Cover crops trap residual nitrogen

Thursday, September 27, 2012

Cover crops will scavenge residual nitrogen (N) and recycle it through their plant biomass.

That's according to Barb Stewart, state agronomist, Iowa Natural Resource Conservation Service (NRCS), Des Moines.

Stewart emphasized this is important in a post-drought year.

"In addition to the obvious negative impact on crop yields, the drought will have left residual nitrate in the soil at harvest time, potentially allowing nitrate to leach out the bottom of the root zone," Stewart said.

If more typical precipitation returns in November through April, the amounts of nitrate lost will be much larger this year than usual, leading to nutrient loading to local waters and eventually to the Mississippi River and Gulf of Mexico, she added.

"Cover crops planted this year will decompose next year, with some of the residual N taken up the cover crops and used by the next cash crop. Some will go towards building soil organic matter," Stewart continued. "Fall-planted cover crops would be a good investment this year, to benefit both their own farms and regional water quality."

Among the best N-scavenging cover crops are oats, cereal rye, or annual ryegrass mixed with oilseed radish, Stewart said.

If a farmer is interested in fall grazing, turnips or crimson clover could be mixed with the oats and cereal rye.

These cover crops will help farmers recoup part of their fertilizer N investment from last season, and will improve soil organic matter and soil biological activity, she said.

Cover crops will also be very useful after soybeans for adding organic matter and trapping N released by decomposing soybean residues, Stewart said.

A portion of a series of presentations by Ray Archuleta, a national soils "guru," to Iowa producer-groups earlier this year said farmers need to increase organic matter and improve soil health.

Along with eliminating tillage activities, Archuleta pointed to legume cover crops as natural fertilizers and grasses as scavengers of nutrients often lost after harvest or during winter.

"Diversity above ground improves diversity below ground, which helps create healthy productive soils," Archuleta said. "Cover crops should be an integral part of a cropping system. They help improve soil health by developing an ecosystem that sustains and nourishes plants, soil microbes and beneficial insects."

In addition to helping to restore soil health, cover crops also protect soil against erosive heavy rains and strong winds, he said.

They can also provide livestock producers with additional grazing or haying opportunities, and winter food and cover for birds and other wildlife, Archuleta added.

Jim Lahn, district conservationist at the NRCS office, in Le Mars, said producers interested in planting cover crops may contact his office: (712) 546-8858 Ext. 3 for additional information.

Lahn also recommends that producers check with their Crop Insurance providers to be assured the planting of cover crops does no adversely affect the insurance coverage.

He also noted that the Farm Service Agency (FSA) considers grazing of certain cover crop varieties as harvesting. Clarification of this situation may also be necessary, he said.