Before you make a final decision on how much fertilizer you’ll put on spring-planted crops, you need to do a little research and use some arithmetic. That’s the advice of Texas AgriLife Extension specialists.

To make certain you apply enough nutrients — or to make sure you don’t apply more than you need — you should take routine soil tests, and then dig a little deeper, as far down as 24 inches, to check for residual nitrogen that can be credited to nutrient demand.

But don’t stop there. Testing irrigation water for nitrates may also reduce overall fertility needs — and cost.

Soil testing is the key to efficient nutrient management.

Mark McFarland, Texas AgriLife Extension soil fertility specialist at College Station, has been promoting deep soil testing for several years, and he encourages farmers to take advantage of residual nitrogen.

“Although fertilizer prices have moderated recently, nutrients are still a significant cost of production,” he said during the grain session of the annual Blackland Income Growth Conference at Waco.

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An 82-0-0 anhydrous ammonia analysis costs about 38 cents per pound; urea is 51 cents per pound; ammonium nitrate (UAN) and phosphorus run 57 cents per pound; and potash is 40 cents per pound.

With significant amounts of residual nutrients in the soil, fertilizer savings may be significant, McFarland says. And many fields have enough in the bank to make a big difference, possibly enough to satisfy a crop’s needs without adding supplemental nitrogen.

“Nitrogen is very soluble,” he says,” so we find more nitrogen below the typical zero-to- 6-inch sampling depth. And nitrogen at depth may still be in the rooting zone and available to the crop.”

The critical depth seems to be 24 inches. “From 6 inches to 24 inches, we see significant amounts of residual nitrogen available to corn, cotton and grain sorghum.”

McFarland and other fertility specialists have looked at the value residual nitrogen provides to cotton for 10 years, and they have five years of data on grain sorghum and corn. All show significant benefits from residual nitrogen.

“From $20 to $120 per acre in savings may be available from residual nitrogen,” he says. “With credited nitrogen, we can grow maximum yields with no supplemental nitrogen.”

Results have been consistent in grain sorghum, with success using residual nitrogen achieved 90 percent of the time. Corn is even better at 97 percent, and cotton is successful with residual nitrogen 98 percent of the time.

Cotton data are from those 10 years of research, while corn and grain sorghum information is based on 5 years.