A phosphorus budget for Wisconsin cropland

ABSTRACT:

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.