Effects of soil aggregate size on phosphorus extractability and uptake by rice (Oryza sativa L.) and corn (Zea mays L.) in two Ultisols from the Philippines

A number of recent studies suggest that soil aggregation may affect short- and long-term phosphorus (P) availability in highly weathered soils. We investigated the effect of natural soil aggregate sizes (from < 0.212 to 4-5.6 mm) on P extractability and plant P availability in low-P and high-P Siniloan soils (Typic Palehumults) from Laguna, Philippines. Mehlich-1 extractable P was always greatest in the smallest aggregates, regardless of whether or not it was extracted without P addition or extracted after 15 days incubation with newly applied P in both intact and ground aggregates. Grinding significantly increased the initial extracted P only in high-P soil. Soil aggregate size had little effect on the extractability of newly added P because the short-term Mehlich-1 P buffering coefficient (PBC), a change in Mehlich-1 extracted P (mg kg^-1 soil) per unit of added fertilizer P (mg kg ^-1 soil), was not significantly correlated with aggregate size. In the greenhouse experiment, plant growth (shoot dry weight, root length and dry weight) and total P in the shoots of both corn (Zea mays L.) and rice (Oryza sativa L.) were markedly increased with decreasing aggregate diameters from 4-5.6 mm to < 0.212 mm, even when the plant had adequate P in the rice experiment in the high-P soil. There was no interaction between P supply and aggregate size on the plant growth response and P uptake in both rice and corn grown in the two soils, suggesting that the effect of soil aggregation on plant P availability of newly added P was small. Although, the smaller aggregates themselves also contained higher total P, finer and longer root growth in these aggregates as a direct effect of aggregate size on root growth mainly contributed to better plant growth and P uptake in these aggregates. The findings of this study suggest that in Siniloan soil, soil aggregation had little effect on short-term PBC and plant P availability of the P newly added to soil over 5 weeks. However, in high-P soil, the current soil test procedures, which require grinding and shaking of soil sample, might overestimate the available P status of the soil.