Speciation of Phosphorus in Recently Acidified Soils of the Columbia Basin, Washington.

Determination of inorganic phosphorus (P) species in recently acidified soils of the 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 recently acidified soils of the CB using both a traditional chemical fractionation schemes and ³¹P NMR spectroscopy.  Several soil samples from the CB ranging in pH (5.2-8.4) together with native calcareous and acidic soils were analyzed.  Chemical fractionation to determine soluble, aluminum (Al), iron (Fe), reductant soluble (RSP), and calcium (Ca) P showed that for the CB soils contained 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 amount of Fe-P extracted increased suggesting a possible transition in soil P chemistry from high to low pH.  Solid state ³¹P 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.  Continued use of the traditional sodium bicarbonate (NaHCO₃) extractable P (Olsen phosphorus, OP) method for making fertility recommendations appears to be the best option for determining plant available P for recently acidified CB soils.