Just like on a hot summer day when opening the front door allows cool air to escape, a soybean plant allows water to escape when it opens up its leaves to the essential process of photosynthesis.
Water escapes happen through microscopic pores on all soybean leaves called stomates, says University of Nebraska-Lincoln Agronomy and Horticulture Professor James Specht, Ph.D. Whenever the stomates open to take in one molecule of carbon dioxide (CO2), on average, about 400 molecules of water is simultaneously lost.
“We calculate that one acre of a 76-bushel-per-acre soybean crop will use during a 140-day period about a half-million gallons of water,” Specht says on a Focus on Soybean webcast on tools to mitigate soybean water stress . “You can’t close the stomates because when you close the stomates to protect against water loss, you’re also closing them from letting CO2 come in.”
To compensate, soybean plants need continuous access to water in the soil to replenish this lost water. Specht suggests three methods that farmers can use when rainfall doesn’t meet their crop’s water needs:
- Irrigation. An obvious tool, but one that not all U.S. soybean farmers can access.
- No-till. This practice retains crop residue, which:
- Helps retain winter snowfall and spring rain in the field for storage in the soil before soybean planting.
- Allows less water to evaporate from the soil surfaces after planting and before the crop forms a canopy.
- Early planting. Planting as early as possible enables the crop to:
- Use more of the solar radiation that is available well before and just after the longest day of the year.
- Use more of the early-spring rainfall for crop development – because some of that water may just evaporate before it can be used by a later-planted crop.
- Form a canopy earlier in the growing season. This helps it capture all of the incoming solar radiation and reduce water evaporation from the soil.