Planting depth recommendations for Ohio are 1.5 to 2 inches deep to ensure adequate moisture uptake and seed-soil contact. Deeper planting may be recommended as the season progresses and soils become warmer and drier, however planting shallower than 1.5 inches is generally not recommended at any planting date or in any soil type. When corn is planted 1.5 to 2 inches deep, the nodal roots will develop about 0.75 inches below the soil surface. However, at planting depths less than 1 inch, the nodal roots develop at or just below the soil surface. Excessively shallow planting can cause slow, uneven emergence due to soil moisture variation, and rootless corn (“floppy corn syndrome”) later in the season when hot, dry weather inhibits nodal root development (Nielsen, 2010). According to some field agronomists, shallow plantings increase stress and result in less developed roots, smaller stalk diameters, smaller ears and reduced yields. In a recent OSU evaluation of planting depths, grain yields were about 14% greater for the 1.5-inch and 3-inch planting depths than the 0.5-inch planting depth in 2011, and 40% greater in 2012. The lower yields of the shallow planting were associated with a reduced final stands and 6 to 7 times as many “runt” plants as the other two planting depths.
In a 2013-2014 Cornell University study comparing planting depth across a range of soil types and plant populations, Cox and Cherney (2015) concluded that optimum seeding depth differed across sites and at times across years within sites. Additionally, the risks of reduced population or grain yield were generally greater at the shallow seeding depth compared with the deeper depth (2.5 inches). Research at Kansas State University (Roozeboom, 2012) that evaluated six planting depths ranging from 1 to 3.5 in. supported planting depth recommendations of 1.5 to 2.5 inches depending on soil conditions
Despite potential risks, many growers continue to plant at depths less than 1.5 inches. There is a perception that seed planted shallower than 1.5 in. will emerge more rapidly due to warmer soil temperatures closer to the surface. This is an important consideration as corn growers across the Corn Belt are planting earlier (Kucharik, 2006) so they can complete planting before yield potential begins to decrease after the first week of May. Particularly in soils that crust, speed of emergence is critical in order to establish plant stands before heavy rainfalls “seal” the soil surface.
Recent work by Deere & Company and the University of Illinois (Armstrong et al., 2016) suggests variable seeding depth planting within fields may improve corn yield especially when soil moisture conditions become less ideal (drier or wetter). Research is underway to improve our understanding of corn response to planting depth across different soil types and conditions. Results of this work may enable more effective use of planting technologies that allow variable planting depths during the planting operation.