The fate of phosphatic fertiliser applied to grassland

Abstract

Phosphorus is an essential element for healthy plant and animal growth. The soils of Ireland were very poor in phosphorus status in the 1950s. Repeated applications of phosphorus over the decades has resulted in the soil phosphorus levels rising from 1 to 9 mg/1 of soil. This paper assesses the fate of applied phosphatic fertiliser over a 30 year period on a grassland site that was rotationally grazed by beef animals for each grass growing season. The treatments were 0, 15 and 30 kg P/ha applied annually in spring. The amount of P that was exported in beef ranged from 6 percent to 20 percent suggesting that most of the P was still in the soil or lost via overland flow. The total loss of P fertiliser via overland flow over 30 years from plots with a Morgan's soil test of 6 and 17 mg/1 were 19 and 130 kg/ha, respectively. Mass balance studies showed over 75 percent of the applied P remained in the soil. A large portion of this phosphorus was accumulated near the soil surface.