SOIL-PHOSPHORUS MOVEMENT AND BUDGET AFTER 13 YEARS OF FERTILIZED CULTIVATION IN THE AMAZON BASIN

Agronomic studies on soil phosphorus dynamics have primarily focused o n the plant-available inorganic P pool. However organic P and less lab ile inorganic pools can contribute significantly to plant P uptake. Th e objectives of this study were to determine the changes in inorganic and organic P pools of varying lability in and below the plowlayer aft er 13 years of continuous cultivation and fertilization on a Typic Pal eudult in Yurimaguas, Peru. The field experiment was established after slash and burn of a secondary forest and included non-fertilized and fertilized treatments. The yearly cropping pattern consisted of an upl and rice (Oryza sativa), - corn (Zen mays), - soybean (Glycine max) ro tation. A modified version of the Hedley et al. procedure was used to sequentially fractionate soil P into increasingly recalcitrant organic and inorganic pools. Plowlayer accumulation of the fertilizer P occur red in all P pools. The greatest increase was in the NaOH extractable inorganic P pool. In the non-fertilized plots, the organic P decreased by 42%. Phosphorus fertilization resulted in significant movement of P below plowlayer. The accumulation occurred mostly in inorganic and o rganic P pools that are not quantified by traditional soil-P test meth ods. In fertilized plots sub-plowlayer total P increased by 90 mu g g( -1) (87%) while resin extractable P increased only 4 mu g g(-1). Phosp horus content of the organic P pools below the plowlayer increased by 24 mu g g(-1) (50%) in fertilized plots. The inclusion of less labile P pools in studies of P movement and the evaluation of P fertilizer re sidual values could lead to a better understanding of P dynamics and h ence better management of P fertilization.