Evaluating Soil Test P Variation Based On Measurement by Molybdate Colorimetry and ICP.

Land grant university phosphorus (P) guidelines are based on measurement of P by molybdate colorimetry, which estimates orthophosphate concentration.  Several studies have reported that P in soil extracts measured by inductively coupled plasma spectroscopy (ICP) can be consistently greater than P  measured by colorimetry, as P measured by ICP includes both free ionic and complexed P species (e.g., organic/particulate P).   We investigated soil test P (STP) variability based on measurement by ICP (STP-ICP) and molybdate colorimetry (STP-Color) in agricultural soils from northern NY (n = 244) and in a set of riparian soil samples from northwest Vermont (n = 283).  For the agricultural soils, STP-ICP and STP-Color were well correlated for all samples, but the ratio of STP-ICP to STP-Color increased at STP-Color values <10 mg/kg.  At low STP-Color levels, STP-ICP was as much as 3.5-fold greater than STP-Color.   Based on stepwise multiple linear regression, the numeric difference between STP-ICP and STP-Color was best explained by aluminum and organic matter content (R² =0.57), suggesting that organic matter-Al-P complexes may be an important component of the non-colorimetric P fraction.  For the riparian samples, STP-ICP was approximately 1.7-fold greater than STP-Color based on linear regression.  Soil organic matter content alone explained 72% of the difference in P measurements.  Results indicate that soil test P categories based on ICP would differ markedly from colorimetric-based determinations, especially in the agronomic responsive range.  Additional research is needed to determine the potential bioavailability of the unreactive P fraction in soil test extracts.