Residual soil N should be considered
after this year's drought, poor crops
How farmers will manage nitrogen is an important question after this year's ongoing drought. There is a high likelihood of excess nitrogen left in the soils because this year's crop didn't take up that nutrient.
Carrie Laboski, a University of Wisconsin soil scientist, talked with farmers and crop consultants at the annual Agronomy and Soils Field Day, Aug. 29 at the Arlington Agriculture Research Station just north of Madison about how to manage nitrogen after a drought.
The effect of this year's widespread drought is variable throughout the state and at this point its effect on crop yields isn't quite known, but she expects those yields to vary widely, too.
"It is likely that there is residual nitrate remaining in the soil profile where fertilizer or manure was applied or from a crop that followed a forage legume."
This residual nitrogen in the soil could be lost before next spring's planting if there is higher than normal precipitation this fall and winter. But if fall and winter's precipitation is lower than normal there is a pretty good chance that there will be some leftover nitrate from this year's application that could be used by next year's crop.
The greatest likelihood of fields carrying excess nitrogen would be where there was a poor corn crop this year, where manure has been applied since 2011 but which may have been sitting there not mineralizing, and where the 2011 crop was a forage legume.
"A lot will depend on winter precipitation if any nitrogen will be left over," she said.
As farmers begin planting their winter wheat, she said, they should think about the fact that the optimum nitrogen rate for wheat is affected by how much nitrate (N) is in the soil profile at planting time.
If wheat will follow a poor corn crop it's likely that a substantial amount of N may remain in the soil profile. "In cases like this the N rate for the wheat should be adjusted using the pre-plant nitrate test (PPNT).
She advised taking 15 cores randomly, for example, in a 20-acre field to determine the residual nitrogen. Areas that have had different management in the past should be sampled separately.
If samples can't be analyzed quickly she suggested freezing or chilling them because if they heat up it will affect the results.
The lab results will report the nitrates in the top two feet and estimate the nitrate in the third foot. It will be reported as N in the soil profile, she added. But this year much of the nitrogen may be in the top foot of soil.
"We side-dressed corn (with nitrogen) in my research plots and then didn't get a drop of rain for six weeks," she said. That N in the top foot of soil will be very accessible to the next crop.
"We need to keep in mind that too much nitrogen on wheat will cause lodging."
If wheat follows soybeans, the amount of nitrate in the soil profile is likely not different than in a growing season with normal precipitation.
A PPNT is not recommended on sand or sandy loam soils because the results are not reliable, she said.
If next year's corn crop will be planted on a field where there is currently the potential for carryover N from this year's fertility program and poor corn crop, N rates should be adjusted for next year - especially if this winter brings normal or sub-par precipitation,
She suggested several options to determine those rates:
• One would be using the low end of the MRTN rate range (an abbreviation for the a profitability measure called "maximum return to N" scale;)
• Or take spring pre-plant nitrate tests and subtract the N credit from the high end of the MRTN rate range or:
• Estimate an N credit by taking the total N rate applied in 2012 and subtracting the actual grain yield, then dividing by two.
Of these methods, she said, using the PPNT will likely provide the best estimate of a nitrogen credit from carryover N; however it does involve soil sampling to two or three feet in the spring prior to planting.
Laboski said she doesn't have a lot of confidence in the last method mentioned.
Overall profitability for next year's wheat and corn crops will increase if residual N is considered when selecting a nitrogen fertilizer rate.
Laboski dug into UW agronomy files and found Larry Bundy's research into nitrate concentrations in the fall of 1988 - the last year the Midwest experienced a severe drought.
At several locations in Wisconsin, 400-450 pounds of N remained in the soil profile after the 1988 growing season. Bundy also did some PPNT testing in the spring of 1989 and found 406 pounds in Columbia County, 222 in Clark, 236 in Wood, 266 in Grant and 192 in Dane County.
For more information see the website: www.soils.wisc.edu/extension.