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The effect of Cl and SO2-4, anions as well as that of the ionic strength on the desorption of soil P were studied in 102 mineral soil samples by extracting them with KCI and K2SO4 solutions at ionic strengths of 0.025 and 0.1. The quantities of salt soluble and water soluble P in the soils were compared. Both sulphate solutions extracted more P in every single sample than either of the chloride solutions. On the other hand, the material could be divided into three groups according to the position of water in the order of extraction efficiency. In the group W&gt;S&gt;Cl, consisting of 53 samples, water was the most effective extractant, in the group S&gt;W&gt;Cl of 37 samples water was less effective than sulphate solutions but more effective than the chloride solutions, and in 12 samples (S&gt;Cl&gt;W) water was the least effective, less effective than even chloride. The groups classified according to P extractability did not deviate from each other in terms of soil texture, pH or org. C %, but the salt solutions tended to be the more effective extractants the poorer the P status of the soil was. The salt soluble and water soluble P which are closely related to each other correlated with the same factors. Exclusive of the group of 12 samples (S&gt;Cl&gt;W), the highest value sof correlation coefficient were found for NH4F-P/Al, i.e. the molar ratio of NH4F soluble P (CHANG and JACKSON's method) to oxalate extractable Al (r=0.89***-0.93***). The absolute differences between amounts of P dissolved in KCI and K2SO4 solutions of the same initial ionic strength were the greater the more water soluble P the samples contained (r=0.58* -0.94***). An increase in ionic strength tended to depress the extractability of P in both salt solutions. Therefore the ligand exchange between sulphate and phosphate or hydroxyl was regarded unprobable. A theory of the extraction mechanism of sulphate was presented. The greater extraction efficiency of sulphate was assumed to be caused by the exchange reactions with H2O groups which affect the ionic strength in a solution and the electric condition near the surface.

Chloride and sulphate solutions as extractants for soil P: I Effect of ionic species and ionic strength on P desorption