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Can organic growers fight weeds and increase soil health?

3rd May 2017

This article comes from the Soil Science Society of America and was originally published on the 5th April 2017. It was written by Rossie Izlar, and can be accessed in full here

To grow crops organically, farmers fight weeds with chemical fre weapons. One of the most common is a disc. Farmers rip out weeds and churn them into the soil wiht these discs. But it takes heavy tractors to do this efficiently and large tractor tyres compress the soil as they roll across the field. The process also depletes soil organic carbon, which plants depend on for nutrients, moisture and healthy bacteria. And, after a field is turned, heavy rain and wind can erode the soil.

University of Missouri soil scientist Kerry Clark and a team including researchers from the USDA Agricultural Research Service are working to get the best of both worlds. Can organic growers fight weeds and increase soil health?

Researchers know that cover crops provide part of the answer. Cover crops anchor the soil with their roots, some event add nutrients to the soil. That would take care of the soil health concern. But the question of weeds remains.

Clark and her team experimented with three methods. One method used cover crops instead of discs to suppress weeds. They compared this method to two other organic options, using cover crops but turning them under, and the standard disc use without a cover crop.

Matting cover crops is key to the first method. Instead of turning the cover crop under, it is flattened with a roller crimper. The flattened cover crop creates a matted barrier over the soil. Afterwards, researchers plant cahs crop seed into the mat, often using discs to cut through it. If the cover crop was health and abundant, the mat will be thick enough to choke out weeds.

Here a delicate balance must be found. If the cover crop doesn't grow well, it won't be able to form a weed-suppressing mat or outcompete the weeds. However if it grows too well. it may compete with the cash crop itself or leave persistent seeds behind. 

Over a two year experiment, the researchers confirmed two principles:

  • there's no silver bullet for producing the perfect crop
  • unexpected weather always gets in the way (the experiment was plagued by drought for one year)

Despite this, the team also confirmed earlier research that found cover crops require almost as much maintenance as cash crops. "We were a little surprised by how much management the cover crops needed," said Clark. 

Ideally the cover crop must be healthy enough to flatten. But if the soil isn't fertile to begin with then the crop won't grow well. The researchers also found that cover crops work better with soybeans than with corn. When it was time to plant corn, the cover crop hadn't matured enough to become a useful mat. The team had to delay corn planting, sacrificing end of season yield.

"The timing doesn't match with corn" said Clark, "but it does with soybeans."

Furthermore soybeans can be planted closely in large numbers, there will always be enough soybean seed to outcompete cover crops. But corn neds more room to grow, so the seeds are placed further apart. Cover crops can quickly overrun the smaller corn population. 

"You'd have to double your corn planting to get a decent yield," said Clark.

Cover crops also tie up nitrogen when planted with corn, so the team had to add extra manure fertiliser to keep the cover crop healthy. The research team also discovered that the longer a farmer avoids using a disc on the field, the more likely perennial weeds will pop up in the field. The team recommends an occasional turn to break the life cycle of these weeds.

Clark said that although she hasn't given up hope for cover crops as a tool against weeds, she's trying other things. 

Read the full scientific paper here