What You Can Do

In short, we need to allow these plants to root down

• to build stronger plants and prevent lodging later in the season,
  
• to help them take up nitrogen that’s been leached below the top two feet, and
  
• to reduce the need for “babying” them. If we irrigate them now instead of letting them root down, they’re apt to show moisture stress in dry, windy conditions.

With this year’s wet weather, there’s been so much moisture in the upper soil profile that plant roots have been shallower than normal, often staying in the top two feet of soil. With corn tasseling in many areas, growers have asked whether corn and soybeans will root down deeper after they enter the reproductive stage. And, if roots don’t tap into moisture in the third foot of the soil profile, how will this affect irrigation scheduling?

There are some old journal articles published in which researchers diagrammed root systems of various plants, including corn throughout the growing season, and documented corn reaching depths of 6-7 feet. Granted, the effective rooting zone where 90% of the moisture uptake occurs is in the top three feet. Corn will continue to send roots deeper, particularly in no-till dryland situations, given the plant has what it needs for rooting — oxygen, available water, and non-restricting soils.

Often, corn doesn't grow roots deeper in wet soils because the lack of oxygen in the soil is restricting growth. 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 is where no-till and improved soil structure help. This also goes a long way in explaining why corn roots deeper in sandy and sandy-loam soils. As there is more air and less water in the spaces, root growth will continue.

As corn tassels, and even fills, the roots continue to grow — but more slowly. 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 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 six inches of soil.

Research Shows How Growth Continues after Tassel

The fact that crop roots continue to grow past tasseling was documented last year in an on-farm research study funded by the Nebraska Grain Sorghum Board. The study was to document crop water use of corn, soybeans, and sorghum in two dryland fields in Nebraska. Each crop trial was randomized and replicated three times in each field and watermark sensors were placed in each plot. The study also allowed researchers to  watch and compare rooting depths at both locations.

The Chester location received 16 inches of rain from mid-May to mid-October. The Lawrence location was in a dry spot and received only 10 inches of rain during the same period. In Lawrence, corn started using moisture from the third foot of soil in the first week of July and by the end of July, it had begun using moisture out of the fourth foot. This shows the crop will continue to root down after tassel. By the end of the growing season, the crop had used all the available moisture in the top three feet and had depleted almost 40% of the available moisture in the fourth foot.

In contrast, by the end of the growing season the Chester site, which received more rain, had recharged the soil profile to above field capacity in the top two feet, at field capacity in the third foot, and 15% depleted in the fourth foot. Even the Chester location, which had 16 inches of rain, eventually rooted down to 4 feet, but didn't really use moisture out of that fourth foot until late September and October.

What about soybeans? In Lawrence last year, the soybeans started using moisture from the third foot the first week in July and from the fourth foot by mid to late July. By the end of the growing season, they had used all the available moisture in the top two feet, 40% of the moisture in the third foot, and 35% of the moisture in the fourth foot. Soybeans used moisture out of the fourth foot at Chester as well, but it took till early August before that happened.

For those of you with watermark sensors in your fields this year, they will also give you an indication of how deep your roots are. When in doubt, carefully dig up some plants to see what's going on with the root system in your field.

In summary, crop roots will move down with the proper balance of air and water in the spaces between the soil particles. We have a great deal of water in the third and fourth feet yet that the crop can tap into, in addition to the nutrients there. Allow your crop to root down before irrigating this growing season and take advantage of what's currently in those deeper depths.

Tom Hoegemeyer
UNL Agronomy and Horticulture Professor of Practice
Jenny Rees
UNL Extension Educator in Clay County