Soil Test Phosphorus and Phosphorus Availability of Swine Manures with Long‐Term Application

Core Ideas Net P additions of 9.8 to 16.1 kg ha −1 were needed to increase soil test P by 1 mg kg −1 . Manure P source availability coefficients were 0.99 for liquid manure, 1.08 for solid manure, and 0.97 for composted manure. The long‐term P availability was identical, regardless of the form of swine manure. The long‐term availability of P from swine manure was similar to that of chemical fertilizer. Understanding the form‐specific long‐term effects of manure (liquid, solid, and composted) application on soil P is essential for nutrient management planning. This 8‐yr study aimed to quantify changes in soil test P (STP, Olsen‐P) with application of three forms of swine manure compared with inorganic fertilizer and to calculate the manure P source availability coefficient (PSAC) as the ratio of the specific manure form to inorganic P in providing crop P availability. The STP content increased linearly with both total and net P addition at the surface (0–15 cm) and subsurface (15–30 cm) soil depths, regardless of P source. The slope of STP vs. total or net P addition in the surface layer was steeper for liquid than for solid manure, whereas in the subsurface layer, composted manure showed a greater slope than either liquid or solid manure. The amount of applied P required to increase STP by one unit in the surface layer was 19.9, 15.7, 31.6, and 20.9 kg P ha −1 for inorganic fertilizer and liquid, solid, and composted manure, respectively. When increases in subsoil STP and grain P removal were considered, the net P additions of inorganic fertilizer and liquid, solid, and composted manure needed to increase STP by one unit were 12.2, 9.8, 16.1 and 10.7 kg ha −1 , respectively. The PSAC values were 0.99, 1.08, and 0.97 for liquid, solid, and composted manure, respectively. The long‐term availability of swine manure P was largely identical among forms and was similar to that of inorganic fertilizer.