/ha.

To increase the phosphorus content in soil by 1 mg, depending on the particle-size composition and soil type, 40 to 120 kg P₂O₅/ha is required.

Under natural conditions, the source of phosphorus nutrition for plants is salts of orthophosphoric acid — phosphates — as well as, after hydrolysis, pyro-, poly- and metaphosphates. The latter are absent in soil but may be components of compound fertilizers.

Orthophosphoric acid, upon hydrolysis, dissociates into the anions H₂PO₄^-, HPO₄^2- and PO₄^3-. Under conditions of a slightly acidic soil reaction, the most widespread and available is H₂PO₄^-, to a lesser extent HPO₄^2-; PO₄^3- practically does not participate in the nutrition of most plants, except for lupine and buckwheat, and to a lesser extent mustard, peas, sweet clover, sainfoin and hemp (B. P. Nikolsky).

All salts of orthophosphoric acid and monovalent cations (NH₄⁺, Na⁺, K⁺) found in soil are readily soluble in water. Monosubstituted salts of divalent cations, calcium Ca(H₂PO₄)₂ and magnesium Mg(H₂PO₄)₂, are also soluble. Disubstituted salts of calcium CaHPO₄ and magnesium MgHPO₄ are poorly soluble in water but soluble in weak acids, including acidic root exudates and organic acids formed during the vital activity of microorganisms. Therefore, dihydroorthophosphates (monosubstituted) and hydroorthophosphates (disubstituted) are a source of phosphorus for plants.

Trisubstituted phosphates (orthophosphates) of divalent cations are insoluble in water and unavailable to most plants. However, freshly precipitated trisubstituted calcium phosphate, formed from mono- and disubstituted phosphates during chemical absorption by the soil, is somewhat better absorbed by plants in the amorphous state. As it ages, these amorphous tertiary phosphates pass into crystalline forms and lose availability to plants.

Orthophosphates of trivalent cations [AlPO₄, Al(OH)₃PO₄, FePO₄, Fe₂(OH)₃PO₄, etc.] are unavailable to plants and make up most of the mineral phosphates of acidic soils.

A source of phosphorus nutrition for plants is phosphates in an exchange-absorbed (adsorbed) state by soil colloids. These anions are displaced by anions of mineral and organic acids (citric, malic, oxalic). In the soil, in the solid phase–solution system, anions are present in sufficient quantities. During respiration, roots release carbon dioxide, which upon dissolution acidifies the reaction and forms bicarbonate ions. The latter displace adsorbed phosphorus into solution from the SAC.

It has been experimentally confirmed that exchange-absorbed anions of phosphoric acid are close in plant availability to water-soluble phosphates. However, the amount of the latter in soil is small; therefore, adsorbed phosphates are of great importance in the balance of phosphorus nutrition of plants.

Some plants are capable of assimilating the phosphate ion of organic compounds, for example, phytin and glycerophosphates, thanks to root exudates containing the enzyme phosphatase. Under the action of phosphatase, the phosphoric acid anion is split off from organic compounds and absorbed by the plant. Such plants include peas, maize, and beans. Phosphatase activity increases under phosphorus deficiency.

In the course of phylogenesis, plants have adapted to nutrition from solutions with very low concentrations. In studies by M. K. Domontovich, all experimental plants (oats, maize, wheat, peas, mustard and buckwheat) could absorb phosphorus from solutions with concentrations from 0.01 to 0.03 mg P₂O₅ per 1 L. The optimal phosphorus concentration for plant nutrition is considered to be 1 mg/L.

Phosphorus cycling in agriculture

In natural biocenoses, phosphorus has no sources of replenishment of reserves in the soil; at the same time, its natural reserves in soils are significant. The meter-deep soil layer contains from 10 to 35 t/ha of various phosphorus compounds (A. V. Sokolov). Because the roots of many field crops penetrate to a depth of 0.9 to 2.8 m, and those of perennial grasses to 3–5 m, mobile forms can be used by plants. The uptake of P₂O₅ by plants from subarable horizons has been experimentally confirmed; its share can be up to 30% of the total removal with the harvest.

The removal of phosphorus with agricultural products averages 25–40 kg/ha per year. Thus, natural reserves in the soil considerably exceed removal.

In natural biocenoses, with their characteristic closed cycle of biogenic elements, phosphorus slowly accumulates in the upper soil layers owing to its redistribution by plant activity.

A feature of the phosphorus cycle in agrocenoses is that most of it is concentrated in the harvest; for example, up to 2/3 of all phosphorus absorbed by plants is concentrated in grain, and the remaining 1/3 is in the non-marketable part — straw. Considering that only a small part of the grain remains on the farm, the removal of phosphorus from farms is significant. In addition, phosphorus is also contained in livestock products, which should also be taken into account in the external farm balance.

In agrocenoses, the phosphorus cycle is relatively simpler than the nitrogen cycle.

Phosphorus losses may be associated with soil erosion in the form of losses of the solid part with wind erosion and runoff with water erosion. On average, losses can be up to 11 kg/ha per year. On soils of medium and heavy particle-size composition, infiltration, as a rule, does not exceed 1 kg/ha per year; on light and peat soils, up to 3–5 kg/ha.

A small amount of phosphorus enters the soil with plant seeds, atmospheric precipitation and dust.

For these reasons, compensation of the expenditure items of the phosphorus balance in agriculture is possible through the use of organic and mineral fertilizers.

In the 1970s–1980s, a positive phosphorus balance developed in the USSR: in many regions, its content in the soil increased. Thus, in the Central region of the Non-Chernozem zone, the amount of mobile phosphates in the soil increased from 5.3 to 12.5 mg/100 g; in Moscow Oblast, from 6.4 to 20.6 mg/100 g.

Phosphorus balance

Phosphorus has no natural sources of replenishment of its reserve in soils. Its reserves in agroecosystems are replenished only through the application of phosphorus and organic fertilizers.

In the atmosphere, phosphorus may be present as dust in small amounts (0.5–1 kg/ha per year). Soil, water and plants are involved in the phosphorus cycle in ecosystems. However, its availability to plants depends on many factors.

Phosphorus losses occur from soil erosion in the composition of fine earth and liquid runoff. Leaching of phosphorus in soils of medium and heavy particle-size composition, as a rule, does not exceed 1 kg/ha; in light and peat soils, up to 3–5 kg/ha.

The phosphorus balance (B_p) is determined by an equation that takes into account the difference between input to the soil with fertilizers, seeds and precipitation and removal with the harvest and losses from leaching and erosion:

where

B_p

— phosphorus balance, kg/ha P₂O₅.

Input items of the balance:

P_f

— input with fertilizers, P_f = (P_m + P_o), P_m — input with mineral fertilizers;

P_o

— input with organic fertilizers;

P_s

— input with seeds (sowing material);

P_r

— input with plant residues;

P_a

— input with atmospheric precipitation.

Expenditure items of the balance:

P_h

— removal with the harvest of main and by-products;

P_r

— removal with plant residues;

P_l

— losses from the soil, P_l = (P_leach + P_erosion), where P_leach — losses from leaching;

P_erosion

— losses from erosion processes.

The input of phosphorus with fertilizers and seeds is determined by chemical composition and seeding rates. The amount of input from atmospheric precipitation for phosphorus does not exceed 0.5 kg/ha. The removal of phosphorus with the harvest (P_h) is determined by the chemical composition and amount of the harvest of main and by-products.

Phosphorus losses from erosion (P_erosion), according to averaged data, are: for phosphorus — 1.5–2 kg/ha. Phosphorus losses from leaching depend on the particle-size composition of the soil, the amount of precipitation, fertilizer rates and crops. Phosphorus losses (P_leach) for loamy soils average up to 0.1 kg/ha; for sandy and sandy loam soils, 1.2 kg/ha.

References

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.
2. Evtefeev Yu. V., Kazantsev G. M. (2013). Fundamentals of Agronomy: A Textbook. Moscow: FORUM.
3. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.