Effects over time of fertiliser P and soil series on P balance, soil-test P and herbage production

Abstract

Quantification of the balance between P input and offtake (P balance), and the consequent effect on soil-test P, is essential for management of sustainable soil and fertiliser nutrient supply. Results of measurements on 31 cut swards showed that change in P balance over 4 years was significantly affected by both P treatment and soil series. The more negative P balances were in high-P soils, and in soils of non-limestone parent material compared with limestone parent material. Initial mean Morgan P of 4.3 and 12.6 mg/l, in low and high index groups (0 to 6.0 and ≥6.1 mg/l) decreased to 1.7 and 4.4 mg/l after 4 years with no P treatment, in response to annual changes of ca. 25 to 35 kg/ha in P balance. Decreases were progressively smaller with increased P input, and smaller in non-limestone than limestone soils. The ratio of negative P balance to change in Morgan P varied from 20:1 to 70:1 depending on soil P index and parent material. Five sites gave a response to P in the final year following annual P inputs of 20 and 40 kg/ha, although Morgan P was ≤3.0 mg/l at 12 sites in the preceding autumn and 3.1 to 6.0 at nine sites. The results showed that both P balance and soil series should be taken into account in efficient fertiliser management, and that data from cut swards can be extrapolated to grazed swards when adjusted for P offtake. However, the results did not support the assumption that inputs balance offtakes in direct proportion.