PARTITIONING SOIL-PHOSPHORUS INTO 3 DISCRETE POOLS OF DIFFERING AVAILABILITY

The Hedley phosphorus (P) fractionation procedure provides an alternat ive for characterizing soil P availability without qualifying the comp onent P species. Interpretation of results involving all fractions is complicated and offers Little practical utility. The objective of this study was to group P fractions of similar availability into three fun ctional pools according to the plant availability of the P fractions. These grouped pools may simplify practical use of fractionation data a nd provide insights into differences in availability and cycling of ad ded P in diverse soils. Eight soils in the greenhouse were treated wit h four levels of P and planted with 14 consecutive crops. Soils were f ractionated using a modified Hedley method. The seven fractions were t hen grouped into three discrete pools of decreasing availability: the readily available, the reversibly available, and the sparingly availab le. Phosphorus extracted by the iron-impregnated filter paper strips ( Strip P) and inorganic P extracted by 0.5 M NaHCO3 (NaHCO3-P-i) were r eadily available in all soils. The reversibly available P fractions di ffered between the slightly weathered and the highly weathered soils. In the slightly weathered soils, the reversibly available P included t he organic P (P-o) (NaHCO3- and NaOH-P-o), NaOH-P-i, HCl-P, and residu al P, In the highly weathered soils, NaOH-P-i and organic P were consi dered reversibly available. No P fractions appeared to be sparingly av ailable in the three slightly weathered soils. The residual P was the sparingly available P in the highly weathered soils. The results sugge st quantitatively that P availability was greater when added to slight ly weathered soils than when added to highly weathered soils.