Effect of Site Class and Rainfall on Annual Range Response to Nitrogen and Phosphorus

Abstract

Maximum forage yield increases from application of fertilizer to annual range in southern California were in the following order according to site class: swale > gentle slope > open slope. The increase in forage yield per pound of applied nitrogen was greatest for low rates of nitrogen at the slope sites and for a much higher rate at the swale site. Residual effect of fertilizer on forage yield, measured the second and third year after application, partially compensated for a first-year lack of response at lower producing sites or during low rainfall years. Yield increase the second year after application of 60 lb. of nitrogen and 26 lb. of phosphorus per acre was 50% of the increase from the same application that year. Range site class and fertilizer residual effects are significant factors in fertilizer application programs on annual range in lower rainfall areas with high annual rainfall variability. Annual range soils that are deficient in available phosphorus by soil test require that the more limiting nitrogen deficiency be corrected before a forage yield response to phosphorus will be obtained.