Long-term effects of phosphorus fertilizer on soil test phosphorus, phosphorus uptake and yield of perennial ryegrass

Increasing phosphorus (P) fertilizer use efficiency in grassland is desirable, since uncertainty exists over the reserves of finite phosphate rock and its future availability. This necessitates revaluation of the current P fertilizer recommendations for grassland to examine the potential to increase fertilizer P efficiency. The present paper reports results from a long-term grassland P experiment (17 years) on two sites in which annual P fertilizer application rates were 0, 15, 30 and 45 kg P/ha/year. The effect of P fertilizer rate on herbage production and mineral concentration in herbage were investigated in addition to the soil test P (Morgan's) trends and P balance over the duration of the experiment for each rate of P fertilizer. The results showed that the P response to herbage yield and P concentration was similar on both sites. The response of herbage yield to P fertilizer was limited to harvests early in the growing season. The P concentration in herbage was lower in mid-season than in spring or autumn. Annual P fertilizer applications > 30 kg P/ha/year were required to maintain soil P levels at their initial levels over the duration of the experiment.