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Fluid Journal : Fall 2014
5 The Fluid Journal Fall 2014 August 2012. In 2013, analyses were conducted on data collected early (18 July 2013) and late (22 August 2013). Consistent K deficiency symptoms were not noted during the season. Red (650 nm), red- edge (730 nm) and near infrared (760 nm) were used to calculate equations described in Figure 3. Analysis of variance was conducted for both reflectance dates and yield data in JMP 10 (SAS Institute Inc., Cary, NC). Independent variables in the model included block, available K2O variety, and the interaction between available K2O, and variety. 2012 results The 2012 response to seed-cotton to changes in variety and available K2O was significant (p≤0.05) as was the interaction between these two terms (p≤0.10). Results suggest increases in available K2O did not significantly increase PHY499 seed-cotton yields, but did increase DP0912 and ST5458 yields. As evident by the plant available K levels and relatively high yields, severe K deficiencies were not noted. Low to moderately sufficient soil K may have contributed to the failure of PHY499 yields to respond to increased plant-available K. Still, the moderately strong response of ST5458 and slight response of DP0912 do not suggest that increased K availability could increase yields within the tested range of plant-available K2O for these two cultivars. Visible K deficiency symptoms were noted during the first week of flower in ST5458 plots but were not consistent across the field until near peak flower. As a result, reflectance was measured at mid-flower (7 August 2012) and after peak flower 22 August 2012). Images of typical canopy response to varying K2O applications can be seen in Figure 3. Responses from both sampling dates were similar. The interaction effects between plant-available K2O and cultivar on the Normalized Difference Vegetation Index (NDVI) readings were significant (p≤0.10 [Figure 4]). However, the Canopy Chlorophyll Content Index (CCCI) was only significantly affected by variety, as available K had no significant effect on CCCI (p≤0.05 [Figure 4]). 2013 results During the 2013 season, neither index responded significantly to available K2O (p≤0.05). This held true for both sampling dates (18 July and 22 August 2013). The response of CCCI to cultivar was significant in both sampling dates (p≤0.05), with PHY499 resulting in significantly greater, and DP0912 resulting in significantly lower CCCI values. In contrast, the response of NDVI to variety was not significant at the first sampling date and, although as significant response was noted in the later sampling date, the only varietal separation was noted with PHY499 (Figure 5). Overview Potassium deficiencies severe enough to impact reflectance during the typical side-dress N application Figure 2. Collecting sensor reflectance data in the cotton field. Figure 3. Reflectance signature curve of N deficient and N sufficient cotton. Red bars represent center of red, red-edge, and near-infrared regions typically used by active sensors. Equations represent calculations of commonly used reflectance indices of the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Red-Edge Index (NDRE), and the more recently developed Canopy Chlorophyll Content Index (CCCI).