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Get N to the roots to get waterlogged corn back on track

Agriculture.com Staff 06/23/2008 @ 10:02am

Fields that looked like lakes a week ago are now revealed to show the scars left by flowing waters that, in most cases, washed away fertilizer, herbicide and even layers of topsoil. These conditions may tempt you to get more fertilizer back on the crop however you can. But, don't waste your time applying to yellowing leaves. Aim for the roots instead, says one crop specialist.

The corn crop in parts of Iowa and Illinois didn't start sustaining damage just with recent flooding. The wet, cool spring leading up to the big deluge left corn roots stunted in compacted soil, and left leaves shortened and yellow, says Iowa State University Extension agronomist John Sawyer. Even in fields where ponding occurred, Sawyer says a fertilizer theme reveals itself on closer observation of different fields.

"Looking at the plant coloration and growth, I can generally see a better response to spring-applied ammonia compared to fall-applied ammonia, especially at lower N rates," he says. "It is too early to tell what amount of N may have been lost this spring, but it is clear either N loss is greater from the fall application, or nitrate from that timing is deeper in the soil and roots have not gotten to the N, or wet soil is restricting deeper root growth, or a combination of all.

"It is clear that the corn is not growing as well on the more poorly drained (lower landscape, wetter) areas compared to the 'higher' and better drained areas, and the corn response to fall and spring applied N is better on the areas where corn is growing better," Sawyer adds.

Conditions like these at this point in the growing season don't necessarily mean the crop is a goner. Sawyer says the corn's root development and subsequent nutrient absorption will continue. How well it does so will determine how the crop bounces back.

Moving forward, sidedressing is about the only way to get any nitrogen back into the field, even if it means surface-applying. But, stick with sidedressing and avoid foliar fertilizer, even in yellowing corn, as root development is the top priority now.

"The best approach is to get sidedressed N into the soil. Injection several inches into the soil places N in the root zone, and helps avoid placement on or near the surface where dry soils can limit root uptake. Injection is not always an option and sometimes it is just necessary to surface apply N materials. In those instances, rain will be needed to move the N into the soil. This (rain) is not exactly what we want right now, but the N needs to get into the soil for plant uptake," Sawyer says. "At this time we want corn to grow root systems. Trying to 'green up' corn with foliar application of fertilizer materials or application of low nutrient rates will not help promote root growth or help plants recover from poor aeration in the root zone.

"Corn will soon reach a rapid growth phase, and adequate N must be present in the active root zone to meet plant demands. Corn requires a large amount of N for high yields, and the way this large N demand is met is through root uptake," he adds.

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