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Planting depth makes difference

One out of five plants in areas across an Iowa cornfield had struggled to emerge. The smaller plants had only three collared leaves, V3, at the time I saw them in late May 2008. The bigger neighboring plants had at least five collared leaves, or V5. Plant-to-plant differences like this reduce yield potential. Weaker plants compete like weeds with the larger, more dominant plants, reducing overall productivity. How could such plant size differences happen?

Rains across Iowa that spring resulted in saturated soils, cool soil temperatures and soil conditions below most standards for planting. Yet the calendar pressed us into thinking we were losing yield potential because of delayed planting. Most of us were guilty of “mudding in” at least some corn that year. The penalty: reduced yield.

Was the “mudding in” to blame for the wide range in plant sizes I saw? Many things can cause plant variability; here the problem was planting depth. The smaller, dominated plants were planted 1 inch shallower than the larger plants. Seeding depth varied by 1 inch from one plant to the next within the same row!

Seeding depths of around 2 inches are optimum for most Iowa conditions. Two root systems exist in corn — the first helping to establish the young seedling, and the second carrying it through the entire season. The seminal roots emerge from the seed, while the nodal roots emerge above these at the junction of the mesocotyl and coleoptile. Planting too shallow results in shallow nodal root formation.

The nodal roots form at a relatively consistent soil depth of ¾ inch regardless of planting depth. This is triggered by light interception as the seedling grows toward the surface. Planting too shallow results in a short mesocotyl, thus little distance between the seminal and nodal root systems.

Avoid shallow planting

Shallow planting may spell trouble. Although soil temperature at the surface warms up faster than soil deeper in the profile, the surface also dries faster. Seed planted into dry soils may not germinate, or worse, may imbibe moisture and then die if rain isn’t forthcoming.

Planting corn shallow also exposes seedlings to more potential damage from either preplant or preemergence herbicides, as well as fertilizer injury. Shallow nodal root formation often shows up as rootless corn syndrome. Even later during the growing season, plants are more susceptible to root lodging when planted shallow due to inadequate root formation below and aboveground.

In addition to all these plant responses to shallow planting, it’s good to remember that planter closing wheels are designed for 2-inch planting depths. In ideal conditions, the best seed-to-soil contact occurs at 2 inches. On average, a 2-inch planting depth ensures the best root formation and potential for uniform emergence.

Nevertheless, the best planting depth varies a bit with soil conditions and with current and impending weather. For example, in cool, early-spring soils, planting at 1½ inch deep may work well. Yet in dry conditions, planting depth may need to be deeper than 2 inches to tap in to consistent moisture. In my experience, more problems occur with shallow planting than with deep planting. Never plant shallower than 1½ inches. Check planter depth settings often. We all tend to want to hurry up and plant, assuming everything is OK. A few years ago we started planting one of our experiments at a depth of ¾ inch before correcting it! Fortunately, we had not planted any of the actual research plots yet.

The planting depth issue in the field discussed in the first paragraphs of this column was likely due to too fast of planting for the conditions. The differences in planting depth were likely due to planter unit bounce. Plant-to-plant uniformity in germination, emergence and growth are essential to maximize yields; every plant should look like every plant.

Elmore is ISU Extension corn agronomist.

This article published in the April, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.