MODERN METHODS FOR DETERMINING THE CONTENT OF MOBILE PHOSPHORUS IN SOILS

There are more than 20 methods for determining available phosphorus in soil in the world. One of the most difficult methodological tasks is to select a solution for soil extracts that would extract only plant-available phosphates from the soil and would not transfer low- and inaccessible phosphorus compounds into the solution. All chemical methods used usually include soil extraction with solutions of different pH, a certain soil : solution ratio, and different interaction times. In Europe alone, there are more than a dozen different methods used as standard methods for determining available phosphorus in soils. In Germany and Austria, the lactate method (CAL) is widely used for routine soil analysis, while in the USA, Canada, and the Czech Republic, the Mehlich method is used for acidic and neutral soils. It uses a multi-element extractant (a mixture of 0.2 M CHCOOH, 0.25 M NHNO, 0.015 M NHF, 0.013 M HNO, 0.001 M EDTA), which allows for the simultaneous extraction of P, K, Ca, Mg, Na, Cu, Zn, Mn and Fe. In European countries, the Olsen method is generally accepted for carbonate soils. It is based on the extraction of available soil phosphates with a 0.5 M NaHCO solution with a pH of 8.5. In Russia, the following standardized methods are used to determine available phosphorus: for sod-podzolic and gray forest soils – the method of Kirsanov (0.2 M HCl, with a soil: solution ratio of 1 : 5), for non-carbonate chernozems – the method of Chirikov (0.5 M CHCOOH, 1 : 25), for carbonate soils – the method of Machigin (1% (NH)CO, 1 : 20). In some countries, for example in Brazil, anion exchange membranes have begun to be used. In general, the availability of phosphorus for plants is mainly determined by three indicators: 1 – the concentration of PO in the soil solution (the “intensity” factor), determined in low-salt extracts (the method of Karpinsky–Zamyatina, Skofield), 2 – the amount of PO in the solid phase of the soil, which can be easily accessed by plants (the “capacity” factor), methods of Kirsanov, Chirikov, Machigin, Olsen, Mehlich, and others, 3 – the ability of the soil to maintain the concentration of PO in the soil solution at a sufficiently high level for a long time (buffer capacity of phosphorus, PBC). The most promising methods are those based on the use of anion exchange membranes, in which, due to the sorption of phosphorus from the solution by anionites, further desorption and the entry of PO from the soil occurs, which allows the most complete imitation of the absorption of phosphorus by the root systems of plants and the most complete convergence of the results of the methods with the efficiency of phosphorus fertilizers and the yield of agricultural crops. This will allow the rational use of phosphorus fertilizers due to the limited reserves of phosphate raw materials.

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