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More Conditions to Consider than Soil Moisture, Corn Seed Depth

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By R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.

Choice of seeding depth for corn is often paid scant attention by growers during the rush of planting their crop. Human nature being what it is, we tend to simply leave the planter’s depth control setting at the same position as it was in previous years. While it is true that a seeding depth of 1.5 to 2 inches is a fairly all-purpose range that works well in most situations, certain conditions merit more attention to seeding depth, the most common factor being soil moisture.

Imbibition of soil moisture by the newly planted seed occurs with the first 24 to 48 hours after planting (Nielsen, 2020a). Therefore, adequate soil moisture at seed depth (not too wet, not too dry) during those first 48 hours helps ensure rapid germination of the seed. If the soil at seed depth is excessively dry, the seed will remain inert until moisture is replenished. If soil moisture is excessive at the seed depth (e.g., saturated), the seeds may die and rot. Equally important is whether soil moisture at seed depth is spatially uniformly acceptable because uneven moisture at seed depth will cause uneven germination and subsequent emergence of the crop.

Many areas of the state are already on the dry side as we approach the serious start of corn planting in 2021. Indeed, large areas of the northern third of Indiana are rated as D0 (abnormally dry) or D1 (moderate drought) drought status by the U.S. Drought Monitor as of Apr 6.

Therein lies the potential challenge over the next few weeks in choosing the proper seeding depth because soil moisture near the surface is already borderline adequate for seed germination in some fields. Usual or excessive spring tillage will further dry out the surface few inches, especially on sunny, warm days. Planting corn at the usual 1.5 to 2 inch seeding depth may place seed into soil too dry for germination or (even worse) into soil that is unevenly moist that will result in uneven germination and emergence.

If rainfall remains a scarce commodity over the coming weeks, growers should assess soil moisture at seed depth in every field they plant. There will be situations where a 2-inch seeding depth will not provide uniformly adequate soil moisture for seed imbibition and germination. There will be situations where growers should place seed deeper to minimize the risks of uneven germination.

How deep can you “chase” soil moisture in planting corn? First of all, recognize that corn is physiologically capable of emerging from seeding depths far greater than that which today’s planters can place seed, because of the innate ability of the seedling mesocotyl to elongate during the emergence process (Nielsen, 2020b).

The risk/benefit of deeper seed placement is influenced by the depth where adequate soil moisture is spatially uniform, soil temperature at seed depth, and the short term (6-10 day) weather forecast for rainfall and temperature. A combination of 1) adequate soil temperature today, 2) inadequate or variable soil moisture at 2 inches, and 3) little to no rainfall expected in the next week or two represents a planting situation that warrants choosing a seed depth deeper than 2 inches. So, if soil moisture at 2 inches is inadequate or spatially uneven, but soil moisture at 3 inches is both adequate and spatially uniform, AND the short term weather forecast is dry, then I would not hesitate changing the planter depth setting to 3 inches (or whatever setting results in the seed actually being placed at 3 inches).

Hold on…. I can already hear the nay-sayers worrying about deep planting in soils that are prone to developing dense surface crusts following intense rainfall events that then interfere with seedling emergence and result in seedlings leafing out underground. My opinion is that the consequence of surface crusting is mostly influenced by the timing of the development of the crust relative to the timing of the emergence process and less so by the depth of seeding. In other words, a dense surface crust can impede penetration of the seedling coleopile whether the seed was planted 1.5 inches deep or 3 inches deep if the crust develops shortly after planting.

Farmers who have the means to irrigate corn have an alternative to changing seed depth when seedbed moisture is not adequate. They can apply small amounts of irrigation water to moisten the upper 6 inches of soil after planting and, if necessary, continue to apply water to ensure adequate soil moisture during the initial development of the nodal root system. For more on this, see this article by Lyndon Kelley, Purdue / Michigan State Extension Irrigation Specialist.

Accurately assessing the spatial uniformity of soil moisture at seeding depth throughout an entire field is obviously difficult. There are some innovative planter accessories in the marketplace that purport to be able to assess soil moisture “on the go” via sensors on the seed firmers and subsequently change seeding depth “on the go” based on these data. I have seen no independent data that validates the accuracy or benefits of this technology. Even though it sounds interesting, I question the spatial accuracy of changing planter depth based on a sensor mounted on a seed firmer that is already firming the seed that was just dropped into the furrow.

Bottom Line: Uniformly adequate soil moisture at seeding depth is important for assuring rapid and uniform germination of a newly planted corn crop. Take time to assess soil moisture at your selected seed depth on the day of planting. If soil moisture is not available or unevenly available at your normal seeding depth, then consider planting deeper than normal if soil moisture is available at those deeper settings.