Getting on Board with Soybeans

Dr. Jeff Schoenau takes a closer look at soybean nutrient requirements and effects on following crops in Saskatchewan

by Jane Caulfield

Every year farmers need to figure out what crops they are going to seed by determining which will ensure a good yield, make efficient use of inputs, and have enough consumer demand and economic return to make the hard work worth it. Over the past few years, more producers are choosing to introduce soybeans into their crop rotations.

Taking notice of the increase in soybean acreage across Western Canada, Dr. Jeff Schoenau, Professor of Soil Science and Saskatchewan Ministry of Agriculture Chair in Soil Nutrient Management at the University of Saskatchewan, decided to take a closer look at the nutrient content, uptake, and dc composition of soybean grain and straw grown in Saskatchewan conditions. Completed as a PhD project by Dr. Jing Xie in the Department of Soil Science, in collaboration with Dr. Tom Warkentin of the Crop Development Centre, the project also aimed to evaluate the contribution of soybean stubble to nutrition and yield of subsequent crops in comparison to peas and lentils.

“For a soybean grower — and there are quite a few first timers out there, particularly in Saskatchewan — this research gives them some guidance for fertility planning for soybean and for crops that follow in rotation,” says Schoenau. “There are promising prospects for soybean production in the northern Great Plains, especially since we have learned that in many ways, growing them provides similar nutritional benefits to following crops compared to other pulse crops.”

Nutrient Requirements and Removals

In an effort to contribute to improve the agronomic management of soybeans, and the subsequent impact of soybeans on soils and following crops, Schoenau compared three short-season soybean varieties to three pea and three lentil varieties at four sites located in the Black and Dark Brown soil zones in Saskatchewan. Conducting the study over a three-year period allowed Schoenau and his team to introduce wheat and canola into the rotation, providing an opportunity to explore how soybean crops affect future crops. 

Research results revealed that the soybeans grown at the four sites in 2014 produced similar or higher grain yield and nutrient uptake compared to the other legumes, making them a good alternative to traditional pulse crops. The effects on yield and nutrition of wheat and canola grown in the two years following the soybeans were also similar. However, the soybeans tended to have higher concentrations of phosphorus, potassium, calcium, magnesium, and sulphur in the grain, suggesting a potential for additional depletion of these elements from the soil over several rotational cycles due to greater
crop removal.

“In the short-term, the requirements for added fertilizer for wheat and canola grown after soybean would appear to be relatively similar to what farmers currently use after growing peas and lentils,” says Schoenau. “But soybean grain had significantly higher amounts of phosphorous and potassium than the other pulse crops. This means that over the long-term, fertilizer adjustments are needed to account for this output to prevent soil depletion.” 

At maturity, all three legume crops in the study showed similar amounts of nitrogen fixed from the atmosphere in the aboveground plant components of grain and straw. The highest overall amount of nitrogen derived from biological nitrogen fixation was found in peas, but soybeans demonstrated amounts that were equal to or greater than peas at some sites. They also discovered that soybeans had the highest proportion of nitrogen derived from fixation of the three pulse crops.

“With about 70 per cent of its nitrogen derived from fixation, soybean was found to be a good nitrogen fixer. However, it requires careful attention to proper inoculation practices that provide sufficient amounts of the correct nitrogen fixing rhizobia responsible for the fixation, since indigenous populations are very low in our soils,”says Schoenau.

Benefits of Soybeans

According to Schoenau, the increasing number of producers choosing to grow soybeans is not entirely surprising. “Last year, for example, was a bad year for some pulse crops because of very wet conditions and disease pressures experienced in some regions of Saskatchewan,” he says. “Soybeans are less susceptible to some of the diseases out there and may fare better under wet conditions, utilizing moisture received later in the growing season.”

Understanding the various agronomic aspects of soybean production will help farmers successfully introduce another crop into their rotation and strengthen their productivity each year. “Adding soybean to your crop rotation contributes to diversification,” says Schoenau. “There are a larger number of
shorter season varieties out there now that are suitable for Saskatchewan’s climate and conditions.”