Partitioning Soil Phosphorus Into Three Discrete Pools of Differing Availability1

The Hedley phosphorus (P) fractionation procedure provides an alternative for characterizing soil P availability without qualifying the component 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 functional pools according to the plant availability of the P fractions. These grouped pools may simplify practical use of fractionation data and provide insights into differences in availability and cycling of added P in diverse soils. Eight soils in the greenhouse were treated with four levels of P and planted with 14 consecutive crops. Soils were fractionated using a modified Hedley method. The seven fractions were then grouped into three discrete pools of decreasing availability: the readily available, the reversibly available, and the sparingly available. Phosphorus extracted by the iron-impregnated filter paper strips (Strip P) and inorganic P extracted by 0.5 M NaHCO3 (NaHCO3-Pi) were readily available in all soils. The reversibly available P fractions differed between the slightly weathered and the highly weathered soils. In the slightly weathered soils, the reversibly available P included the organic P (Po) (NaHCO3- and NaOH-Po), NaOH-Pi, HCl-P, and residual P. In the highly weathered soils, NaOH-Pi and organic P were considered reversibly available. No P fractions appeared to be sparingly available in the three slightly weathered soils. The residual P was the sparingly available P in the highly weathered soils. The results suggest quantitatively that P availability was greater when added to slightly weathered soils than when added to highly weathered soils.