Soybean School: Making grain with roots and rain


It was a dry start to the year for soybeans in Ontario as a lot of seed went into dry conditions, but the crop seems to have come along. A dry spring in Ontario can make for easier planting though, and coupled with timely rains the season’s start hasn’t been too bad.

In this Soybean School episode, Bernard Tobin is joined by Dale Cowan, AGRIS Co-operative agronomist, for a chat in the field about when soybeans need moisture, and how much.

Soybeans need to be planted into moisture, of course, but they also need accumulating moisture throughout the growing season.

Snow and rainfall combined, the area Tobin and Cowan are in can expect anywhere from 600 to 1,000 mm or somewhere just under 24 to 40 inches of rainfall for the whole year. Normally, the season is started under a moisture surplus, and then during the summer there’s a moisture deficit.

“This whole idea of understanding water usage in the plant, helps us understand some of the growth habits and where the yield potential’s gained and lost, and why things like fertility and weed control are so important — things that help conserve the moisture, keep the plant health up, so it can get through the season,” says Cowan.

Evaporation, or loss through of water from the soil surface, and transpiration, the traveling of water through roots, then leaves, and out through the stomata of leaves, is the main use of water by the plant and how nutrients get into the plant for growth and photosynthesis. (Story continues below video)

Soybeans take up about half their weight in water during germination, but it’s not until the plant gets to about the first trifoliate that it needs about 0.05 inches per acre per day, says Cowan.

“Here we are, a V4 plant, we’re now getting to the point where we’re almost using 0.2 inches of water per acre per day,” says Cowan. A typical soybean plant, depending on the season, can use between 18 and 23 inches of water for the whole season.

“The point to remember is almost half that water need is needed at reproductive stage, when we’re trying to fill those pods and make the beans,” adds Cowan.

Managing soil moisture becomes critical, especially in the moisture deficit year, like parts of Ontario might find themselves this summer. Cowan says that residue cover reduces evaporation and practices that build organic matter increase water infiltration and water holding capacity, too. Every one per cent of organic matter can hold one inch of water, he says.

The phrase “August rains make grains” really rings true in the case of soybeans. Cowan says the 90 per cent of soybean roots are in the top three inches of the soil, which is why good moisture content later in the growing season is so important to keep roots active, fixing nitrogen, and delivering it and other nutrients to the plant.

“There’s a ten-day period in there where the daily uptake of nitrogen is three pounds of N per acre per day, and potash is almost three pounds per acre, so you need moisture to do that,” says Cowan.

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Soybean School (view all)Season 10 (2021) Episode 17

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