A soil scientist at Cornell University suggests organic farmers use tillage to control weeds without sacrificing their soil structure.
Charles L. Mohler, senior research associate at Cornell's Department of Crop and Soil Sciences, described a technique of fallow tillage as part of a crop rotation approach. He presented several rotation practices during a webinar sponsored by the USDA's Natural Resources Conservation Service.
Crop rotation cannot solve all of a farmer's weed and pest problems, he said, but manipulating the weed seed bank over several years can be effective. He described a New England farmer who incorporated summer tilled fallow into his rotation scheme by stirring the seed bed and promoting weed germination. He tilled shallowly to kill the weeds, then repeated the process.
"People think, oh well, you're going to destroy the soil," he said. "But it's not necessarily the case."
The farmer incorporated "enormous amounts" of coarse organic matter into the soil, which protected it from the rain. "They put a huge amount of water on it, but they could not get it to puddle."
Doug Collins, soil scientist at Washington State University, agreed the described technique wouldn’t necessarily impact the quality of soil with lots of organic matter. Other strategies might include till-and-irrigate to encourage weed seed germination, then follow with a summer cover crop, which would add nitrogen to the soil instead of leaving it fallow.
For weed control, he emphasized, "Make sure whatever you’re growing is not going to seed."
Mohler offered several other tactics organic farmers can use to deal with weeds:
• Follow a weedy crop with one that’s easy to keep weeded.
• Rotate the seasonality of the crops to disrupt the weeds.
• Avoid using a cover crop that can cause weed problems, for example hairy vetch before wheat.
• Rotate annuals with sod crops. Repeated mowing depletes the perennial weeds, and annuals can’t reproduce.
Mohler is co-editor and lead author of a guidebook available online — www.sare.org/Learning-Center/Books/Crop-Rotation-on-Organic-Farms— to which he often referred during his presentation.
There is no one-size-fits-all recipe for crop rotation, he said. Variables include number of crops, size of farm and planting-harvesting schedule. Some farmers have an ad-hoc plan for the next rotation crop and cash crop, but shy away from long-term plans because of changing weather, markets or labor supply.
"Our purpose is not to tell farmers what to do," he said, "but to help them organize information and make decisions in an orderly series of steps, to facilitate checking for problems."
For a simple operation, with just a few crops, low variation in acreage among crops and one cash crop per year, he suggested focusing on rotation planning. For a complex operation, with many crops, high variations and continuous cropping, a focus on crop sequencing works better.
Mohler recommended writing down "family return times," the minimum time needed between nightshades, brassicas and cucurbits, for example. Return times of at least four years, except for grasses, improve disease control.
Mapping the planning units should account for characteristics critical to crop management, such as tendency to flood or early availability, and those units should be grouped together. Units should also be grouped by similar histories over the past three years.
In planning and mapping out future crop sequences, he suggested beginning with crops already in the ground, then considering in order which families are close to their return time, high-value crops that require special field conditions, other valuable crops that grow anywhere, less-valuable crops that require special conditions and less-valuable crops that grow anywhere.
"Check next summer’s crop and mix, then plan two summers from now," he said. "Go back and fill in cover crops between next summer and two summers from now."
After putting the plans on maps, walk the fields and "farm the land in your head," he said.