The Cowlands long-term field trial experimental field showing layout of six replicate plots for each of six P treatments. Starting in 1968 P was applied annually at 0 (P0), 15 (P15) and 30 (P30) kg ha−1 to each of 12 replicate plots. N (240 kg ha−1) and K (20 kg ha−1) were applied annually to all plots. In 1999 the P application was altered such that half (i.e. 6) of the plots from P0 now received 30 kg P ha−1 year−1, half from P15 received 5 kg P ha−1 year−1 and half from P30 received 0 kg P ha−1 year−1. Thus, since 1999 there were six replicate plots of six P treatments: 0_0, 0_30, 15_15, 15_5, 30_30, 30_0 (kg P ha−1 year−1 1968–1999 _ kg P ha−1 year−1 1999–2009).