Changes in Soil Test Phosphorus Concentration After Application of Manure or Fertilizer

The availability of P from injected manure application has received limited study. The objectives of this study were to determine the effect of time on P availability in soils amended with manure or fertilizer, evaluate the relative availability of manure P compared with fertilizer P, and determine the impact of previous manure history on P availability. Liquid swine manure or fertilizer was applied to Minnesota soils, from seven different mapping units with various manure histories. The application rates were equivalent to the amount of total P applied in undisturbed manure injection zones. Soils were analyzed for available P after incubation periods of 1, 2, 3, 6, and 9 mo. As the duration of incubation increased, fertilizer P became less available in six of the seven soil series, while soil test P concentrations in manured soils were unchanged. Phosphorus from liquid swine manure was more available than fertilizer P from 1 through 9 mo of incubation. It is postulated that the decomposition of manure resulted in concentrations of organic acids that effectively reduced P sorption to the soil and increased P availability. Prior manure history had a mixed impact on the availability of P from subsequent applications of manure or fertilizer. Further studies need to be conducted to determine if similar results can be found in a field setting.