INFLUENCE OF PREVIOUS CROPPING PRACTICES ON THE RESPONSE OF SPRING WHEAT TO APPLIED N

Nitrate (NO3) leaches from the rooting zone into ground waters during late fall and early spring in Atlantic Canada, particularly from soils cropped to potatoes, both in the year of potatoes and in the followin g year in the rotation. Therefore, it is imperative to maximize N fert ilizer efficiencies in order to minimize environmental degradation. Si x field trials were conducted in the upper St-John River Valley of New Brunswick to determine the effect of two previous crops, oats and pot atoes, on the amount of residual NO3 in the spring following their har vest, and to determine if spring soil NO3 could be used as a single cr iterion to predict fertilizer N requirements of wheat. At all six fiel d trials, spring soil NO3-N (0-0.45 m) was greater with potatoes as th e previous crop (average of 86 kg NO3-N ha(-1)) than with oats (averag e of 56 kg NO3-N ha(-1)). As a result, the economically optimum fertil izer rate (Nop) for spring wheat was lower on soil previously cropped to potatoes (108 kg N ha(-1)) than oats (135 kg N ha(-1)). Nop, howeve r, was poorly correlated with the amount of spring soil NO3-N. We conc lude that conventional spring soil tests based on NO3-N are inappropri ate to predict fertilizer N requirements of wheat in Atlantic Canada. Fertilizer N recommendations in Atlantic Canada, however, may be impro ved by taking previous crop into consideration.