PHOSPHORUS SORPTION AND AVAILABILITY INDEXES AS AFFECTED BY PROPERTIES OF CALCAREOUS SOILS

The Olsen solution is usually considered the best extractant for estimating P availability in calcareous soils, but predictability of the response to P fertilizers is often low under field conditions. In this study, soil characteristics influencing P sorption and extractability were evaluated. Forty-one soils varying in CaCO3, pH, and clay content were selected from pastures to minimize the effect of recent P additions. A P sorption index (PSI) determined from a single addition of 150 mg P/100 g soil was related to soil Ca and CaCO3, but the correlation coefficients were rather low (r = 0.46 and 0.38, respectively). A P availability index (PAI), determined from the increase in extractable soil P after adding 50 mg P/kg to a suspension and allowing it to dry, was correlated quite well with cation exchange capacity and clay content (r = -0.61 for each) in soils with pH < 8.8. The PAI also had a positive relationship with the density of the processed soil sample (r = 0.60). The relationship between PAI and soil Ca (r = -0.51) was also better than that between PSI and soil Ca. Inclusion of initial soil P and organic carbon along with CEC increased the predictability of PAI from 37% to 59%. In soils with pH > 8.8, soil pH was the dominant factor controlling the PAI (r = 0.92).