Relating Extractable Soil Phosphorus to Phosphorus Losses in Runoff

TLDR

Phosphorus runoff from agriculture can cause eutrophication, especially when soil P becomes the main source after over‑application. The study tested whether the correlation between soil test P and runoff DRP/BAP depends on the extraction method used. Soil from 54 grass plots (0–2 cm depth) was extracted with six methods, then simulated rainfall (100 mm h⁻¹) produced runoff collected for 30 min to measure DRP and BAP. Distilled water, iron‑oxide paper, and acidified ammonium oxalate extracts yielded the strongest correlations (r² ≈ 0.82–0.85) with runoff DRP/BAP, whereas load correlations were weak (r² < 0.18).

Abstract

Phosphorus in agricultural runoff can cause accelerated lake and stream eutrophication. Where producers have applied P at rates exceeding crop uptake, soil P has sometimes become the main source of P in runoff. We hypothesized that soil test P (STP) correlation to dissolved reactive P (DRP) and bioavailable P (BAP) in runoff varies, depending on the extraction method. To investigate which STP extraction method would be best for predicting DRP and BAP concentration and load in runoff, soil samples were taken from the 0- to 2-cm depth of 54 grass plots (5% slopes) on Captina silt loam (fine-silty, siliceous, mesic Typic Fragiudult). The STP was extracted by six methods and the ranges of results (mg kg−1) were: 54–490 (Mehlich III), 27–592 (Bray-Kurtz P1), 25–169 (Olsen), 14–110 (distilled water), 23–170 (Fe oxide paper), and 105–1131 (acidified ammonium oxalate). The soil P saturation ranged from 16 to 80%. Simulated rain was applied at 100 mm h−1 and runoff was collected for 30 min. The concentration of DRP in total runoff ranged from 0.31 to 1.81 mg L−1, and BAP from 0.37 to 2.18 mg L−1. The r2 values for STP by each extraction method correlated with runoff DRP and BAP, respectively, were: 0.72 and 0.72 (Mehlich III), 0.75 and 0.73 (Bray-Kurtz P1), 0.72 and 0.72 (Olsen), 0.82 and 0.82 (distilled water), 0.82 and 0.82 (iron oxide paper), 0.85 and 0.82 (acidified ammonium oxalate), and 0.77 and 0.76 (P-saturation). All correlations were significant (P < 0.001), but the high r2 values of those obtained from distilled water, iron oxide paper, and acidified ammonium oxalate extractants indicate better precision for predicting DRP and BAP concentrations in runoff. Correlations of STP with DRP load (range: 43.4 to 472.8 g ha−1) and BAP load (54.2 to 542.0 g ha−1) were not useful (r2 < 0.18), possibly because runoff volumes were highly variable.