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It's a good year for soil testing

Jeff Caldwell 09/26/2012 @ 1:45pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

With harvest running at full steam in much of the nation's middle, other jobs like soil testing may seem far from your mind. But, it's more important this year than usual to make sure you get a good picture of your soil nutrients before shuttering the farm for the winter.

The drought dragged down corn yields this year, but it wasn't altogether blanketing; there are pockets where yields have come in closer to average. That's a sign that the nutrients left in your soils are just as variable, making it even more important to do a good job of soil testing this fall, says University of Nebraska agronomist and soil fertility specialist Richard Ferguson.

"The big question is how drought will affect nutrient requirements for next year," he says in a university report. "The net effect of crop nutrient removal on soil nutrient availability will vary from field to field, and with locations within fields."

So, what can you expect when you get into the field? First off, don't expect a huge change in the amount of phosphorous (P) and potassium (K) left in your corn fields after harvest. This doesn't necessarily mean the plants' uptake didn't change (if affected much by drought, chances are that uptake was lower than normal), but it also doesn't mean the amount left behind will change as much as the difference in nutrient uptake.

"There may be some tendency toward higher soil test values, but soil tests for these nutrients are an estimate of plant availability over the growing season," Ferguson says. "There is evidence that soil test K can be influenced by soil drying, and some states have started analyzing soil K from field moist samples."

If you irrigated, though, the levels of remaining nutrients will be closer to normal, Ferguson adds.

A lot of corn fields whose normal fate would have been grain harvest this fall were cut and chopped for silage earlier in the year. Doing so has its own effects on nutrient removal, namely in taking more nutrients out than harvesting for grain, Ferguson says. In this case, soil testing not only helps determine what's left in the field but also what's in the forage gleaned, with the latter important to getting a clear picture of feed value.

"Growers should be aware of nutrient removal resulting from biomass harvest. Significant amounts of P, K, and micronutrients, as well as carbon, can be removed from the field with biomass harvest. For example, a 150 bu/ac corn crop will uptake approximately 64 lb P205 and 42 lb K2O in grain, but will have approximately 36 lb P2O5 and 144 lb K2O/acre in stover," Ferguson says. "The value of these nutrients should be considered when pricing baled stalks for livestock feed."

What if you graze corn stalks instead? "If stalks are grazed rather than baled, expect much of the nutrient content in grazed residues to return to the field, minus the harvested weight of cattle once they are taken off the field," Ferguson adds.

Why does this all matter? With a likely price tag for fertilizing an irrigated corn crop at between $100 and $150/acre, knowing where you stand with existing soil nutrient levels heading into spring can help cut down on your fertilizer bill for the 2013 crop. But, how much you can save will depend a lot on when you conduct your testing. Ferguson recommends waiting until spring if you can.

"Dry conditions this fall may make sampling difficult, and spring sampling may result in a more accurate prediction of nitrate-N availability, depending on weather conditions this winter," he says.

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