JWSS - Journal of Water and Soil Science

Understanding the distribution of different P forms in soil aggregate fractions is important in evaluating the risk of P run-off and leaching in the agricultural soils. The objective of the present research was to determine the effect of aggregate size on soil phosphorus availability and fractionation in 10 calcareous soils. Micro aggregate (< 0.25 mm), macro aggregate (> 0.25 mm) and whole soil were separated by dry sieving. Olsen P, total P, organic P, and inorganic P fractions in micro aggregate, macro aggregate, and whole soil were determined. Soil inorganic P fraction was  determined by a sequential extraction procedure including: dicalcium phosphate (Ca₂-P), octa calcium phosphate (Ca₈-P), apatite (Ca₁₀-P), P absorbed by Al oxide (Al-P), P absorbed by Fe oxide (Fe-P) and P incorporated in to Fe oxide (O-P). The results showed that the amounts of (Olsen P), (Ca2-P), (Ca8-P), (Ca10-P), (Organic P), (Al-P), (Fe-P), (O-P) and (Total P) in 70, 60, 40, 40, 60, 70, 60, 50 and 70 percent of soils, respectively, in the micro aggregates were significantly higher than those of the macro ones. Finally, by increasing the P content, particularly the smaller sized aggregates, it was likely that the eroded material would favor greater P loss.