SOIL FIXATION AND AVAILABILITY OF PHOSPHATE FERTILIZER

The addition of low quantity (up to 50 mg/kg) of phosphorus (P) to two calcareous soils in the Argentine semiarid-subhumid pampean region increased the formation of DCPD (Brushite) and decreased the P (Paer) extractable by an anionic exchange resin. This latter effect was not observed when the quantities of added P were over 50 mg/kg. The pH decreased with P addition up to a point (50 mg/kg) after which it returned to its original value. The percentage in weight of peds smaller than 74 mu m increased with the added P up to certain point which decreased to values nearing the original ones. To obtain a positive Availability Index (F1) by incubating for six months the soil with additional P, it is suggested that there are at least two points where the increase of labile P (Paer) takes place.