Desorption of Water Soluble Phosphorus from Soil : development of a Consecutive Extraction Method

Abstract: A consecutive extraction method was developed which allows to determine the total amount of water soluble phosphorus in soil. For that, soil was shaken with deionized water, which was removed and replaced after one hour; this procedure was repeated ten times. The concentrations measured at each extraction step allow to quantify desorption of total water soluble phosphorus. Seven soils of the Ultuna long term soil organic matter experiment, Sweden, were used to test the method. Two additional soils from a horse paddock were used for method development. Soils treated with farmyard manure and sewage sludge showed the highest release of total water soluble phosphorus, whereas calcium nitrate and ammonium sulfate showed the lowest. Fallow, green manure and peat treatments showed intermediate release. The amount of total water soluble phosphorus was controlled by pH, total P and P-AL. The increase in potentially releasable water soluble P is about 20 per cent of total P but 55 per cent of P-AL in average among all soils tested. Data were compared with an earlier phosphorus fractionation of four of the soils used showing that all resin P and part of sodium bicarbonate P was released by consecutive extraction with water. The relative decline in consecutive P release was inversely related to the P quantity/intensity ratio. The estimation of total water soluble phosphorus obtained by the method showed that the actual availability of P in soil to plants seems not to be limited by chemical binding and release of P to the soil solution, but by the ability of plants to obtain P from the soil solution.