Cover Crops Help Retain Nitrate, Prepare for Future Crops

By CnAgri2012-09-27 19:46:16 Print

Drought-decimated corn crops are likely to leave residual nitrate in soils after harvest, making this year ideal for farmers to plant cover crops, says a Purdue Extension agronomist.

Cover crops can "scavenge" residual nitrate and recycle it through biomass. The process helps reduce nutrient loss through leaching and runoff, and makes some of those nutrients available for the next cash crop.

"This year is a great example of when a cover crop is needed to trap the much larger amount of residual nitrate that will be present after the poor corn crop," said Eileen Kladivko, Purdue University professor of agronomy. "Farmers who lose residual nitrogen also are losing the opportunity to trap that N and keep it in their fields for subsequent crop use."

According to Kladivko, cover crops will benefit individual farms by building soil organic matter and potentially reducing next year's nitrogen application. The practice also benefits regional water quality.

Although much of the Midwest is still in drought, rain has returned to many areas. With rainfall, residual nitrate will leach out the bottom of the root zone along with other essential nutrients. Those nutrients end up in local waters and eventually in the Mississippi River and the Gulf of Mexico.

Kladivko expects the amounts of nitrate lost to be much larger than usual this year. She said numerous studies have shown the highest nitrate losses in drainage waters following dry years.

Cover crops can help reduce these large losses.

Cover crops also can help farmers recoup part of the nitrogen fertilizer they applied to fields this season and provide benefits for future growing seasons.

Poor cash crop yields equal less crop residue being returned to the soil. Cover crops can offset some of that loss by protecting against soil erosion and providing food for soil organisms.

As the cover crops decompose during the next year, some of the nitrate taken up will be released back into the soil for use by the next cash crop, and some nitrate will help build soil organic matter.

The amount of nitrate uptake this fall and the amount of release next season depends on several factors, such as the amount of residual nitrate in the soil currently, the type of cover crop, the amount of growth this fall or spring if it's a winter-hardy cover crop, the stage of cover crop at termination time and the decomposition rate of the cover crop in the spring.

While most farmers think of using cover crops after corn, they also can be useful after soybeans. As with cornfields, cover crops can add organic material and trap nitrate released by decomposing soybean residues.

Although the amount of nitrate taken up by cover crops is difficult to predict, Kladivko said it could be 50-100 pounds of nitrate per acre in a drought year. The amount released to next year's crop is also hard to predict, but it could potentially be up to half of the nitrate in the aboveground biomass if the cover crops are terminated while in the vegetative state and they have a low carbon-to-nitrogen ratio.

Kladivko recommended planting cover crops such as oats, cereal rye, annual ryegrass or oilseed radish if the primary objective is to scavenge nitrate and build organic soil matter. If producers want to do fall grazing, she recommended planting turnips or crimson clover mixed with oats and cereal rye.

Whichever cover crop is planted, farmers need to plan for their future cash crop in the spring. Planning for cover crop termination is essential if the additional crop is to be beneficial and not hinder the planting of the next crop.

The Midwest Cover Crops Council has additional resources and tools to help farmers select and plant cover crops at their website, http://www.mccc.msu.edu.