Grid sampling fields in 1-4 acre portions is becoming the norm for Precision Agriculture in many areas of the midwest. This system is being questioned however in lower rainfall areas. In these areas, grid soil testing and variable rate fertilizer application are hard to justify as they exist. In low rainfall areas, farmers know from experience that yields vary across rolling landscapes. Soil water holding capacity is one factor that limits yields in these areas. In areas where water limited yield, higher levels of nutrients, such as nitrogen, may remain. Initial soil formation processes, erosion, tillage, drainage and many other factors have also created differences in soil test levels with topography. Some farmers have attempted to manually adjust fertilizer rates on the go, but the success has been limited to the ability of the operator to guess the correct rate to apply. Soil testing fields by topography (i.e. high, middle, low) has shown differences in soil nutrient levels by landscape position (see examples).
New technologies such as GPS (Global Positioning Systems) now enable dealers and consultants to sample by landscape position and record the locations. This soil test information, along with a yield map or some other guide to yield potential, can then be used to vary rates of fertilizer accordingly. Any fertilizer product can be applied variably using this information and GPS equipment. Will this system provide as much information as griding fields in 1-4 acre portions? “NO” Smaller grids provide more detailed information. Will this system provide better information than representing a field with one composite soil sample? Common sense tells us this should be much better information than a composite sample, yet cost less than grid testing. Research being conducted by university and industry researchers will provide us with the solid data needed to test ideas such as this in a few years. There will also be a lot field scale research from equipment manufacturers and fast adapting farmers.