The end-of-season stalk nitrate test provides a method of assessing the N available to the corn (Zea mays L.) crop during the latter part of the season. This study was conducted to determine how stalk nitrate test results and interpretations are affected by sample composition. Stalks were collected from three field sites and separated into phytomers (node plus internode above), which were subdivided into three or five segments after length was measured. Nitrate-N concentration of phytomers decreased linearly from the soil to the ear. Within a phytomer, segments also decreased acropetally (from base to apex). Node tissue NO3-N concentration did not differ from that of the internode segment immediately above the node. Weighted means were used to compute NO3-N concentration of stalk samples collected 5 cm higher (from 20 to 40 cm above the soil) or lower (from 10 to 30 cm above the soil) on the stalk than in the original method (from 15 to 35 cm above the soil). Although the three samples (10-30, 15-35, and 20-40 cm) differed in NO3-N concentration, the difference was only about 15% compared with the 25% difference in sampling position (+/-5 cm of 20-cm sample length). The phytomer nearest the soil had 35 to 40% greater NO3-N concentrations than the section of stalk 15 to 35 cm above the soil. Critical values delineating yield-limiting, adequate, and excessive N availability should be modified if stalk sections other than the standard 15 to 35 cm section are used. However, the qualitative nature of the stalk nitrate test and the range of NO3-N concentrations observed with reasonable corn cultural practices (1000x) make this test quite robust and precise definition of sample composition and critical values less necessary.
