Soil Testing Critical After A Drought Year

Carryover nitrogen in the soil will be likely; deep soil sample to about 2 feet.

Published on: Oct 4, 2012

Planning for next year may not be at the top of your priority list right now, but as harvest wraps up, soil sampling should be near the top of their list.

"This has been among the most challenging growing seasons in 50 years, with rainfall amounts far below average. Irrigation has moderated drought impacts for many Nebraska growers, but at a significant cost," says Richard Ferguson, University of Nebraska-Lincoln Extension soils specialist. "The combination of drought conditions, heavy irrigation, and widely varying yields means that soil testing is more important than ever this year."

Irrigated corn yields across Nebraska are predicted to be moderately lower, 3-8% below average, while dryland yields will be substantially reduced, from 32 to 67% below average. These estimates are based on weather data from 2012, and assume no yield-limiting factors other than temperature and solar radiation for irrigated production, Ferguson says.

Soil Testing Critical After A Drought Year
Soil Testing Critical After A Drought Year

Many areas of Nebraska had high temperatures during pollination, which may further reduce yield from modeled predictions. The big question is how drought will affect nutrient requirements for next year. The net effect of crop nutrient removal on soil nutrient availability will vary from field to field, and with locations within fields.

For very low-yielding, dryland corn fields, expect residual nitrate levels to be high. This will primarily be from unused fertilizer nitrogen. Soil mineralization of nitrogen will have been quite low in dryland fields due to low moisture. Limited crop nitrogen removal and little or no leaching means that unused nitrogen will remain in the soil.

Soil Testing Critical After A Drought Year
Soil Testing Critical After A Drought Year

"Unless we have an unusually wet winter and spring, this nitrate-N will be mostly available for crop use next year," he says.

The only way to accurately account for residual nitrate-N is through deep sampling, to a depth of at least 2 feet, according to Ferguson. This may be difficult until soil moisture is restored.

"Unusually high residual nitrate levels following soybean harvest are not likely since soybean typically depletes soil nitrate-N to reduce energy needed for biological N fixation," Ferguson says.

For irrigated fields, total irrigation amounts for 2012 were double or more compared to 2011 applications, with many more irrigation events for sprinkler-irrigated fields. Generally nitrate leaching with irrigation is not a concern with sprinkler-irrigated fields, due to irrigation amounts being one inch or less per event. Such frequent irrigation, combined with warm soil temperatures all summer, may have led to above-average mineralization of nitrogen from organic matter, Ferguson says. Consequently, residual nitrate-N levels for sprinkler-irrigated fields may still be higher than normal, even if yields are not that much lower.

Furrow-irrigated fields generally will have larger amounts of water applied per irrigation event, which can result in nitrate leaching below the root zone, depending on how irrigation was managed.

"Expect residual nitrate-N levels to be quite low for furrow-irrigated fields. However, variability in residual nitrate can be very high with furrow irrigation," Ferguson points out. "It will be critical to sample residual nitrate from the upper, middle and lower third of the field relative to the location of irrigation pipe, since more leaching will occur in the upper third of the field.