SENSITIVITY OF ISOTOPICALLY EXCHANGEABLE PHOSPHATE IN SOIL SUSPENSIONS TO THE SUPPORTING SOLUTION

The isotopically exchangeable quantity (E-t) of phosphate measured in 1:10 soil/solution suspensions with deionized water has been identifie d as the reserve of plant-available phosphate in agricultural soils. W e studied the effects of two supporting solutions, 0.01 M CaCl2 and de ionized water, on E-t values in 27 calcareous agricultural soils. For three soils, E-t was also measured after adding increasing phosphate q uantities as KH2PO4, and equilibrating for 24 h, to assess changes in E-t with phosphate concentration in solution (C-P). In unamended and p hosphate-enriched soils, C-P(CaCl2) was smaller than and related to C- P(H2O) The initial rapidly and the subsequent slowly isotopically exch angeable phosphate were both affected by the use of CaCl2 solution. Si nce both the rapid and slow isotopically exchangeable phosphate are C- P dependent, the overall effect of CaCl2 solution was analyzed conside ring two components: (i) a nonspecific effect attributed to the decrea se in C-P and (ii) a specific effect attributed to phosphate reactions (i.e., adsorption or precipitation) in calcareous soils. Modification s in E-t are the result of both nonspecific and specific effects. The change in E-t due to the use of 0.01 ill CaCl2 instead of deionized wa ter varies with the soil sample, and can be positive, negative or nil depending on the time of exchange. For instance, after 24 h of exchang e, the ratio {[E-24h(CaCl2) - E-24h(H2O)]100/E-24h(H2O)} ranged from - 30 to 340% for the 27 soils. The 0.01 M CaCl2 solution thus introduces complex and unpredictable changes in isotopic exchanged phosphate, an d its use is not recommended to estimate plant-available phosphate in soil.