Understanding the spatial variability of soil mineral nitrogen (SMN) and crop growth is an important step for implementing precision nitrogen (N) management technologies for canola production. A 3 yr field experiment in Ontario investigated the within-field spatial variability of SMN in relation to growth parameters and yield. Each year, large strips in a commercial field were randomly assigned a preplant N treatment (0, 50, 100, and 150 kg ha^-1), with three replicates of each. Our data showed that SMN varied widely among field-strips receiving different treatments and also within strips receiving the same N rate, indicating significant spatial variability in N availability at the field- and strip-scale. Some crop measurements exhibited wide variations in parallel with the SMN dynamics. At the early flowering stage, SMN contents displayed a strong relationship with plant height and branch numbers. Although grain yield showed a positive response to N, the inconsistent yield increase with increasing N supply was likely due to the inherent variations in soil N supply among years and fields, indicating an inefficient use of the uniformly applied preplant fertilizer N by the crop. The strong associations between SMN and crop parameters or yield provided a substantial evidence for implementing in-season variable rate N application.