Availability of native and fertilizer P in Brazilian soils

Inorganic forms of phosphorus were determined in two Red-Yellow Latosols, which were incubated with P fertilizers for a month, after equilibration with 32P for 60 hours. The methods used were soil P fractionation combined with the 32P isotopic dilution technique. The fertilizers applied were: H3PO4, 10-30-0 suspension, MAP and triple superphosphate, at the rate of 92 mg P kg-1 of soil. In both soils, the isotopically exchangeable inorganic phosphorus fractions decreased in the following order: water soluble P > Al bound P > Fe bound P > calcium bound P > occluded-P. The water- soluble and Al bound P were the main source of available P for the newly fertilized soil. The Fe bound phosphate was also an important source of available P in both soils when fertilizer was not applied. The soil P fixing capacity affected the availability of native and added phosphorus. (author)