Date of this Version
The end-of-season corn (Zea maysL.) stalk nitrate-N test was developed as a post-mortem to determine if excessive or insufficient N was available to the corn crop during the latter part of the season. The stalk section specified for the test was very specific, the 20 cm-long section between 15 and 35 cm above the soil. Under production conditions, it may not always be possible to collect this precise stalk section. The objective of this study was to determine how nitrate concentration varied within the stalk from the soil level to the ear node, and how this variation could affect interpretations of the stalk nitrate test. Field grown (140 kg N ha-1 ) corn stalks were collected and separated into phytomers (the node plus leaf, internode, and bud developing from it). Phytomers were further divided into six segments; the node and five equal length segments of the internode. All samples were analyzed for NO3-N with a nitrate-ion specific electrode after extraction with 0.04 M (NH4)2SO4. Nitrate concentrations of individual samples varied from less than 100 to greater than 8000 mg NO3-N kg-1 dry weight, and increased down the stalk from ear to soil. Generally, the nitrate concentrations of segments within a phytomer were similar. These results indicated new critical values, approximately 35% greater than the original ones, may be needed to determine if limiting or excessive amounts of N were available to the crop, i.e. 950 vs. 700 and 2700 vs. 2000 mg NO3-N kg-1 for insufficient and excessive levels, respectively. However, the general interpretation of test would remain unchanged because stalk nitrate concentrations vary so widely under field conditions from less than 100 to greater than 5000 mg NO3-N kg-1.