The fractional composition of phosphorus in the alluvial light humus saline soil of Western Transbaikalia

The article presents the results of studying the fractional composition of mineral phosphorus in alluvial light humus saline soil and its changes during the sorption of phosphate ions from an aqueous solution of KH2PO4 (concentration of 1 mmol R/l) in laboratory environment. For determining the fractional composition of mineral phosphates in the soil we have used the method of Chang and Jackson modified by Ginzburg-Lebedeva. An analysis of the obtained data has shown that the composition of mineral phosphates of the studied soil is dominated by poorly soluble iron (Fe-P) and calcium (Ca-PIII) phosphates, which are difficult to access for plants. The Fe-P content ranged from 490-720 mg R/kg, accounting for 24,6-46,5 % of the total fractions; after phosphate ion sorption it increased to 640-900 mg R/kg (38,1-55,0 %). The amount of Ca-PIII fraction ranged from 350-970 mg R/kg and increased after sorption to 390-670 mg R/kg of soil, amounting to 21,2-36,7 %. The fraction of loosely bound phosphates in the control samples varied from 30 to 160 mg R/kg of soil, amounting to 2,4-8,7 %. The Ca-PIII fraction was 9,3-20,1 % of the total amount of mineral phosphates, aluminum phosphates - 3,5-7,1 %. The groups of phosphates that characterize the available reserve for plants (Ca-PI and Ca-PI) and iron phosphates (Fe-P), the amount of which has increased not only in absolute, but also in relative terms to the sum of fractions, are subject to the greatest changes during the sorption of phosphate ions by the soil. We have also revealed a decrease in the Al-P fraction. In general, the absolute content of the mineral phosphate fraction can be represented as the following decreasing series (weighted average for a layer of 0-50 cm): Ca-III > Fe-P> Ca-III > Ca-PI > Al-P. During the sorption of phosphate ions the distribution of different phosphate fractions over the soil profile undergoes some changes. The maximum concentrations of Ca-PI, Ca-PII, and Al-P have been found in the upper humus layer with a decrease down the profile. The intra-profile distribution of Ca-PIII phosphates is characterized by a slight increase in the lower horizons.