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Fluid Journal : Fall 2014
6 The Fluid Journal Fall 2014 Dr. Raper is Assistant Professor of cotton and small grains at the University of Tennessee, Dr. Oosterhuis is Professor of Crop Physiology at the University of Arkansas, Dr. Espinoza is Extension Soil Specialist at the University of Arkansas, and Taylor Coomer is a graduate research assistant at the University of Arkansas. Figure 4. Images of deficiency symptoms noted on 23 Aug 2012 in all three tested varieties at low (0 lb K2O/acre) and high (90 lb K2O/acre) rates. Figure 5. Response of the Normalized Difference Vegetation Index (NDVI) and the Canopy Chlorophyll Content Index (CCCI) to different varieties. window were not noted in either 2012 or 2013. Still, the significant interaction of variety and plant-available K on NDVI noted in 2012 suggests that cultivar-specific models may have to be developed to characterize specific NDVI responses to varying soil-K concentration if K deficiencies occur during the sidedress N application window or if NDVI will be used to drive foliar N or K applications. Failure to detect deficiencies of K during the 2013 growing season may be partially explained by an exceptionally mild summer, which was suspected to support exceptional root growth and therefore greater K uptake under the low-moderate soil test K values. Still, the significant impact of variety on CCCI and lack of impact of available K2O on this index suggest a simple varietal correction term could be developed and implemented if the index was used to drive variable rate N applications, regardless of application type (early-mid soil application or late foliar). Summing up The data collected suggest that the use of some cultivar-adjustment factor will have to be implemented if sufficiency reflective data are collected on a nearby field of a different cultivar or if a threshold value approach is used to drive variable rate N applications. Neither tested index provided strong, reliable insight into plant K status during a period in which a soil or foliar K application could be made, but this may be partially attributed to the presence of only slight K deficiencies in either tested year. Additional research is needed to better characterize index responses in soils of lower K status or under seasons that are more favorable for the development of K deficiencies.