Nitrogen (N) application for corn (Zea mays L.) production is often based on a yield goal, with adjustments to soil residual N content and other N credit, such as legumes and manure. It is generally clear that in arid and semiarid regions, soil mineral N test such as preplant soil N (PPSN), supplemented with pre-sidedress soil N (PSSN), is a reliable indicator for crop N requirement. In more humid areas such as Ontario, however, limitations with PPSN and PSSN arise from the extensive spatial and temporal variability in soil mineral N concentrations, which depend on numerous N-cycle processes, including mineralization, immobilization, nitrification, denitrificatin, leaching and plant uptake. A field study was conducted near Ottawa, Canada for three years to determine changes in soil available N from preplanting to shortly after the sidedress stage for corn (mid July). In each year, soil samples from fields with different cropping history, soil texture and management levels were taken at 7 to 10 d intervals. The concentrations of NO₃^- – and NH ₄⁺ - N were determined colorimetrically from the KCl extract of fresh soil samples. It was found that soil NO₃ ^- - N of PSSN was a linear function of PPSN. However, the slopes differed dramatically among cropping sequences, soil textures and precipitation. Our data suggest that caution must be taken when deciding the rate of fertilizer N sidedress based on PPSN test, especially in more humid environments.