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Study shows soil building benefits of organic farming

By Jean Caspers-Simmet
simmet@agrinews.com

Date Modified: 09/23/2013 9:33 AM

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GREENFIELD, Iowa — Farmers switching to organic crops not only reap premium prices, they also build healthy soil and sequester carbon, according to a study at Iowa State University's Neely-Kinyon Research Farm near Greenfield.

Kathleen Delate, ISU agronomy and horticulture professor, outlined the results of the Long-Term Agroecological Research Experiment during a recent field day at the farm. The experiment is one of the longest running replicated comparisons of organic and conventional agriculture in the country. Results from 2002 to 2010 were published in April in Crop Management.

The experiment started in 1998 with funding from the Leopold Center for Sustainable Agriculture and is also used as a demonstration plot for USDA studies.

"Farmers interested in transitioning to organic production will be happy to see that with good management, yields can be the same, with potentially higher returns and better soil quality," said Delate, who leads the project.

The U.S. organic ag industry continues to grow and was a $31 billion industry in 2011, Delate said. To market a crop as organic, it must be grown on land that has received no synthetic chemicals for three years prior to harvest.

"Our research focuses on best management practices for enhancing soil quality and pest management for transitioning and certified organic farmers," Delate said. "Through timely weed management and crop rotations, we have demonstrated comparable organic corn, soybean, oat and alfalfa yields compared to conventional crops."

The 17-acre LTAR experiment compares four crop rotations using identical varieties that are repeated four times in 44 plots. The conventional rotation receives synthetic nitrogen, herbicides and insecticides at ISU recommended rates. Organic corn plots received composted hog manure during the first years of the study and now receive chicken manure. Weeds are managed by tillage, longer crop rotations, cover crops, alleleopathic chemicals from a rye cover crop alfalfa and hand weeding.

All organic fields are planted to organic seed from Blue River Hybrids or Albert Lea Seedhouse.

Results from 2002 to 2010 show that organic corn yields have been equivalent to conventional corn that received 120 to 145 pounds per acre of synthetic nitrogen. Organic soybean yields were also similar to conventional yields.

During the 2002 to 2010 period, corn planted to a conventional corn-soybean rotation yielded 180 bushels per acre. Organic corn in a corn-soybean-oat/alfala-alfalfa rotation yielded 178 bushels and organic corn in a corn-soybean-oat/alfalfa rotation yielded 167 bushels. Conventional soybeans in a corn-soybean rotation yielded 49 bushels compared with the county average of 46 bushels. Organic soybeans in a corn-soybean-oat/alfalfa rotation yielded 51 bushels and organic soybeans in a corn-soybean-oat/alfalfa-alfalfa rotation yielded 50 bushels. The 13-year average yield for organic oats is 97 bushels per acre compared to the county average of 63 bushels. The 13-year average yield for organic alfalfa is 4 tons per acre compared to a county average of 3.5 tons.

Cynthia Cambardella, with the USDA-ARS National Lab for Ag and the Environment in Ames, found that soil quality has been maintained with the organic system. Soils under organic plots in LTAR have more total soil organic carbon, total nitrogen, biologically active organic nitrogen, particulate organic matter carbon, higher phosphorus, potassium and calcium concentrations, and lower soil acidity than conventional soils. Organic soils are similar in organic nitrogen, aggregate stability and bulk density to conventional soils.

Soil organic carbon, total nitrogen and extractable potassium and calcium were 5.7 percent, 9.5 percent, 14.2 percent and 10.8 percent higher in organic soils respectively.

"Soil properties related to biologically-active organic matter were up to 40 percent higher in organic soils," Cambardella said. "In general, organic soils are showing enhanced soil function, greater nutrient use efficiency, more labile nitrogen and cations and higher carbon sequestration. Organic soils are more efficient at supplying nitrogen to growing crops than conventional soils that have conventional fertilizers applied. This is due to extended rotations, legumes, cover crops and application of compost."