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. 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 2011-2012 Ohio evaluation of planting depth, 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 reduced final stands and six to seven times as many “runt” plants as the other two planting depths.
Despite potential risks, many growers continue to plant at depths less than 1.5 inches. The rationale for this is typically that the seed 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 so they can complete planting before yield potential begins to decrease after the first week of May. Some studies have documented faster emergence rates with shallower planting depths, but the comparisons have often included deeper planting depths than the recommended ranges and results are highly influenced by temperature and rainfall in the given season. Improving our understanding of corn response to planting depth across different soil types and conditions may enable more effective use of planting technologies that allow variable planting depths during the planting operation. However, research on the effects of soil temperature and moisture flux in the seed furrow at different planting depths in relation to seed emergence is limited.
In 2017 and 2018, we conducted studies in two OSU research farm fields at South Charleston, each with different soil types — one a Strawn-Crosby complex with a silt loam texture (2.0% organic matter), and the second a Kokomo loam (3.8% organic matter) to study the impact of varying planting depth (1, 2, or 3 inches) on emergence rates and grain yield. In each field, soil moisture and soil temperature sensors were installed in each plot, which continuously recorded average temperature and soil moisture every twenty minutes until the end of the emergence window.
Emergence was most uniform with 2-inch planting or deeper. Deeper planting depths slightly delayed the date of first emergence compared to the shallowest planting depth. This was partly due to greater temperature fluctuation in shallow planting depths resulting in faster accumulation of soil GDDs driving faster emergence. Shallow planting was the most subject to moisture content of the soil causing an extended emergence window in 2018 (5 to 6 days to reach >95% emergence) compared to the deeper planting depths (3 to 4 days to reach >95% emergence).
A three-day emergence period (72 hours from first plant emerged) did not limit yield. However, after three days, emerging plants exhibited 8% to 15% lower yields and greater variability in yield.
Increasing planting depth was correlated to yield increases. Deeper planting was associated with more yield per plant (3% to 5% yield increase from 1 to 3 inches). Yield uniformity also improved with increasing depth, and was most optimum at 2 or 3 inches for the lower organic matter soil and most optimum at 2 inches for higher organic matter soil.
Key take away points:
- Soil GDD accumulation drives early emergence.
- Uniform emergence was highly dependent on soil moisture content.
- Emergence of all plants within three days is critical to maintain yield.
- Yield per plant tends to increase with planting depth.