A phosphorus budget for Wisconsin cropland

Phosphorus budget information far Wisconsin cropland was compiled between 1970 and 1995 in order to identify changes and trend in budget parameters through time. The main P inputs were animal manure and commercial fertilizer, and the major P removals were crop uptake and loss through runoff and erosion. The difference between inputs and removals was accounted for as a change in the amount of P stored in the soil. Phosphorus inputs from manure have decreased from a high of 120 million lbs (54.5 million kg) in 1975 to about 98 million lbs (44.5 million kg) in 1995. Since 1985, inputs from P fertilizers have decreased by about 30%. Phosphorus removals through crop uptake have increased since 1970 due to increased crop yields. Estimated annual P losses in runoff and erosion represent about 1.4% of the P applied each year or an average loss of about 0.32 lb P ac−1 (0.36 kg P ha−1) of cropland Excess P additions to cropland have decreased by 74% since 1975, with the 1995 excess amounting to 31 million 16 (14.2 million kg) of P, or 3.3 lb of P ac−1 (3.7 kg of P ha−1) of cropland. Most of this decrease is due to reduced use of P fertilizers and increased crop removal of P due to higher yields. Most of the excess P input contributes to increased amounts of P stored in soils. This change in storage of soil P is confirmed by the increasing average soil test P values for the state.