Phosphorus needs of grassland soils and loss to water.

Abstract

Soil test phosphorus (STP) levels for optimum grassland production are considered to be in the region of 4 to 6 mg phosphorus (P) kg soil for the Morgan test used in Ireland (equivalent to 16 to 24 Olsen test). Many grassland soils have higher soil P levels because of excess inputs in fertilizer and purchased animal feed, compared with outputs in milk, meat and other produce. Continued build up of soil P has implications for P loss from soil to water and eutrophication. Recent field experiments in Ireland show a good relationship between STP and dissolved reactive P (DRP) in water from four grazed grassland fields and are in good agreement with four studies from the literature. The equation of best fit for the five studies is: DRP = 0.002(STP), with R~ = 0.92 (P < 0.001). The results indicate that for good water quality STP should be at the lower end of the range for optimum grassland production.