Available soil phosphorus is maintained by less labile form that, in the long term, can become available to the plants. The aim of this research was to determine the depletion of various inorganic phosphorus fractions after successive extraction with resin. Soil samples were collected in four long-term experiments under no tillage or conventional tillage systems with different crop sequence. The soils (heavy clay Rhodic Hapludox, clay Rhodic Hapludox and clay loam Rhodic Paleudult) were collected from three depths, 0-2.5, 2.5-7.5 and 7.5-17.5 cm. Inorganic phosphorus was fractioned by a modified Hedley technique before and after successive phosphorus extraction by membrane resin. The fraction of phosphorus extracted with 1.0 mol L-1 HCl is not labile. Those extracted with 0.5 mol L-1 NaHCO3 and 0.1 mol L-1 NaOH can be considered labile, regardless the soil type, method of cultivation and crop sequence. In the Oxisols, the inorganic phosphorus fraction extracted by 0.5 mol L-1 NaOH is also labile.
soil phosphorus; labile forms; quantity factor