McFarland said a better approach for nitrogen management is to identify what’s available before applying nutrients. Nitrogen carryover from one year to the next may be more significant and more useful than some farmers might think.

“Nitrogen is soluble, so it moves below the typical 6-inch sampling depth. Much of that nitrogen below 6 inches is still plant-available.”

As nitrogen prices have risen in the past few years, interest in residual nitrogen has increased. McFarland recommends farmers look at nitrogen carryover just before planting. “Sample to depth,” he says.

Sampling should include the typical 6-inch depth for a routine nutrient analysis. But producers also should collect a second sample from 6 to 12, 6 to 18, or 6 to 24 inches. Significant amounts of nitrogen likely will be present in the deeper sample and can be credited to reduce fertilizer nitrogen needs for the next crop.

“Deeper sampling requires good soil moisture conditions, but with a little extra effort a hand probe can go that far,” McFarland said.

Much of that residual nitrogen will be available to corn, grain sorghum or cotton. Tests across Texas have shown that crediting residual nitrogen measured to 24 inches as part of the annual fertilizer requirement produces yields equal to adding the full amount based solely on yield goal.

Multiple tests have indicated that many samples to depth included a minimum of 40 pounds of nitrogen, available to the plant, per acre. “There is economy to be gained in using that nitrogen,” he added.

Cotton will respond to residual nitrogen, as shown by tests on more than 100 sites across Texas. “Consistently, we could achieve maximum yields with little, if any, supplemental nitrogen. We did a lot of early work on cotton with funding from Cotton Incorporated. Over the last few years, we’ve also tested corn and grain sorghum. Preliminary data show very similar results.”

When residual nitrogen measured in soil samples taken to 6, 12, or 24 inches was deducted from the fertilizer application, corn and grain sorghum produced as much yield as a full rate of nitrogen based on the yield goal.  

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.