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Fluid Journal : Fluid Journal 2008-2009
season N variability in corn (Figures 1 and 2). While each sensor yielded different levels of NDVI, the overall trend of the NDVI readings for each sensor generally increased with increasing corn growth stage. Table 1 shows that both sensors were able to account for similar levels of variability when NDVI was regressed on yield. The r-square for each sensor generally increased with each successive crop growth stage. The highest r-square from NDVI readings was collected at the V14 crop growth stage. The Crop CircleTM sensor achieved an r-square of 0.71 and the GreenSeekerTM achieved an r-square of 0.75 at the V14 crop growth stage. These results suggest that both sensors used in this study correlated relatively well with corn grain yield at later (V14) corn growth stages without the aid of any other ancillary crop or soil variables that often are destructive to the plant. However, these correlations still leave a significant hole in determining accurate N requirements. Ancillary Data Analysis for the ancillary crop and soil data is presented in Figures 3, 4, 5, 6 and 7. The corn grain yield data by applied N are also presented there. SPAD leaf chlorophyll content, soil N content, and corn grain yield are a function of applied N rate. Leaf N content and plant height are the function of an applied N rate and corn growth stage interaction. SPAD leaf chlorophyll content increased with increasing N application rate. The 175 lbs N/A rate had the highest SPAD reading, the 50 lbs/A rate had the lowest (except for the check), and the 100 lbs/A rate was intermediate. Corn leaf N content increased across applied N rates at all growth stages except V8, which increased up to 100 lbs/A and then decreased at 175 lbs/A. Leaf N content also decreased with each successive growth stage, the V8 growth stage having the highest leaf N content at all N rates, with V10, V12, and V14 each decreasing in leaf N. Soil N content at each N application rate did not follow any trend as results appeared to be random. The soil N variable has many other factors contributing to it rather than just applied N. Residual soil N, organic matter mineralization, and other factors likely contribute to soil N, meaning applied N rate alone may not necessarily be a good indictor of this variable. Plant height also increased with corn growth stage but not necessarily with applied N rate. At all growth stages there was an increase above 0 lbs/A rate, but at the V8 and V10 growth stages Figure 1. Crop CircleTM NDVI readings by corn growth stage over N rate. Figure 2. GreenSeekerTM NDVI readings by corn growth stage over N rate. Figure 3. SPAD readings by N rate. N Rate NDVI 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 p-value = <0.0001 LSD 0.05 = 0.31 0 50 100 175 V14 V12 V10 V8 N Rate NDVI 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 p-value = <0.0001 LSD 0.05 = 0.32 0 50 100 175 V14 V12 V10 V8 N Rate p-value = <0.0001 LSD 0.05 = 2.5 SPAD 50 45 35 30 25 15 40 20 10 0 50 100 175
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