Phosphorus Availability in Soils Amended with Wheat Residue Char

Plant availability and risk for leaching and/or runoff losses of phosphorus (P) from soils depends among others on P concentration in the soil solution. Water soluble P in soil measures soil solution P concentration. The aim of this study was to understand the effect of wheat residue char (biochar) addition on water soluble P concentration in a wide range of biochar amended soils. Eleven agricultural fields representing dominant soil texture classes of Swedish agricultural lands were chosen. Concentrations of water soluble P in the soils and in biochar were measured prior to biochar incorporation to soils in the laboratory. Experiments with three dominant soil textures- silt loam, clay loam and an intermediate loam soil with different rates of biochar addition (i.e., 0.5, 1, 2 and 4%; w/w) showed that the highest concentration of water soluble P was achieved at an application rate of 1%. At higher application rates, P concentrations decreased which coincided with a pH increase of 0.3 - 0.7 units. When the eleven soils were amended with 1% (w/w) biochar, water soluble P concentrations increased in most of the soils ranging from 11 to 253%. However, much of the water soluble P added through the biochar was retained (33 - 100%). We concluded that - wheat residue char can act as a source of soluble P; and low and high additions of biochar can have different effects on soil solution P concentration due to possible reactions with Ca and Mg added with biochar