Variable Rate Fertilizer Application Could Pay Off

The field yield may have been cut in half, but not all parts of the field yielded the same.

Published on: Oct 31, 2012

You likely are tempted to cut fertilizer application rates in corn fields that didn't yield well if you're on a program where you apply every year and pay attention to soil test reports. It may be the right thing to do, but Jeff Nagel, agronomist with Ceres Solutions based in Lafayette, says you ought to think through the scenario first. Once you do, it's possible that this could be the year when variable-rate application would more than pay for itself, especially in terms of phosphorus and potassium.

Nagel says a 180 bushel per acre corn crop removes 80 pounds of potash and 145 pounds of phosphate per acre. A half-crop, or 90 bushels per acre, assuming it was harvested for grain and not forage, removes 40 pounds of potash and 72 pounds of phosphate, roughly half of what a normal, solid crop would have removed.

Variable Corn: This combine is moving through 160 bushel corn. Other areas made only 70 bushels per acre. Variable-rate fertilizer application might pay in a field like that.
Variable Corn: This combine is moving through 160 bushel corn. Other areas made only 70 bushels per acre. Variable-rate fertilizer application might pay in a field like that.

The situation is similar for soybeans, with 55 bushel beans taking away 130 pounds of potash per acre and 95 pounds of phosphate. The average yield in Indiana is predicted to be about 36 bushels per acre. A 36-bushel per acre crop would remove 87 pounds of potash and 64 pounds of phosphate.

Nagel says if you know your soil tests and they are in the optimum range, then you could cut back if you had poor crops based upon these removal rates. However, if the whole field made 90 bushels per acre but your yield monitor, assuming it was well-calibrated, said part of the field made 200 bushels per acre, then that part of the crop removed as much or more nutrients than it would in any other normal year.

If you have yield monitor data so you can prepare a variable rate application map based upon the principle that more yield removed more nutrient, then this might be the year to do it, Nagel says.