By Tom Block
In addition to punishing yields, drought conditions in much of Iowa this summer had profound impact on soil health, including limited nutrient uptake by crops. Agronomists advise farmers to use cover crops to help rebuild soil health and reduce the risk of unused soil nitrogen leaching at the end of the growing season.
"Cover crops have many benefits that are critical, especially during drought conditions," said Mahdi Al-Kaisi, a professor of agronomy and extension soil and water specialist at Iowa State University.
One of those benefits is to scavenge residual nitrogen and recycle it through their plant biomass.
"There certainly are some cases where some nitrogen is left out there," said Iowa Agriculture Secretary Bill Northey. "If it’s there, it sure would be nice to be able to hold on to that with cover crops."
Farmers learned from the 2012 drought, when heavy rains the following spring flushed residual nitrogen out of the soil, Northey said.
"We did see some challenges holding on to the nitrogen coming out of that dry year. The better that we’re able to hold on to that nitrogen, the better it is for farmers and the environment."
Cover crop usage has grown rapidly since then as farmers realize the water quality and soil health benefits of the conservation practice, Northey said. Farmers planted more than 600,000 acres of cover crops last year, up from 100,000 acres in 2012. The number of farmers trying cover crops for the first time through a state-cost share program continues to grow, Northey reported.
"We had 1,000 farmers sign up as first-time cover crop users this year, bringing their own dollars," he said.
The increase in acres has fostered learning about different cover crop species and seeding methods that work in a vastly different environments and farming systems, Northey said.
"We’re building the opportunity for more and more conversations and understanding about what works in a wet year or a dry year," he said. "Everybody has a different system, and everybody is learning from each other. Some of those options have come in, where five years ago they didn’t exist."
Best scavengers
Agronomists say the best nitrogen-scavenging cover crops include oats, cereal rye or annual ryegrass mixed with oilseed radish. Turnips or crimson clover can be added to the mix for farmers interested in fall grazing for their livestock.
"Cereal rye is a go-to option to scavenge nitrogen following corn," said Midwestern BioAg President Gary Zimmer. Oats are another good option for scavenging nutrients, and they are easy to manage and terminate, he said.
As cover crops decompose next year, some of the nitrogen taken up will be released for use by next year’s crop, and some will go towards building soil organic matter.
Over the long term, cover crops promote better soil biological and physical conditions that can help soils recover from drought, said Al-Kaisi.
"It is well documented that cover crops increase soil water infiltration and recharge of the soil profile by improving soil aggregate stability and soil porosity," he said.
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