Phosphorus Source Effects on Soil Test Phosphorus and Forms of Phosphorus in Soil

Phosphorus (P) is often supplied to field crops in organic forms such as manures or biosolids, but P availability and appropriate application rates may differ between sources. An incubation study was conducted using a Ringwood silt loam soil and seven P sources. The P sources were low, medium, and high P manure (feces) from a dairy feeding study, whole manure, fiber manure from a liquid–solid separator, biosolids from a municipal sewage treatment facility, and inorganic P applied as calcium phosphate (CaHPO4). Phosphorus sources were applied at rates of 0, 101, 202, and 404 kg total P ha−1 and incubated at 25°C. Five soil samplings were taken at 16-week intervals and analyzed for deionized water extractable P, Mehlich 3, Bray–Kurtz P1, ammonium oxalate extractable P, P saturation, bioavailable P, and anion exchange membrane extractable P. In general, the low P and fiber manures supplied the least available P, CaHPO4 the most, and medium, high, whole, and biosolids contributed intermediate amounts of P as determined by the soil P tests. The 101 and 202 kg total P ha−1 rates did not differ from each other, but were significantly lower than the 404 kg total P ha−1 rate. Except for fiber at 101 kg total P ha−1, all treatments significantly increased soil test P compared to the control. The amount of P available did not change over time except at the 404 kg total P ha−1 rate where available P usually increased with time. Correlations among soil P tests indicated significant positive relationships for each test. Bray–Kurtz P1 was most highly correlated to Mehlich 3 (r2=0.92) and least correlated to anion exchange extractable P (r2=0.13). Bioavailable P and deionized water extractable P were similarly correlated to Bray–Kurtz P1 with relatively high r2 values of 0.83 and 0.84, respectively. Ammonium oxalate extractable P and P saturation had lower r2 values (0.78 and 0.74, respectively), but were still positively correlated to Bray–Kurtz P1 and could have useful predictive value. These results indicate P availability in soil varies with the type and composition of the P source. Strong correlations among agronomic and environmental soil P tests suggest that routinely used agronomic tests or the simple DI water extractable P test could be used in place of more time consuming and expensive environmental tests to assess the P status of soils and to determine risks of various fields to release P in runoff.