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

Field experiments were conducted at 92 sites for barley (Hordeum vulgare L. cv. Gait) and at 60 sites for rapeseed (Brassica campestris L. cv. Span) i n central and north-central Alberta to determine the influence of nitrate-n itrogen (NO3-N) level in soil (0-60 cm) on yield response to fertilizer nit rogen (N) and on Net Present Value (NPV) of returns from applied N. Grain y ields of barley in the unfertilized plots increased with increasing amount of NO3-N in soil up to gs kg N ha(-1) and tended to decrease thereafter. Re gression analysis (quadratic) showed that yield response of barley and rape seed to applied N decreased with increasing N rate and with increasing leve l of NO3-N in soil. The coefficient of determination (R-2) was highest (0.7 9* 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 sys tematic yield increase from applied Fd 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 s tudy concluded that for maximum economic returns from N fertilization, soil testing to determine NO3-N levels is an essential practice.