Soil and Phosphorus Source Effects on Phosphorus Buffering Capacity.

The use of phosphorus (P)-based nutrient management planning makes it important to understand the effects of P additions from various sources on changes in soil P status as measured by soil P tests.  The impact of P inputs and removals on soil test P (STP) status is often expressed in terms of the soil P buffer capacity (PBC), which describes the amount of P removal or addition necessary to achieve a one-unit change in STP value.  The objectives of this study were: 1) to determine soil PBC values where P is added in dairy manure, biosolids, and inorganic P and 2) to determine differences in PBC of several Wisconsin soils.  An incubation was performed in which potassium phosphate, dairy manure, and a biosolid were applied at 110 kg total P ha^-1 to four soils representing major soil groups in Wisconsin.  Differences between soils were evaluated using Bray P1, Mehlich 3P, water extractable P, degree of phosphorus saturation, and PBC.  The P fertilizer had the lowest PBC (3-5 kg P ha^-1) and the biosolid produced similar or slightly higher PBC values than the dairy manure (7-11 kg P ha^-1 for the biosolid and 6-9 kg P ha^-1 for the diary manure).  Additionally, PBC differed among the soils used, and changes in STP were related to initial STP level but not to P source characteristics such as WEP, TP, or the molar ratio of P to Fe and Al.  These data validate the need to incorporate PBC values that are appropriate to specific P sources and soils for Wisconsin P Index runoff risk calculations.  Accurate PBC values will also improve fertilizer recommendations and should provide better economic and environmental resource management.