Speciation of Phosphorus in Anthropogenically Acidified Soils

Determination of inorganic phosphorus (P) species in anthropogenically acidified soils of Washington's Columbia Basin (CB) is vital in understanding which available P extraction methods are most appropriate so that fertilizer recommendations can be made accurately. The objective of this work was to determine dominant P species in anthropogenically acidified soils of the CB using chemical fractionation and 31 P nuclear magnetic resonance (NMR) spectroscopy. Soil samples from the CB ranging in pH (5.2–8.4) together with native calcareous and acidic soils were analyzed. Chemical P fractionation to determine soluble, aluminum (Al), iron (Fe), reductant soluble P (RSP), and calcium (Ca) showed that CB soils contain more than 60% of inorganic P as Ca‐P. Significant differences in other fractions were most pronounced in the Fe‐P form. As soil pH decreased the proportion of Fe‐P extracted increased suggesting a possible transition in soil P chemistry concomitant with change from high to low pH. Solid‐state 31 P NMR spectroscopy was limited by the low soil P content and interference from paramagnetic ions. No definitive determination of inorganic soil P species could be made. Despite these limitations, some generalized inferences can be made using the spinning side band (SSB) patterns. Because the traditional sodium bicarbonate (NaHCO 3 ) extractable P (Olsen phosphorus [OP]) method assumes a dominant Ca‐P form, fertility recommendations based on OP appears to be the best option for determining plant available P for soils that have been acidified by human involvement.