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Fluid Journal : Fluid Journal 2005-2007
of the U.S. where fertilizer N recommendations are not based on the level of production (yield goal) have fewer opportunities for variable rate N management unless they have considerable spatial variability in other N sources (i.e., mineralization, manure). Sharing technologies. Some innovative producers have integrated band application of nutrients and reduced-till into a concept called "strip tillage." The goal is to place liquid starter fertilizer and most of the annual N application (usually anhydrous ammonia) about 6 to 8 inches directly below the row where the next crop will be planted. Variable-rate technologies have made it possible to address the spatial aspects of N and P availability in the soil. Ammonium polyphosphate (10- 34-0) is being injected to spatially address crop P needs (Capstan or Exactrix technologies) and then variable rate N is applied to complement the N in the 10-34-0. An example in 2004 from Nebraska is where a producer used imagery (bare soil and vegetation) and yield maps to delineate management zones. Plant-based N management. Producers should not rely solely on a plant-based N management strategy for corn production. This is because several key physiological processes are initiated in plants before leaf color and plant vigor become reliable indicators of relative nutrient status or yield potential. Discussion of by-plant fertilization is in its infancy and largely driven by technologies that are capable of monitoring the biomass and chlorophyll status of individual plants and then fertilizing accordingly. The reality is that individual plants share both above- and below-ground environments with their neighbors. The appropriateness of applying N fertilizer to plants based on their potential to produce grain is not in question because the scale of management will ultimately be driven by profi tability and the ease of implementing the technologies. For the time being, the greatest potential for in- season N management lies with the introduction of several new active crop canopy sensors (GreenSeeker and Crop Circle) that can be used to assess relative crop vigor. The unique feature of active sensors is that they work equally well any time of the day and do not necessarily need to involve the generation of a recommendation map (i.e., all operational aspects are transparent to operators other than some safety checks). Dr. Schepers is soil scientist and Dr. Shanahan is agronomist with the USDA/ARS in Lincoln, NE. Spring 2005 Fluid Journal 3
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