Investigation of acid hydrolysis reactions of polyphosphates and phytic acid in Bray and Mehlich III extracting solutions

Soil test phosphorus (STP) (Bray P-1, Mehlich III) was developed to predict the plant available P in agricultural soils in the USA. The acidic STP extracting solutions were intended to extract phosphate from alkaline earth metal-based solubility products and or adsorbed species on amorphous metal hydroxides or phyllosilicates in soils. However, the accuracy of STP in predicting plant available phosphate has often been questioned due to potential acid hydrolysis of poly- and organic-P in soils during the STP extraction. The objective of the study was to investigate the acid hydrolysis reaction of reference poly- and organic-P compounds (1.25–3.75 mM) in Bray and Mehlich III extraction solutions using spectrophotometric methods. The results showed that both tripolyphosphate and pyrophosphate readily underwent the acid hydrolysis reaction over 2 h, whereas phytic acid did not hydrolyze. The results suggest an overestimation of dissolved reactive phosphate from these polyphosphates should be considered when evaluating the STP data.