Management of soil phosphorus fertility determines the phosphorus budget of a temperate grazing system and is the key to improving phosphorus efficiency

The phosphorus (P) efficiency of fertilised grazing systems on P deficient soils is often very low. P budgets (P input vs P output) were developed to examine P use in a long-term experiment in which grazing systems were managed with contrasting soil test P concentrations: (i) no P-fertiliser (P0; Olsen P = 2-5 mg/kg), (ii) near-optimum soil P fertility (P1; Olsen P = 10-15 mg/kg), (iii) supra-optimal P fertility (P2; Olsen P = 20-25 mg/kg), or (iv) with variable P fertility. Pastures were grazed with either 9 or 18 sheep/ha. P was exported as liveweight gain in sheep removed from the fields. Fertilised fields accumulated 89-93% of their P input over the whole P-budgeting period (1994-2006). However, this included P that was contributing to a “build up” in soil fertility (1994-2000). The efficiency of P fertiliser use was better demonstrated by P budgets during a soil P fertility “maintenance” phase (2001-2006) in which P inputs and soil test P concentrations of the grazing system treatments were relatively stable. When the amounts of P associated with the small changes in soil fertility were accounted for, the accumulation of P was 43-52 kg P/ha (83-87% of P inputs) in P1 fields and 87 kg P/ha (88% of P inputs) in P2 fields over the six-year period. Differences due to stocking rate were relatively small. Audits of the total P in sheep camp soil and field soil demonstrated that sheep camps were not a major sink for the P that was accumulating in the grazed fields. P was mainly accumulated in soil in the non-camp area of fields when they were fertilised and this was the major reason for low P-balance efficiency. It was concluded that the annual rate of P accumulation in fertilised soil (due mainly to P-sorption reactions) was higher when soil is being maintained at higher extractable-P concentrations. Consequently, strategies that can achieve equivalent pasture production with lower concentrations of extractable-P in the soil should reduce the amount of P fertiliser necessary for high production.