Variable Rate Technology (VRT) or Variable Rate Application (VRA) has been in existence for several years. In fact, one form or another of VRT has always been around, even before the advent of satellite technology. The soil would be tested at different areas and those spots would be identified and flagged. Once the soil analysis was determined, recommendations would be made and then a certain amount of fertilizer would be applied. It was a manual form of variable rate, but none the less, the concept was true.
As technology advanced, variable rating fertilizer became easier and more accurate. This process protects the environment and saves the grower money, most often in reduced input costs from fertilizers.
The idea begins with using GPS (global positioning satellites) to grid a field. We recommend a 2.5 acre grid, however, 3.3 and 4.4 are also alternatives. Every 2.5 acres, a soil sample is secured. The samples are taken back to the lab and analyzed for nutrients. Once the nutrients are determined, a variable rate map can be generated.
Several mapping programs have this capability. SGS utilizes Agvance, SSToolbox, Summit, AgFleet or SGS F.A.R.M. (link to article 1) and SMS. These programs are not only able to generate the nutrient and variable rate maps, but several of the programs can generate a variety of file exports to accommodate the many and varied fertilizer applicators.
The three main nutrients that will involve variable rate are lime (determined by the pH), potash (determined by the potassium value and tweaked with cation exchange capacity) and phosphate (determined by the phosphorus value).
Variable rate of a nutrient is primarily driven by the soil fertility. We can use this soil analysis and enhance the procedure with soil type and yield data.
The final rate of fertilizer is determined by the buildup and the maintenance for the crop. A maintenance rate is a fixed value that is determined by the crop to be planted. The buildup is determined by the yield goal of the crop. Soil type can give us a good idea of the yield goal for a crop on a particular field. Each soil type has a productivity number, or optimum yield goal, associated with it. This can be used as a yard stick in determining the yield goal to plug into the buildup equation. We can also use yield data to better judge the productivity of the specific areas within the field.
Each part of the country, even parts of a particular state, will have slightly different equations that govern the application. An excellent example is southern Illinois. The Illinois Agronomy Handbook is the starting point for the state of Illinois recommendations. We then factor in the many University of Illinois and Southern Illinois University studies conducted specifically in the southern portion of the state, to come up with equations that are useful to that region. Most every state in the U.S. has a similar handbook and state university studies that allow for recommendation equations for the benefit of that particular area of crop production.
Variable rate technology enables the grower to use the correct amount of fertilizer at the right position in the field to get maximum production. The fertilizer is applied where it is needed but not where it isn’t needed. It saves the grower money, it maximizes yield, and it saves the environment from excess fertilizer.
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