Soil aggregation as a determinant of phosphorus availability in tropical upland soils

Phosphorus limitation is widespread in tropical upland soils primarily due to high soil P fixation. A recent study on a Hawaiian Ultisol indicated the P fixation and release are highly dependent on soil aggregation. Tropical upland soils vary in agggregation, but present-day P recommendations are based on soil test P and P isotherms on ground whole soil samples. Soil aggregation patterns were examined and the influence of aggregation patterns were examined and the influence of aggregation on short-term extractable P on three upland soils in Southeast Asia (Matalom, Leyte, Philippines; Siniloan, Laguna,Philippines; and Sitiung, Indonesia). Soil samples were collected 30 d after P application from experiments with four or five levels of fertilizer P. Soil samples were separated into five aggregate size fractions (2.0 mm, 2.0-1.0 mm, 1.0-0.5 mm, 0.5-0.25 mm, and 0.25 mm) using the dry sieving method. Mehlich 1 (double acid) extractable P in each aggregate fraction was also calculated. Soils differed significantly in their aggregation pattern. Matalom soil was less aggregated as 0.25 mm fraction constituted 30 percent of the soil by weight. Siniloan and Sitiung soils, in contrast, were more aggregated with greater than 34 and 36 percent, respectively of the soil weight made up of aggregates larger than 2 mm. Soil aggregate size significantly (P0.01) influenced extractable P; within each P level, the smaller the aggregate size, the greater the amount of extracted P. Thus 58 percent of the total extractable P in the Matalom soil was in the smallest aggregate size fraction. In the Siniloan and Sitiung soils, the total extracted P tended to be evenly distributed among the aggregate size fractions. Phosphorus isotherms on ground whole soil samples indicated similar fertilizer P requirements for Matalom and Siniloan soils although these soils differed markedly in their aggregation pattern. Soil aggregation seems to be important characteristics that determines P availability to crops in tropical soils and present-day soil test procedures are likely to overestimate P requirements in well-aggregated soils