Compare yield and soils maps
You can layer soil test maps over yield maps, and you may see something interesting. However, not everyone agrees upon how much you should depend upon these conclusions.
“There is no simple answer,” says Chuck Barr, Senesac Inc., Fowler. “Yield maps are the result of all the things the crop has dealt with over an entire season.
“Only one of those things is soil fertility. In fact, soil fertility is likely not the overriding factor contributing to yield difference. One is more likely to see yield difference from water management and soil type before they do soil fertility.”
• Many factors besides soil fertility impact yield differences.
• Yield maps may be part of a starting point for yield recommendations.
• Continuously higher-yielding areas may produce lower soil test results.
Note the distinction between soil test maps and soil-type maps. Each is a different layer you can examine and compare to yield maps. Most people rely on soils maps from the Natural Resources Conservation Service. Gary Steinhardt and other soil scientists demonstrated that soil-type maps are a good guess, but not totally accurate. They did what’s called a first-order soils-type determination at the Davis-Purdue farm near Farmland and discovered 11 soil types in a field where four types were mapped.
“In the order of things that can make or break a crop, water management and soil type are above soil fertility,” Barr says. “When doing this sort of comparison, one has to be very careful before placing blame or giving credit to soil fertility for either poor or good yields.”
Meanwhile, Steve Gauck, Greensburg, thinks comparing soil test results to yields has advantages. He’s a certified crop adviser and a Beck’s Hybrids seed rep. “This is a great way to see how soil fertility affects yield,” Gauck says. “You should be able to tell if a lower-yielding area is because of soil fertility, or something else. You can also tie in the need for lime based upon yields.
“My advice is to evaluate good and bad areas, compare and see what is the same or different. If you’re not already grid soil sampling, doing so will give you smaller areas to sample and compare. Use this as a starting point for making fertilizer recommendations, not as the only method of deciding what to apply.”
Still another view
Dave Taylor, Richmond, a CCA with Harvest Land Co-op, also sees it as a practical thing. He takes a different twist, however.
“Expect in many cases where yields have been high for many years, soil test levels will be low,” he says. “Base the yield portion of the fertilizer recommendation off actual yield information, not field average yield goals. Yields can vary greatly within a field. Higher nutrient removal over many years is actually drawing soil fertility levels down in higher-yielding portions of the field.
“Be cautious in reading too much into comparisons between yields and soil test information. Soil testing and yield monitoring are not exact sciences. Both have errors. Yet they give very good direction in management of the acre.”
This article published in the November, 2010 edition of INDIANA PRAIRIE FARMER.