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.
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.
Test irrigation water
Water is also a potential source of nitrates, says Paul DeLaune, assistant professor, environmental soil science, at the Texas AgriLife Research Center at Vernon.
He discussed water quality at the Red River Crops Conference at Altus, Okla.
“Nutrients in irrigation water may offer a fertility advantage,” he says. But producers must sample irrigation water sources to determine the amount of nutrients available.
Some studies show available nitrogen levels equal to $48 per acre in nutrient savings. He also warned that nitrate concentrations in irrigation wells may be environmental issues, if allowed to get into drinking water or freshwater streams. As either resource or drawback, producers need to know if they have nitrates in their irrigation — and if they do, how much.
“As a resource, nitrates in irrigation water can be credited to crop needs,” DeLaune says. “Producers may supply a significant amount of nitrogen in the growing season through irrigation water, and it is immediately available.”
Applying 12 inches of water at 20 parts per million of nitrogen in irrigation water equals about 55 pounds per acre. Depending on the crop and other conditions, producers may reduce fertilizer application by 40 percent, just by crediting nitrate in irrigation water.
A 2011 trial showed potential to supply 100 percent of crop needs through irrigation water. “We irrigated heavily because of a season-long drought,” DeLaune says.
He breaks down the figures: Crediting from 39 pounds to 149 pounds of nitrogen per acre from well water, and using a cost of 60 cents per pound, savings would be from $23 to $89 per acre. Adding residual soil nitrate to the formula could increase available nitrogen significantly.
Determining how much residual nitrogen is available in the soil and how much is available through irrigation water offers both economic and environmental advantages, specialists say. Crediting residual nitrogen reduces the need for supplemental fertilization and reduces production costs. Reducing the amount of nitrogen applied to the soil also decreases potential for leaching into underground water and into nearby streams.
The critical factor, however, is doing the testing and then measuring the advantages of what’s already available.