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.