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Sample soils now to determine residual nitrogen

Depending on where you farm, 2012 was either too dry, too wet or, as Goldilocks told the three bears, just right.

It will go down in the record books as one of the driest years in decades in the Midwest, Arkansas, western Kentucky, Mississippi, Missouri, west Tennessee and parts of Texas, and not so bad in the remainder of the Southeast.

If you happen to be in one of the former or even in the latter, in some cases, this fall could be the best time to determine if crops that were hit by the 2012 drought left any residual nitrogen or other nutrients in your soil profile.

“While the drought’s impact on crop production has been sobering, more than yield has been affected,” says T. Scott Murrell, north central director with the International Plant Nutrition Institute, an organization which works with farmers to help improve nutrition management for the world’s crops.

“The cycle of every soil nutrient has been affected. This makes taking nutrient measurements more important than ever to insure you apply the next application of nutrients at the right time, right place, right rate and from the right source.”

Residual soil nitrates can be higher after a drought. This happens because of decreased downward soil water movement and reduced N uptake by drought-stressed plants. The effect of the 2012 drought on phosphorus and potassium is largely dependent on how the crop was managed. While less P and K may be removed at harvest as an outcome of lower grain yields, Murrell notes, P and K removal could be greater than planned if the plant was cut for silage instead of harvested for grain.

With regard to N, IPNI scientists say farmers should be incorporating the 4R Nutrient Stewardship program and consider these questions: (1) How much applied N is left in the soil profile?; (2) How much of that N will remain in the soil until next spring?; (3) What can be done to prevent its loss?; and (4) What adjustments need to be made to fertilizer rates for the next crop?

Sample and analyze

Clifford S. Snyder, nitrogen program director with IPNI and a former soil scientist with the University of Arkansas, says sampling is key, especially after a drought. This is because drought reduces crop uptake and removal of nutrients.

Whether or not residual N will be available for next season’s crop depends greatly on the precipitation that occurs after harvest.

In the Midwest, nitrate losses are associated with wet periods during fall, winter and early spring when roots are not actively taking up N.

Soil testing and plant analysis remain the best tools for quantifying the drought’s impact and adjusting nutrient management. Because states vary considerably in the suggestions for sampling, Snyder indicates consulting local guidance on sampling and discussing analytical results with a trusted and knowledgeable agronomist to ensure informed strategies are created to meet local conditions.

“It’s helpful to collect samples at least two feet to three feet deep in most soils and analyze them for nitrate-N,” he says. “It’s hard to do this manually, so consider using a hydraulic or powered sampler.

“When collecting samples to that depth, it is helpful to partition samples into nominal depths (0 to 6, 6-12, 12-18, 18-24, etc.) to identify where the concentration of nitrate is in the soil profile and determine risks and magnitude of loss from the root zone.”

Taking samples during a drought year and examining the changes can provide valuable information for improved interpretations when dry weather strikes again. Additionally, before beginning the 2013 planting season and through the late spring, a soil nitrate test should be used to adjust N application rates to more closely match what the plant needs but the soil lacks.

Planting a cover crop?

A catch crop is a cover crop planted for the express purpose of taking up residual soil nitrate. USDA Agricultural Research Service research in central Iowa shows that a winter cover crop of wheat following corn can reduce tile drain nitrate-N losses as much as 60 percent.

“A well-established cover crop can result in uptake and retention of more than 40 pounds to 60 pounds of N per acre,” Snyder says. “Just N savings alone are valued at more than $15 to $25 per acre.”

A good cover crop can serve as an “N bank” and release plant N via microbial decomposition of crop residues after the cover crop is mechanically or chemically terminated. In addition to conserving N, cover crops can protect water quality.

The time to kill cover crops will depend on the crop to be planted and soil moisture. For corn, cover crops should be killed two weeks before planting, but if soils are dry, earlier killing may be needed. Soybeans are less sensitive and cover crops may be killed up to three days ahead of planting unless it is dry, then earlier killing is needed as well.

Information available

Murrell and Snyder say this latest drought has given farmers tough nutrient management challenges. They should be addressed with as much information as possible and with the 4R stewardship practices in mind. IPNI has assembled additional nutrient management guidance at More information about the 4R Nutrient Stewardship Program is available at and through the Fertilizer Institute’s website,

“The effects of drought on crop nutrition are profound,” Murrell says. “Soil testing and plant analysis, combined with sound nutrient stewardship, remain our best tools to quantify drought impacts and adjust nutrient management for next year’s crop.”

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