PHOSPHATE AVAILABILITY IN SOILS AT WATER ACTIVITIES BELOW ONE

Water activity (aw) can influence chemical equilibria in soils, but little is known about the effects of this variable on the availability of phosphorus (P) and other fertilizers. To evaluate P availability as a function of aw samples of 24 calcareous Vertisols and Inceptisols from Spain were incubated with monocalcium phosphate at different aw's for periods up to 180 days. The P availability index (AI), defined as the ratio between the increase in extractable P and added P, depended on aw and soil properties. At aw's lower than those required for plant growth (<0.989), AI was markedly lower than in soils stored at low (0.6) aw. This indicates that P transformation can proceed in relatively ''dry'' soils of arid and semiarid areas and also cautions against the possibility of chemical changes in apparently dry soils that are stored but still have aw's in the 0.8-1.0 range.