Soil total nitrogen concentration explains variation in pasture response to spring nitrogen fertiliser across a single farm.

Soil total N (TN) has been shown to be a useful surrogate measure of soil N supply in pasture soils. Since soil N supply can modify fertilizer N response, we hypothesized that TN would be a good predictor of the relative pasture dry matter (DM) response to applied fertilizer N across a single property. We chose a single property to exclude confounding effects from large variation in weather. Nitrogen response experiments were placed in 20 paddocks on a single farm to test this. Each experiment comprised a single application in spring of 25, 50, 100 or 200 kg N/ha (plus a nil-N control). Range of soil TN (0–75 mm) was 0.3–1.3 %. The full response to applied N was captured in the first two pasture harvests (49 days), even at the highest N rate. Nil-N yield increased with increasing TN, but the maximum yield with 200 kg N/ha was constant across sites. Consequently, the response to fertilizer N (difference between yield at 200 N and nil-N) decreased with increasing soil TN concentration, with TN explaining c. 53 % of the variation in fertilizer N response (P\0.001); this increased to 72 % when all N rates were included. The experiment suggests that use of TN, as a surrogate estimate of soil N supply, can improve the prediction of pasture DM response to applied fertilizer N in spring to a ryegrass/clover sward. Further work is required to test this in other seasons and at other sites.