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 25degreesC. 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 Pha(-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 PI was most highly correlated to Mehlich 3 (r(2) = 0.92) and least correlated to anion exchange extractable P (r(2) = 0.13). Bioavailable P and deionized water extractable P were similarly. correlated to Bray-Kurtz P1 with relatively high r(2) values of 0.83 and 0.84, respectively. Ammonium oxalate extractable P and P saturation had lower r(2) values (0.78 and 0.74, respectively), but were still positively correlated to Bray-Kurtz PI 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.