The economics, McFarland says, make sense with savings of $20 to $33 by crediting residual nitrogen, “with the same yield. Some values are even higher, depending on the events of previous years in a particular field.”

If drought limited the previous year’s crop growth, plants removed little nitrogen from the soil. A Nueces County field showed a $93 per acre advantage following two years of failed crops.

In research trials on corn, 28 of 29 locations—97 percent—produced equal yields with residual nitrogen credits, compared to application of recommended nitrogen amounts. For grain sorghum, 17 of 19 sites—89 percent—produced equal yields with credited nitrogen compared to recommended rates.

“Consider deep sampling,” McFarland said. “At 6 inches, sample for nitrogen, pH, phosphorus, potassium, calcium, magnesium, sulfur and micronutrients. At depth, sampling for just residual nitrogen is cheaper, just $4 per sample instead of $10.”

Farmers may gain some advantage, however, by testing for other nutrients at depth.  Potassium and sulfur found in deeper samples also can be credited and provide additional savings.