By Roger Elmore, Department of Agronomy
One out of five plants in areas across the Iowa corn field struggled to emerge. The smaller plants had only three collared leaves, V3, at the time I saw them in late May 2008. The larger neighboring plants had at least five collared leaves, 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 plant size differences like this happen?
A 2-leaf differential among plants in this corn following corn field.
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. 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 I witnessed? Many things can cause plant to plant variability; here 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!
Root placement changes with shallow 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.
The root development of a young corn seedling.
Consequences of shallow planting
All of this may spell trouble. Although soil temperatures at the 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 percent of its weight in water before germination begins; soybean absorbs 50 percent. When planted into dry soils, whether shallow or not, corn emerges more erratically than soybean. 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 when planted shallow due to inadequate root formation below and aboveground.
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 2inches.
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 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.
Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at firstname.lastname@example.org or (515) 294-6655.