Corn Stalk Nitrate Concentration Profile

knowledge of crop N need may be used by producers to guide future fertilizer-N management thereby improving The end-of-season stalk nitrate test provides a method of assessing profitability and reducing environmental degradation. the N available to the corn (Zea mays L.) crop during the latter part ofthe season.This studywas conductedto determinehow stalknitrate To be a useful crop management tool, methods for test results and interpretations are affected by sample composition. collecting and analyzing samples must be straightforStalkswere collectedfrom threefieldsites andseparated intophytom- ward and reasonably flexible so producers and consulers (node plus internode above), which were subdivided into three tants can adapt to field conditions as they gather samor five segments after length was measured. Nitrate-N concentration ples. It is unrealistic to assume that samples will always of phytomers decreased linearly from the soil to the ear. Within a be exactly 20-cm long and can always be collected bephytomer, segments also decreased acropetally (from base to apex). tween 15 and 35 cm above the ground under field situaNode tissue NO3–N concentration did not differ from that of the tions. For example, if samples are collected after cominternode segment immediately above the node. Weighted means bine harvest, the specified stalk segment (Binford et al., were used tocompute NO3–N concentration ofstalk samples collected 1990) may be destroyed or severely damaged and there5 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 fore inappropriate for use in assessing late-season N 15 to 35 cm above the soil). Although the three samples (10–30, 15–35, nutrition of the crop. The objective of this study was to and 20–40 cm) differed in NO3–N concentration, the difference was determine the distribution of nitrate by phytomer (node only about 15% compared with the 25% difference in sampling posi- and the internode above) within stalks of corn and how tion (5 cm of 20-cm sample length). The phytomer nearest the soil stalk nitrate test results and interpretations would be had 35 to 40% greater NO3–N concentrations than the section of stalk affected if samples were collected from a portion of the 15 to 35 cm above the soil. Critical values delineating yield-limiting, stalk different from that specified in the original method adequate, and excessive N availability should be modified if stalk (Binford et al., 1990). sectionsotherthanthestandard15to35cmsectionareused.However, the qualitative nature of the stalk nitrate test and the range of NO3–N MATERIALS AND METHODS concentrations observed with reasonable corn cultural practices (1000) make this test quite robust and precise definition of sample At physiological maturity, stalks were collected from 20 composition and critical values less necessary. corn plants grown in a rainfed, general production field at the Agronomy Havelock Farm, Lincoln, NE, to conduct initial analyses (designated the 20-stalk experiment in the remainder of the paper). NC hybrid 4880 was planted in late April at T he end-of-seasoncorn stalk nitrate test (hereafter about 49 000 plants ha 1 . Nitrogen was applied pre-plant at referredtoasthestalknitratetest)wasproposedand 140 kg ha 1 as anhydrous ammonia. Weeds were controlled advocated by Binford et al. (1990) as a method of deter- bya combinationof pre-emergenceherbicides andcultivation. mining if excessive or insufficient N was available to the Crop yield was 6.35 Mg ha 1 . Stalks were randomly assigned corn crop during the latter part of the season. In the test, to 1 of 10 replications. Leaves were removed and stalks air20-cm segments of corn stalks (between 15 and 35 cm dried before processing. During processing, stalks were cut above the soil) are collected from several plants (10), immediately below each node. Each phytomer was further dried, ground, and analyzed for nitrate-N. Nitrate-N con- divided into six pieces: the node and five equal-length pieces centrations 700 mg kg 1