RELATING EXTRACTABLE SOIL-PHOSPHORUS TO PHOSPHORUS LOSSES IN RUNOFF

Phosphorus in agricultural runoff can cause accelerated lake and strea m eutrophication. Where producers have applied P at rates exceeding cr op uptake, soil P has sometimes become the main source of P in runoff. We hypothesized that soil test P (STP) correlation to dissolved react ive P (DRP) and bioavailable P (BAP) in runoff varies, depending on th e extraction method. To investigate which STP extraction method would be best for predicting DRP and BAP concentration and load in runoff, s oil 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 Frag iudult). The STP was extracted by six methods and the ranges of result s (mg kg(-1)) were: 54-490 (Mehlich III), 27-592 (Bray-Kurtz P1), 25-1 69 (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 c ollected for 30 min. The concentration of DRP in total runoff ranged f rom 0.31 to 1.81 mg L(-1), and BAP from 0.37 to 2.18 mg L(-1). The r(2 ) 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 0.7 3 (Bray-Kurtz P1), 0.72 and 0.72 (Olsen), 0.82 and 0.82 (distilled wat er), 0.82 and 0.82 (iron oxide paper), 0.85 and 0.82 (acidified ammoni um oxalate), and 0.77 and 0.76 (P-saturation). All correlations were s ignificant (P < 0.001), but the high r(2) values of those obtained fro m distilled water, iron oxide paper, and acidified ammonium oxalate ex tractants indicate better precision for predicting DRP and BAP concent rations in runoff. Correlations of STP with DRP load (range: 43.4 to 4 72.8 g ha(-1)) and BAP load (54.2 to 542.0 g ha(-1)) were not useful ( r(2) < 0.18), possibly because runoff volumes were highly variable.