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Fluid Journal : Fluid Journal 2005-2007
Spring 2005 Fluid Journal 2 many producers fi nd it difficult to justify sampling at greater than the management zone level. This partially explains why the interest in real-time nitrate monitoring is waning. Current technologies for on-the-go assessment of residual N are limited to electrical conductivity (i.e., Soil Doctor or Veris) and experimental techniques using a nitrate electrode. N mineralization. Organic matter is a source of N that keeps giving over time as crop residues and manure decompose. This process (mineralization) is regulated by temperature, moisture conditions, and the availability of an energy source for the soil micro- organisms (i.e., carbon in crop residues and manure). A rule of thumb suggests that the N credit given for mineralization should be 20 to 40 lbs/A/yr for each one percent organic matter in the soil. The spatial variability of soil organic matter within a field and the cost of acquiring the information will dictate whether N mineralization is considered a spatial or uniform input. Sometimes remote sensing is used to generate a proxy map of soil organic matter content from which a map of estimated potential mineralization is generated. Bare soil imagery. Aerial photographs provide an inexpensive way to assess the extent of spatial availability in N mineralization within a field. Patterns in soil brightness in images can be readily calibrated to generate a map of soil organic matter or potential N mineralization. Incorporating this information into a variable-rate fertilizer N recommendation should be an easy and inexpensive task in that the same map can be used from year to year. Vegetation imagery. Aerial photographs of living vegetation have adequate spatial resolution to compare with patterns of soil color and soil types. However, the spatial resolution of many sources of satellite images is probably at the extremes for being useful. Remote sensing specialists recommend that users should strive to have 2.5 pixels (picture elements) per width of the application or harvesting equipment. This translates into about three row-widths for eightrow equipment or about five rowwidths for 12-row equipment. Resampling images using GIS tools to increase pixel size (e.g., adjusting the pixel size to a common scale) so that images can be more readily compared with yield maps is quite acceptable. However, resampling a course-resolution color image (e.g., Landsat satellite images) doesn't really gain anything unless it is merged with a higher resolution image (e.g., black and white image) to proportionately scale the color of the larger pixels into smaller ones to generate a map with finer resolution. Nitrate in irrigation water. Irrigation of corn usually occurs predominantly during the last half of the growing season. N uptake is usually about 60 percent complete at the time of tasseling and 90 percent complete three weeks later. Therefore, timing can be an issue when considering an N credit for nitrate in irrigation water. A general guideline is that each acre-inch of irrigation contains 0.227 lb N per 1.0 ppm NO3-N in water. As such, an acre-inch of irrigation water containing 30 ppm NO3- N supplies nearly 7 lbs/A of N to the crop. Considering that evapotranspiration for corn is 0.35 inch/day during July and August, and if all of the water is supplied by irrigation, then 30 ppm nitrate-N water carries about half of the maximum daily N uptake for corn. In comparison, precipitation in the Midwest usually contains <1.0 ppm N (the fi rst tenth inch may contain 3 to 4 ppm N but shortly thereafter the N concentration usually declines to <0.1 ppm). Soil-based N management. Much of the information that goes into a soil- based variable-rate N management scheme does not change very much over time (e.g., pH, organic matter content, mineralization, electrical conductivity, drainage, etc.) The parts that change are residual soil inorganic N (nitrate source) and biomass production/yield (N sink or removal in grain). The extent to which either of these considerations can be justified as having a spatial component depends on the producer and situation. Herein lies the value of yield maps or vegetation images over several years. Areas of a fi eld that have relatively stable yields over time should serve as the base when making variable rate N management decisions. These parts of the field are less likely to be affected by excess precipitation, nitrate leaching, and drought, none of which can be predicted when N applications are made. Producers perpetually fear that their fields, or parts of fields, might come up short of N if an exceptional year evolves (ideal growing conditions with high N uptake or excess precipitation that results in N losses). In reality, Mother Nature tends to take care of herself in that fertile areas that are subject to nitrate leaching and denitrification will also have a higher potential for N mineralization. Regions Variable rate has made it possible to address spatial aspects of N and P availability in the soil.
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