Interactions Between the Effects of Nitrogen, and Phosphorus, Potassium and Sulphur on Grass Production

Interactions Between the Effects of Nitrogen, and Phosphorus, Potassium and Sulphur on Grass Production

The interactions between rates of nitrogen (N) and rates of phosphorus (P), potassium (K) or sulphur (S) on yield of ryegrass dominant swards were measured with no return of mown clippings in Southland, New Zealand. In the second year of the trial, the yield response to 176 kg N ha^-1 compared with no N increased only slightly with increasing soil Olsen P from 8 to 91 µg g^-1 soil. At 352 and 704 kg N ha^-1 there was a large increase in the yield response (from 5 to 9.4 and 11 t DM ha^-1 respectively) up to an Olsen P of 55 µg g^-1 soil. At 176 kg N ha^-1, there was a linear yield response (from 3.5 to 6.5 t DM ha^-1) as ammonium acetate extracted soil K increased from 80 to 120 µg g^-1 soil. A larger yield response to K (from 3.5 to 13.5 t DM ha^-1) was measured up to but not above 240 µg K g^-1 soil at 352 and 704 kg N ha^-1. The yield response to 352 and 704 kg N ha^-1 increased with soil S status (from 8.2 to 11.8 t DM ha^-1) up to 15 µg calcium phosphate extractable S g^-1 soil. These results demonstrated that yield responses to increasing rates of N do not increase above a threshold level of other soil nutrients.