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Hold off watering wet young crops
Corn and soybeans that spent the spring in saturated fields need time to put down deeper roots during the summer, even when the weather turns hot and dry. University of Nebraska agronomist Tom Hoegemeyer urges you to resist the temptation to start irrigating before the crop develops roots that reach down deeper into the field since doing so results in healthy plants.
Delaying irrigation (only if it doesn't unduly stress the crop) not only allows plants to root down but also builds stronger plants that are less prone to lodging later in the season. As an added benefit, deeper rooted (down 3 to 4 feet and deeper) crops take up nitrogen that leached below the top 2 feet of the soil. “If we irrigate them early in the season instead of letting them root down, they're apt to show moisture stress in dry, windy conditions,” Hoegemeyer says.
Lack of oxygen restricts root growth
Hoegemeyer goes on to explain that often corn, for example, doesn't grow roots deeper in wet soils because the lack of oxygen in the soil restricts growth. Crop roots will move down with the proper balance of air and water in the spaces between the soil particles. As the absorbed water continues to percolate down through the soil filling the soil moisture profile, roots will tend to stay in the shallower area where there is more air between soil particles. This goes a long way in explaining why corn roots down deeper in sandy and sandy loam soils (which contain more air). As the soil dries and more air becomes present between particles, root growth will continue.
Rooting continues at tassel
As corn tassels and even fills, the roots continue to grow–but more slowly, Hoegemeyer explains. The small roots and root hairs tend to proliferate where water and oxygen are both available. The plant's ability to do this well varies with the hybrid. Some varieties are better able than others to continue root growth during grain filling. But it's not likely that any will stop growing at tasseling. Some hybrids produce large numbers of brace roots, and these continue to produce small roots, root hairs, and take up moisture if there is reasonable moisture in the top 6 inches of soil.
Farmers employing watermark sensors (soil moisture sensors) can depend on these devices to “give an indication of how deep the roots are,” Hoegemeyer says. “When in doubt, carefully dig up some plants to see what's going on with the root system in your field.
“After an overly wet spring, there can be a great deal of water in the third foot and fourth foot (at irrigation time) that the crop can tap into, in addition to the nutrients that are down there,” he explains. “Allow your crop to root down before irrigating to take advantage of what's currently in those deeper depths.”