Economics of nitrogen fertilization of barley and rapeseed as influenced by nitrate‐nitrogen level in soil

Abstract

Field experiments were conducted at 92 sites for barley (Hordeum vulgare L. cv. Galt) and at 60 sites for rapeseed (Brassica campestris L. cv. Span) in central and north‐central Alberta to determine the influence of nitrate‐nitrogen (NO3‐N) level in soil (0–60 cm) on yield response to fertilizer nitrogen (N) and on Net Present Value (NPV) of returns from applied N. Grain yields of barley in the unfertilized plots increased with increasing amount of NO3‐N in soil up to 88 kg N ha‐1 and tended to decrease thereafter. Regression analysis (quadratic) showed that yield response of barley and rapeseed to applied N decreased with increasing N rate and with increasing level of NO3‐N in soil. The coefficient of determination (R2) was highest (0.79* for barley and 0.64* for rapeseed) when soils contained less than 12 kg NO3‐N ha‐1 and it decreased to 0.03 when soils contained more than 66 kg NO3‐N ha‐1. The NPV was influenced by rate of applied N, soil test NO3‐N level, cost of fertilizer N, and product price. The economic optimum rates of N fertilization declined as soil NO3‐N level increased. There was no systematic yield increase from applied N on soils with more than 88 kg NO3‐N ha‐1 for barley and with more than 72 kg NO3‐N ha‐1 for rapeseed. Our study concluded that for maximum economic returns from N fertilization, soil testing to determine NO3‐N levels is an essential practice.