Submitted to: Plant Nutrition Colloquium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/4/2001
Publication Date: 11/1/2001
Citation: Wilhelm, W.W., Varvel, G.E., Schepers, J.S. 2001. Corn stalk nitrate concentration profile: implications for the end of season stalk nitrate test. pp. 708-709. Plant Nutrition Colloquium Proceedings. Interpretive Summary: The end-of-season corn (Zea mays L.) 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) 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 nitrate-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 nitrate-N/kg 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 nitrate-N/kg for insufficient and excessive levels, respectively. 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 nitrate-N/kg.
Technical Abstract: The end-of-season corn stalk nitrate-N test was developed by scientists at Iowa State University to help farmers determine if they had applied insufficient, appropriate, or excessive amounts of nitrogen fertilizer to their corn. The developers of the test found that if the concentration of nitrate in a specific portion of the stalk (an 8inch- long section between 6 inches and 14 inches above the soil) was less than 700 ppm, yield may have been limited by amount of available N; whereas, if the nitrate concentration was greater than 2000 ppm, more N than needed to support maximum yield may have been applied. If the nitrate concentration was between these values, an appropriate amount of N was likely available to the crop. By appropriate, we mean that yield was not limited by N availability, nor was so much N applied that movement of nitrate to surface or ground water would occur. Growers, consultants, and crop advisors have used the test successfully. However, by defining the specific section of stalk for the test so precisely, collection of samples is difficult. We analyzed the nitrate concentration of many small sections of stalks from the soil surface to the ear to find out if the concentration varied greatly and, if samples for the end-of-season stalk nitrate test were collected somewhat differently, would the values defining insufficient, appropriate, and excessive N application change. Concentration of nitrate was greatest near the soil surface and declined as the samples came for parts of the stalk closer to the ear. For convenience, many growers and researchers may want to collect samples representing one node (about 6 inches long) from just above the soil surface.