It was May 2008 and Iowa State University Extension corn agronomist Roger Elmore was looking at an Iowa corn field. One out of five plants in areas across that field struggled to emerge. "The smaller plants had only three collared leaves, V3, at the time I saw them in late May 2008," says Elmore. "The larger neighboring plants had at least five collared leaves, which is V5 stage of plant growth. 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 plant size differences like this happen?"
Spring rains across Iowa in the spring of 2008 resulted in saturated soils, cool soil temperatures and soil conditions below most of our standards for planting (see ICM article on May 2008 conditions). "Yet the calendar pressed us into thinking we were losing yield potential because of delayed planting," says Elmore. "Most of us were guilty of 'mudding in' at least some corn that year. The penalty: reduced yield."
So was the "mudding in" to blame for the wide range in plant sizes Elmore witnessed? Many things can cause plant to plant variability, he says. In this case the problem was planting depth. The smaller, dominated plants were planted one inch shallower than the larger plants. Seeding depth varied by one inch from one plant to the next within the same row!
Elmore offers the following recommendations and guidelines for planting depth. The first step to achieving a uniform stand is to plant at the right depth, he notes.
Root placement changes with shallow corn planting
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 of an inch regardless of planting depth. This is triggered by light interception as the seedling grows toward the soil surface. Planting too shallow results in a very short mesocotyl and, as a result, very little distance between the seminal and nodal root systems.
Consequences of shallow corn planting
All of this may spell trouble. Although soil temperatures at the soil surface warm up faster than deeper in the profile, it also dries faster. Seed planted into dry soils may not germinate, or worse, may imbibe moisture and then die if rain is not forth coming. Corn absorbs 30% to 35% of its weight in water before germination begins; soybean absorbs 50%. When planted into dry soils, whether shallow or not, corn emerges more erratically than soybeans. Planting corn shallow also exposes seedlings to more potential damage from either pre-plant or pre-emergence 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 if planted shallow due to inadequate root formation below and above ground.
In addition to all of these plant responses to shallow planting, it is 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.
The bottom line or rather the "bottom of the seed furrow"
On average, a 2-inch planting depth ensures the best root formation and potential for uniform emergence. Planting deeper delays 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, the planting depth may need to be deeper than 2 inches to tap into consistent moisture. In my experience, more problems occur with shallow planting than with deep planting. Never plant corn shallower than 1½ inches.
Also, check planter settings often. All of us have the tendency to want to hurry up and plant and assume 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. Check planter depth settings every time you enter a new field or plant in different conditions.
The planting depth issue in the field discussed in the first paragraphs of this article was likely due to planting too fast 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.