CORN STALK NITRATE CONCENTRATION PROFILE

Authors: Wilhelm, Wallace; Varvel, Gary; Schepers, James

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2005
Publication Date: 12/1/2005
Citation: Wilhelm, W.W., Varvel, G.E., Schepers, J.S. 2005. Corn stalk nitrate concentration profile. Agronomy Journal 97:1502-1506.

Interpretive Summary: The end-of-season stalk nitrate test was proposed several years ago as a method of determining if excessive or insufficient N was available to the corn crop during the latter part of the season. Excessive N applications reduce profit and risk contaminating ground water with nitrate. Insufficient N applications limit yield, and in turn, revenue from the crop. The test gives farmers a way to evaluate their N management practices during the past season and information to help manage future N application. The original method called for a specific segment (from 6 to 14 inches above the soil surface) of the stalk shortly before or after harvest to be analyzed for nitrate concentration. Under field conditions it may not always be possible to collect this segment. For example, parts of this section of the stalk may be destroyed during combine harvest. In addition, under research conditions, the specified segment may be inconvenient to collect. Our objective was to determine how test results and interpretations changed if the stalk segments analyzed differed from the 8-inch piece specified in the original procedure. We initially thought that if a sample contained more than one node instead of all internode tissue or one node and one internode, the test results would be affected. However, we discovered that the amount of node and internode tissue in the sample did not change the results. We also found that nitrate concentrations were greatest in a section of stalk collected at the soil surface and decreased rapidly as sections were collected closer to the ear. For example, if the section of stalk analyzed was collected from 8 to 16 inches above the soil, instead of specified 6 to 14 inches, test results were reduced by 15%. However, the qualitative nature of the end-of-season stalk nitrate test and the wide range of stalk nitrate concentrations seen with modern corn cultural practices (from <100 ppm to nearly 10,000 ppm) make this test robust and quite tolerant of small differences in segment of stalk analyzed.

Technical Abstract: The end-of-season stalk nitrate test provides a method of estimating if excessive or insufficient N was 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 (1) by sample composition and (2) if samples were collected from a portion of the stalk different from that specified in the original method (from 15 to 35 cm above the soil surface). 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 and recorded. Nitrate-N concentration of each segment was determined with a NO3 electrode. Nitrate-N concentrations 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 the 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 cm, 15-35 cm, and 20-40 cm) differed in NO3-N concentration, the difference was only about 15% compared to 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 for 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 end-of-season stalk nitrate test and the wide range of stalk NO3-N concentrations observed with reasonable corn cultural practices (3 orders of magnitude) make this test quite robust and precise definition of sample composition and critical values less necessary.