Evaluation of oxalate/oxalic acid for extracting plant available phosphorus in unfertilized acid soils

The extractability of phosphorus (P) was studied in an acid, unfertilized soil at an abandoned farm and in 40 different soils from former agricultural fields and pastures. Oxalate/Oxalic acid extraction was compared with a number of other methods in order to evaluate the suitability of oxalate/oxalic acid as a method for extracting P from soil. The extracting efficiency and a rough estimate of the origin of the extracted P was also investigated. Total and molybdate reactive P extracted with weak (1 mM), acid (9.35 mM, pH 2.3), and strong (50 mM) oxalate correlated well with Na2SO4 + NaF, Olsen P, and Bray 1 extractable P (0.96 > r > 0.77). Total and molybdate reactive P fractions determined by different oxalate methods were also closely correlated (0.99 > r > 0.84). Only total organic P did not correlate with the other P fractions extracted. However, the large strong oxal extractable P fraction was correlated to many of the weaker extractable fractions. A large share of this fraction is probably organic. The Na2SO4 + NaF extraction, Olsen P, and Bray 1 were also closely correlated. The more loosely adsorbed fractions are possibly replenished to some extent when the soluble phosphate is consumed by plant uptake. A large share of total P extracted with acid (9.35 mM) and strong (50 mM) oxal was organic. The molybdate reactive fraction, however, was fairly constant up to 9.35 mM oxalate in the solution, neither depending on solution concentration nor on pH. Total P extracted with oxalate was related to the logarithm of the extractant concentration. The advantage of the oxal method is that the pH can be adjusted to fit a desired soil pH, and both extractant concentration and analytical method could be chosen to determine either loosely bound P or a substantial amount of total P that might be potentially available.