For decades, soybean acreage has increased in Canada and has been internationally fuelled by growing world demand for the oilseed crop. Soybeans have also proven to be a friend to farmers, making a profitable contribution to the bottom line and adding another cropping option to diversify and extend rotations.

But what happens when farmers plant too many soybeans? On this episode of the Soybean School we tackle this question with help from Agriculture and Agri-Food Canada’s (AAFC) Craig Drury.

For the past 20 years, Drury, a soil management and soil biochemistry scientist, and his team have been running long-term rotational trials at AAFC’s Harrow Research Station in Essex County, Ont. He concedes that soybeans are a tremendously important cash crop, but cautions that farmers need to be aware of the impacts that growing too many soybeans can have on yield and soil health.

The crop rotation trial, planted on a Brookston clay loam soil, was designed to evaluate the impact of crop rotations on yield and soil organic carbon changes. It includes 17 crop rotations that are planted every year. In the video, Drury shares soybean yield results from various rotations, including continuous cropping, 2-year, 3-year, and 4-year results highlighting the impact of rotation diversity. (Story continues after the video.)

Overall, based on a five-year average, continuous soybeans delivered the poorest yield at 41 bu/ac. When wheat was added to create a soybean-soybean-wheat rotation, soybean yield increased 30 per cent to 53 bu/ac; and jumped 50 per cent to 64 bu/ac in a 3-year corn-soybean-wheat rotation. Drury adds that this more diverse rotation also had huge impacts on other crops in the rotation, including corn, which showed a 40 per cent yield increase over continuous corn.

Drury also notes the cumulative effect of diversity. Over 19 crop years, adding corn to the rotation added 194 bushels of soybean production. When wheat was added as the third crop in the rotation, soybean yield increased by 285 bushels.

In the video, Drury looks at the impact soybeans have on soil organic carbon levels and how diverse rotations can make soils more resilient and protect crops against a range of weather and environmental conditions. He also comments on what happens to soil planted to continuous soybeans, including reductions in organic matter and the impact on nitrogen use efficiency.

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