Soybeans could replace pulses in rotation

Growers who want to extend their pulse rotations but do not want to reduce the amount of nitrogen fixed in their soil by a legume should evaluate soybeans as a cropping option.

Disease pressure in the prairie pulse crops has waned over the past two growing seasons under drier conditions, but pea and lentil growers will not soon forget how devastating aphanomyces was to the region’s 2014 and 2016 crops.

Soybeans generally do well under the same moisture conditions that allow pulse diseases to thrive, so in wet years the crop may be a viable option.

In dry years, however, soybean does not perform well compared to peas and lentils.

“Certainly it’s been recognized that in these last couple dry seasons, 2017 and 2018, the soybeans don’t particularly like the terminal drought in the summer,” said Jeff Schoenau, a soil fertility professor in the University of Saskatchewan’s soil science department.

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“They really respond well to the later season rains, especially the ones in August, compared to the peas and the lentils.”

Growers accustomed to growing pulses such as peas and lentils should keep in mind that soybeans are a high user of phosphorus and potassium, according to research that Schoenau helped complete a few years ago.

“When you have soybean in rotation for a few cycles, looking down the road one is going to need to pay particular attention to phosphorus and potassium fertility, and a greater potential for depletion,” he said.

Schoenau noted that nutrient removal by crops depends on the yield of the crops. He also said weather was favourable to soybeans when the trial was completed.

He said soybeans proved to be a strong nitrogen fixer in the study.

“In terms of nitrogen derived from biological nitrogen fixation in the air by the rhizobium in the roots, we found the soybeans to be good nitrogen fixers under good growing conditions,” he said.

The soybeans were double inoculated in the study, which Schoenau said is a good strategy to use because not much rhizobium lives naturally in prairie soil.

The study also looked at the short-term effects of the release of available nutrients, with an emphasis on nitrogen and phosphorus, to crops that followed soybeans, peas and lentils in the rotation.

“When we tracked it at two sites in Saskatchewan, not a big difference (in nutrients available to subsequent crops) amongst the grain legume, and that would include soybean,” Schoenau said.

There was little difference in how crops following soybean were affected compared to peas and lentils.

“In three of our sites we grew cereal wheat after the soybean,” he said. “In one site we grew canola afterwards and it seems like in terms of the relative difference of having soybean stubble, pea stubble, lentil stubble, growing canola or wheat didn’t make a lot of difference.”

This study was on loamy textured soil and did not look at the relative effect of different textured soils on the crops.

However, Schoenau said soybeans would likely do better in soil with higher water holding capacity because they require moisture throughout the season.

“Soil that has better water holding capacity, heavier textured soils, with higher clay content, I think could be desirable for the soybean, particularly to store some moisture that would be available to the crop later on in the season,” he said.

Contact robin.booker@producer.com

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