Using a Nitrate Specific Ion Electrode to Determine Stalk Nitrateâ•fiNitrogen Concentration

tion procedure (Lachat Instruments, Milwaukee; Method 12-107-04-1-B) to determine NO3 concentration The end-of-season stalk NO3 test has been used to determine N in aliquots of filtered extracts prepared by shaking sufficiency in corn (Zea mays L.). Nitrate concentration is commonly known weights of ground stalk material for 30 min in determined with flow-injection analysis (FIA), which is accurate but 100 mL of 2 M KCl. Though accurate, these analytical uses hazardous chemicals and is time-consuming. Use of a simpler procedures are expensive, time-consuming, and employ method of NO3 determination, such as the NO3 specific ion electrode (SIE), may save time and costs, and reduce hazards. The objective hazardous chemicals (strong acids and bases and Cd). of this study was to compare estimates of stalk NO3 concentration by Given that the goal of the stalk NO3 test is to deterFIA and NO3 SIE. For FIA, NO3 was extracted with 2 M KCl, and mine if stalk NO3–N concentrations are less than 700 the extract was filtered before analysis. For SIE, NO3 was extracted mg kg21 or greater than 2000 mg kg21, it seems logical with 0.04 M (NH4)2SO4, and the extract was analyzed without filtration. that a somewhat less accurate procedure could provide The slope of the linear regression between concentrations estimated essentially the same information, with the possibility of by SIE and FIA did not differ from 1.0. Use of the NO3 SIE, compared saving time and laboratory resources and avoiding with FIA, reduces costs, sample processing, and use of hazardous safety and environmental hazard issues. A candidate chemicals. procedure that is less expensive and less time-consuming, but may be less accurate, is the use of a NO3 SIE. The object of this study was to compare stalk NO3 conT end-of-season corn stalk NO3 test was proposed centration determined by the flow-injection method and and advocated by Binford et al. (1990) as a method NO3 SIE techniques. of determining if excessive or insufficient N was available to the corn crop during the latter part of the season. In the test, 20-cm segments of corn stalks (between 10 MATERIALS AND METHODS and 30 cm above the soil) are collected from several Shortly after physiological maturity, stalk samples were colplants (≈10), dried, ground, and analyzed for NO3–N. lected from 10 corn plants in a crop sequence 3 inbred line 3 Nitrate N concentrations less than about 700 mg kg21 N rate experiment initiated to determine the optimum rate plant tissue indicate that available N limited grain yield; of N fertilizer application for hybrid seed production fields NO3–N concentrations above 2000 mg kg21 indicate that (Wilhelm and Johnson, 1997). Twenty-two (Table 1) of these excessive amounts of N were available to the crop (Binsamples were selected for use in this study to compare methods ford et al., 1992). Other researchers have evaluated the of determining stalk NO3 concentration. Samples were seproposed test and concur that when end-of-season stalk lected a priori to represent the range of treatment combinaNO3 concentrations are great (.2000 mg kg21), excestions in the study, and therefore were assumed to provide sive levels of N were available to the crop (Varvel et samples covering the range of stalk NO3 concentrations found al., 1997). These studies suggest that the end-of-season in producers’ fields. corn stalk NO3 test can be used as a postmortem to Stalk segments were 10 to 20 cm in length and came from determine if yield-limiting or excessive N was present. the base of the stalk, from 0 to 25 cm above the soil surface. Historical knowledge of crop N need may be used by At sampling time, all plants in a 3.1-m segment of row were producers to guide future fertilizer-N management, cut at the soil surface and moved to the field edge. Ten of thereby improving profitability and reducing environthese plants were selected at random and a stalk segment was mental degradation. taken from each. Each stalk segment was composed of one In the initial publications on use of the end-of-season node and one internode (Fig. 1). Individuals collecting the stalk NO3 test, Binford et al. (1990, 1992) reported using samples estimated the fraction of total length of internode the MgO–Devarda alloy steam-distillation procedure between the lowest node and the cut end of the stalk on each (Keeney and Nelson, 1982) and the Lachat1 flow-injecsampled plant. The length of internode above the lowest node needed to represent the complement of the fraction below 1 Mention of commercial products in this paper is solely to provide the node was estimated and the stalk cut at that point. In the specific information for the reader. It does not constitute endorsement example shown in Fig. 1, about 0.3 of the internode below by the USDA-ARS or University of Nebraska’s Agricultural Research the lowest node remained on the stalk as it was removed from Division over other products that may be suitable. the field. To collect the equivalent of one internode, 0.7 of USDA-ARS, 119 Keim Hall, Univ. of Nebraska, Lincoln, NE 68583the internode above the lowest node was estimated and the 0934. Joint contribution of the USDA-ARS and the Agric. Res. Division of the Univ. of Nebraska. Published as Journal Series no. 12603. Received 4 May 1999. *Corresponding author ([email protected]). Abbreviations: FIA, flow-injection analysis; SIE, specific ion electrode. Published in Agron. J. 92:186–189 (2000).