Soil sampling for nitrate in late spring when corn plants are 15 to 30 cm tall can be used to obtain information needed for site-specific management of N during corn production. However, variability in soil nitrate concentrations causes sampling problems when using the test to assess N availability. Although variability of soil nitrate concentrations has been studied, there is a lack of information on the uncertainty associated with assessment of nitrate concentrations for small research-sized plots. Also, few studies have been conducted to develop guidelines for efficient sampling of fields for soil nitrate in late spring. Knowledge of the variability of nitrate concentrations within and among small areas of seemingly uniform soil within a field is essential for site-specific management of N;Studies were conducted to learn more about the variability of nitrate concentrations within small plots of seemingly uniform soil and variability among these small plots within a field. Results from these studies show that there is a high degree of uncertainty associated with the determinations of nitrate concentrations from samples collected from small plots of seemingly uniform soil. Such uncertainties must be considered whenever soil nitrate results are interpreted. This problem, however, can be minimized by collecting several samples, looking for trends established by most samples, and giving little credence to samples that do not follow the observed trends. The results also show that much of the within-field variability in nitrate concentrations can be explained by differences in Kjeldahl N concentrations. These findings suggest that within-field differences in soil organic matter concentrations, which are correlated with Kjeldahl N concentrations, offer a practical way to divide fields into 'sites' for soil nitrate testing. This is helpful because differences in organic matter contents can be visually identified when soil samples are collected.