| Fertilizer or manure applications are increasing soil P in agricultural fields. This study assessed the impact of P application on soil P fractions and P loss with surface runoff based on P rate field trials and both indoor and field rainfall simulations. Bray P, Mehlich-3 (M3) P, Olsen P, total P (TPS) and soil P saturation (Psat) estimated by M3 or oxalate extractable P, Fe, and Al molar ratios increased with cumulative P application (0 to 1098 kg P ha−1 after 4 to 23 yr) at plots from 11 trials. Soil test P, TPS, and Psat were linearly correlated. Routine soil P tests can approximate long-term effects of P application on TPS and Psat for soils and P ranges considered. In an indoor rainfall simulation study, Marshall, Nicollet, Fayette, Tama, and Harps (calcareous) soils were incubated with 0, 50, 125, 300, and 600 mg kg−1 P. Phosphorus application increased soil and runoff P concentrations linearly, and often at higher rates for the calcareous soil, but P relationship differences in soil or runoff were small or nonexistent across four noncalcareous soils. Three P tests, two environmental P tests (FeO-impregnated paper and water extraction), TPS, and Psat (M3, oxalate, and a P sorption index) increased linearly with P rate and were highly correlated. Environmental soil P tests and Psat indices correlated no better with dissolved reactive P (DRP), bioavailable P (BAP), or total P (TPR) than routine soil P tests. A field study assessed runoff P losses following liquid swine manure application (up to 220 kg ha−1 P) and simulated rainfall. When manure was not incorporated and rainfall was applied within 24 h, increasing P rates increased runoff P linearly and the fraction of TPR as DRP or BAP increased. When manure was incorporated, runoff P did not increase or increased only slightly. A 10 d rainfall delay sharply decreased runoff P in non-incorporated treatments, sometimes to lower levels than with incorporation. Incorporating manure when the probability of immediate rainfall is high reduces the risk of P loss in surface runoff; however, this benefit decreases with time and incorporation could lead to greater erosion and TPR loss in the long term. |
