Impact of Soil-pH on Soil Test Phosphorus Extraction Results.

Information regarding the bioavailability of soil phosphorus (P) is generally gained through the use of chemical soil extractions, where extractable P is frequently interpreted as bio- or plant-available P. The availability of P is believed to be strongly influenced by soil-pH, however, research into the relationship between P extractability and soil-pH is lacking. This study was conducted to investigate the effects of soil-pH on the results of several well-established soil-P extraction methods. These effects were investigated in soils from four sites in Oklahoma; a Dale silt loam, Grant silt loam, Teller fine sandy loam, and Easpur loam from Chickasha, Lahoma, Perkins, and Stillwater, OK, respectively. Soil-pH was manipulated during previous trials through the use of alum and hydrated lime applied at rates necessary to achieve a range of target soil-pH. Extractable-P was determined using various extraction methods, including Bray-P1 (BP1), Mehlich-3 (M3P), water soluble (WSP), and total-P (TP; by acid digestion). The correlation between TP and soil-pH was not statistically significant at any site (p=0.7812). BP1, M3P, and WSP exhibit curvilinear relationships with soil-pH. The correlation between M3P and BP1 was statistically significant (r=0.91, p=2.2e^-16). The correlation between M3P and WSP was not significant at Lahoma or Stillwater, but was significant at Chickasha (r=-0.3629, p=0.0043) and Perkins (r=0.4981, p=0.0023). The strong correlation between BP1 and M3P is expected, and suggests that one may be used to reliably predict the other. The relationship between WSP and soil-pH is well-explained by current theory regarding P solubility and soil-pH; where maximum P extractability (solubility) occurred in soils between pH 6 and 6.5, and decreased in both lower and higher pH soils due to interactions with aluminum and iron, or calcium, respectively. The relationships between M3P and BP1 and soil-pH were unexpected; where extractability generally increased with a decrease in soil-pH. Based on these results, M3P is likely an acceptable predictor of BP1, and vice-versa, but may not be a good predictor of WSP.