Abstract
The effect of increasing sulphate concentration on P desorption was studied in 102 mineral soil samples with: a) solutions of a constant ionic strength I=0.1 adjusted with KCI and b) 0.1 M and 0.033 M sulphate solutions of different ionic strengths. Further, the efficiency of chloride and sulphate solutions of equal anion concentration (0.1 M) was compared. At a constant ionic strength, P desorption was enhanced as the sulphate concentration of the extractant increased. Even despite a much higher ionic strength the extraction power of the K2SO4 solution was greater than that of the KCI solution of the same anion concentration. Comparison of the 0.1 M and 0.033 M K2SO4 solutions revealed in 53 soil samples the extractability of P to be depressed by an increase in sulphate concentration (and ionic strength). In 49 samples, on the contrary, the 0.1 M K2SO4 solution extracted equal or greater P amounts than did the 0.033 M K2SO4. It was observed that the lower the P coverage on the oxide surfaces in the soil was, the more obviously the promoting effect of increasing sulphate concentration overruled the depression induced by increasing ionic strength, and the more superior the K2SO4 solution was as compared to the KCI solution.
Highlights
Indifferent anions depress the desorption of P from soil to solution (RYDEN and SYERS 1975)
The ionic strength created by sulphate is, found not to decrease the desorption as much as the same ionic strength created by chloride (HARTIKAINEN and YLI-HALLA 1982 a)
The effect of anion concentration on P desorption was studied by extracting the soil samples with K 2S0 4 and KCI solutions of the equal anionic concentration 0.1 M
Summary
Indifferent anions depress the desorption of P from soil to solution (RYDEN and SYERS 1975). WIKLANDER and ANDERSSON (1974) have demonstrated the mobilization of P under field conditions to be promoted in autumn when the ionic strength is diminished. Their analyses of soil solution showed that in spring, after fertilization, the concentration of cations and anions at field capacity was 91—97 mmol/1, at wilting point as high as 211 —307 mmol/1. Fertilization caused a high concentration of Cl' ions, the ratio of Cl' to SO4 being about 10:1. In Finland, a common fertilization practice is to add potassium to the soil as KCI. The fertilizers sold during the period of July 1979 to
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