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Cover crops provide water quality benefits in organic vegetable system

By Jean Caspers-Simmet
simmet@agrinews.com

Date Modified: 09/23/2013 9:35 AM

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GREENFIELD, Iowa —An organic vegetable research study at the Iowa State University Neely-Kinyon Research Farm is showing big benefits from cover crops in terms of decreased nitrate leaching and improved soil carbon and organic matter.

The USDA-Organic Transitions Project is looking at the effect of cover crops, compost, no-till and mulch on organic vegetable production. Funded by USDA in 2010, the project examines ways to encourage organic transition by developing recommendations for vegetable cropping systems that maximize soil quality, foster carbon sequestration and minimize nutrient loss, said Kathleen Delate, ISU agronomy and horticulture professor.

Delate is leading the project with Cynthia Cambardella, who is with the USDA-ARS National Lab for Ag and the Environment in Ames.

Delate and Cambardella are collecting data through plot lysimeters, which are devices used to collect water and measure nitrate concentrations below the rooting zone.

The study looks at three tillage comparisons — tilled followed by straw much, tilled without straw mulch and organic no-till. Two organic fertility treatments are also being researched —composted animal manure alone and composted animal manure with cover crops.

Fall cover crops were planted in October 2012 at a rate of 25 pounds of hairy vetch and 90 pounds of rye per acre. Treatments were replicated four times for a total of 48 plots. Cover crops were disked under.

"Because we had extensive spring rains, the cover crops were not disked until June 8 and not rolled until June 20," Delate said. "We planted June 24, three weeks behind schedule."

Compost is applied at a rate of 100 pounds of nitrogen per acre each spring and organic fertilizer is side-dressed after vegetable crop establishment at 50 pounds of nitrogen per acre.

Excellent pepper and sweet corn yields were obtained during 2012 in mulched and tilled treatments, with lower yields in organic no-till, contrary to high yields in earlier organic no-till tomato experiments at the Neely-Kinyon Farm when rainfall was more normal, Delate said.

The highest pepper yields were in plots that were tilled, mulched and had compost applied. Highest sweet corn yields were in plots that were tilled and had compost applied.

This year's plots are planted to Luscious sweet corn and Defiant tomatoes. The vegetables will be used by the University of Iowa Dining Service for the Iowa Organic Conference.

Cambardella said cover crops reduced nitrate leaching throughout the growing season, which indicates more efficient nitrogen cycling.

"Long after the cover crops were no longer growing they were having a major impact on nitrate leaching below the rooting zone," Cambardella said. "That is exciting. Nitrogen cycling efficiency is improved vastly in these systems. More nitrogen is staying in the system and available for the plants to use with the cover crop. It is not something that I had hypothesized. I figured there'd be a difference early in the season because cover crops are known to scavenge nitrogen at the end of the growing season and early in the season, but I did not have any educated guesses related to what would happen during the growing season."

Cambardella found soil organic carbon was greater with cover crops and compost. Tillage stimulated the mineralization of organic phosphorus from the compost. Biologically active organic matter was 20 to 50 percent higher with cover crops. Mulched plots had lower soil nitrate than non-mulched plots.

Cambardella is also measuring the release of carbon dioxide from the soils. She measures the respiration rate of the soil every two weeks to learn more about the efficiency of the plots in cycling nitrogen, which is related to carbon sequestration.

"The farm's organic grain plots show organic systems can store carbon," Cambardella said. "These plots will help us understand the mechanisms that control that."