Corn stalk nitrate research update
The cornstalk nitrate test (CSNT) is recommended as an end-of-season assessment of an N management program (N source, timing, placement, and rate). For this diagnostic test, 15 or more 8-inch stalk segments (beginning 6 inches above the soil surface) are taken from representative areas of a field within a few weeks after blacklayer and analyzed for nitrate-nitrogen (NO3-N). Accumulation of NO3-N in the lower corn stalk results from N availability exceeding crop N utilization. More details on the test can be found in Extension Bulletin AY-322-W.1
The CSNT was developed at Iowa State University in the late 1980’s and the interpretation of cornstalk NO3-N concentrations used in Iowa differs very little from that used in Indiana. Iowa State Univ. considers less than 250 parts per million (ppm) NO3-N to be low, 250 to 700 ppm to be marginal, 700 to 2,000 ppm to be optimal, and greater than 2,000 ppm to be excessive.2 Previous research conducted in Indiana in 1996 and 1997 concluded that NO3-N concentrations between 450 and 2,000 ppm were associated with optimal N availability while concentrations greater than 2,000 ppm NO3-N indicated N availability was excessive.1
click image to zoomFigure 1. Stalk nitrate-N relationship to relative yield for 23 location-years of N trials conducted in Indiana from 20070-2009 and 2011-2012. Within each location and year the yield of an individual N rate treatment was related to the predicted maximum yield at that location in that year. The relationship between cornstalk NO3-N and relative yield from the most recent Indiana N response trials (23 location-years conducted in 2007-2009 and 2011-2012) (Fig. 1) are quite similar to the earlier findings in both Iowa and Indiana (despite 30+ years of hybrid improvement) suggesting similar interpretations are relevant today. Most of these studies were conducted with at-planting or sidedress N application as 28% urea-ammonium nitrate. Although the timing and form of N were not found to alter the relationship between cornstalk NO3-N in earlier Iowa and Indiana research recent research conducted by the On-farm Network, Iowa Soybean Association suggests the fall application of manure may need to be evaluated differently (more on this later).
The recent Indiana data was categorized by cornstalk NO3-N and for each category the average relative yield and the average difference in fertilizer N rate relative to the N rate needed to maximize yield was determined. Seventy-two percent of cornstalk samples had NO3-N concentrations below 250 ppm (Table 1). Relative yield in this NO3-N category ranged from 25 to 110% of maximum yield (Figure 1), averaging 82%. All relative yields less than 80% of maximum yield were associated with cornstalk NO3-N concentrations less than 250 ppm (Figure 1). However, many N rate treatments producing maximum yield also had less than 250 ppm cornstalk NO3-N. Therefore, a low level of cornstalk NO3-N does not necessarily mean the crop was short of N.