Clarke McGrath has been getting questions from farmers this week wondering about planting depth for corn, given the soil conditions this spring. Some farmers are talking about "ideal" conditions, and some are talking about "dry." McGrath, who authors the "CSI:Iowa" column on crop-related topics in Wallaces Farmer magazine each month, is an Iowa State University Extension field agronomist based at Harlan in western Iowa.

Related: Don't be afraid of deep planting for corn

How Deep Should You Plant Corn?

The ideal planting depth for corn is generally 2 to 2.5 inches (2 inches is considered "perfect" by Iowa State University agronomists). "We don't recommend aiming for less than 2-inches as your planting depth. We like to have some margin for error," says McGrath. There are a few reasons for that strategy.

Crop residue can cause the planter's row units to bump up so they plant shallower, riding up on the residue. "It seems that the residue this year is not as well broken down as much as it was the last few years, as I look at fields this spring," says McGrath.

Also, you need to consider the condition of the soil, he says. Loose seedbeds can easily settle a quarter-inch to a half-inch with a rain, so corn originally planted at 1.5 inches could end up closer to being only 1 inch deep, a very bad deal. "If the last tillage pass you make before planting is made at an angle to the planting direction, you may get a similar 'bumping' effect of the planter's row units—the same effect as the heavy crop residue can give," says McGrath. "Consider running your last tillage pass the same way you plant. Also, watch your planting speed. Planting at speeds on the high end of the scale--faster than around 5 to 5.5 mph--can tend to lift the planter and produce shallower seed placement, as well as contribute to variable spacing between plants."

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Try to avoid rootless corn syndrome, a problem aggravated by shallow planting
Corn that is planted shallow has a greater potential for root development problems. "For example, if seed is placed about 1.25 inches or less as the planting depth, the nodal roots will be situated barely below, or even on the soil surface," says McGrath. "The unstable plants and related problems that can occur from this are commonly referred to as 'rootless corn syndrome.' This is something you want to avoid."

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Another factor to consider, when determining proper planting depth for corn in your field, is that soil near the surface dries out faster and has greater temperature fluctuations. "This can cause the seminal and nodal root system of the young plants to develop symptoms that resemble herbicide injury such as stunted, stubby or swollen roots, all of which can be less effective in supporting the corn plant," he explains. "Roots forced to develop close to the soil surface can be aborted if conditions are hot and dry where development of a corn plant is taking place."

Shallow planting can also increase the potential for chemical injury. With a shallow seed placement, the growing point of the corn plant is closer to the soil surface and may be exposed directly to higher concentration of herbicides.

What if you have very dry topsoil? Should you plant deeper than 2 inches?
"We almost always recommend a planting depth of 2 inches for corn, but there is an exception," says McGrath. "If you are dealing with very dry topsoil in mid-to-late May, it is advisable to plant deeper to enable the seed to absorb the amount of moisture needed to germinate (about 35% of the kernel dry weight). Looking at the calendar, we are hoping this isn't an issue this year. We are now still plenty early in the planting season—it is only the second week of April. So we may not have to get too excited about this. But just in case, if you do get into a very dry topsoil situation in later on, here are our recommendations."

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

How deep? It depends on several factors, probably the most important is your soil type. In heavy, high clay content soils, probably no deeper than 2.75 to 3 inches is the best bet, while in lighter silt soils you could plant the corn closer to 3.5 inches deep and find the right combination of soil moisture and emergence if the soils are warm and dry.

ISU agronomists continue to conduct depth of planting studies to look at this question. Across the 1.75-inch to 3-inch planting depths the last few years, the results are statistically the same most of the time, he says. "So, in summary, we recommend you target a 2 inch planting depth or slightly deeper unless it's extremely dry later on this spring, then do some digging and if needed, put the seed a little deeper."