COMPARISON OF 7 PHOSPHORUS SORPTION INDEXES

Seven methods of estimating or predicting phosphorus (P) sorption capa city were compared for 47 neutral to acid Western Australian soils. Tw o methods, the P buffer capacity (PBC) and the Fox and Kamprath proced ure, provided reliable indices of P sorption from well defined P sorpt ion isotherms, but they are not quick routine methods because several levels of P addition are required. The other five routine procedures i ncluded two versions of the P retention index (PRI), determined by add ing one level of P, and three soil properties, oxalate extractable iro n (oxalate Fe), oxalate extractable aluminium (oxalate Al), and pH mea sured in sodium fluoride [pH (F)], that are known to indicate P sorpti on capacity. All the indices were well related to one another. The oxa late Fe index was the least well related to PBC whereas oxalate Al, on e of the PRI indices, and pH (F) were closely related to PBC and could be used as quick, economical procedures to assess the P sorption capa city of soils.