Shallow Thoughts
Plant More Than Knuckle Deep
DECATUR, Ill. (DTN) -- Traditionally, the general rule of thumb has been to plant corn at a depth equivalent to the second knuckle on the index finger. Problem is, not all knuckles are created equal, nor do they take this spring's variable conditions into consideration.
Cool temperatures and dry soil profiles still prevail across much of the Corn Belt, despite some recent rains. Without fully charged soil moisture profiles, small rains may tempt farmers to plant shallow with the hope of a quick warm up and subsequent emergence.
Getting that seed in contact with soil that contains moisture is critical every year, according to Bob Nielsen, Purdue University agronomist. However, last season's widespread drought leaves questions about current field conditions.
"I've been harping all winter that growers really need to make that planting depth decision for every field, based on moisture conditions in that field at the time," Nielsen told DTN in an interview.
Ideally, he said the correct seeding depth should be based on the conditions of the seedbed and the 10-day weather outlook at the time of planting. The conditions of every field may vary dramatically enough as to warrant a slightly different seeding depth for each one. Or it could be the grower will end up using a common seeding depth setting for every field this year. The point is, he said, growers need to spend some time evaluating each field at the time of planting and avoid using the setting they finished up with last year.
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An all-purpose seeding depth for corn that is practical under many conditions is 1 1/2 inches to 2 inches. Planting shallower increases the risks associated with a rapidly drying seedbed, shallow crown root development and (in some locations) feeding damage by birds and rodents.
Nielsen said that under dry or potentially dry seedbed conditions, growers should not hesitate to increase seeding depth to 2 to 3 inches if that depth is where uniform moisture exists.
Physiologically, corn can easily emerge from those seeding depths due to the ability of the mesocotyl (first internode) to elongate in response to planting depth, Nielsen said. If dry conditions exist at shallower depths and the short-term (10-day) weather forecast is dry, the risk associated with deeper planting is less than the risk of inadequate or uneven moisture at shallower depths, he noted.
"The main thing is don't plant too shallow," Nielsen said. A minimum corn planting depth of 1.5 inches to 2 inches is recommended, depending on soil moisture and texture. Dry, sandy soils may require slightly deeper planting.
"If soils continue to dry out and the seed zone gets dry, growers should be looking to increase those planting depths to 2.5 to 3 inches.
"I know some growers don't like to plant that deep, but there were many cases last year where that would have helped avoid the issues with 'floppy' or 'rootless' corn," he said.
It is also possible to plant too deep. Slower and more variable emergence, soil surface crusting and exposure to seedling disease and insects due to increased emergence time are possible with extremely deep planting. A recent Iowa State University release by Roger Elmore and Mark Hanna stressed that proper seeding depth depends on soil moisture, texture and type of tillage system. For example, they indicated if necessary, corn can be planted as deep as 3 to 3 1/2 inches on clay soils, 4 to 4 1/2 inches on silt soils, and 5 to 6 inches on sandy soils. Soil type affects germination in part because of differences in temperature and water holding capacity and their physical properties.
Nielsen said deeper planting does not result in a larger or more extensive corn root system. The mesocotyls compensate for deep planting by elongating and nodal roots form at a relatively consistent soil depth of 1/2 to 3/4 inch of the soil surface. This is triggered by light interception as the seedling grows toward the soil surface. So deep planting doesn't result in more nodes formed underground, Nielsen said. However, planting too shallow results in a very short mesocotyl and shallow nodal root formation -- which leads to the floppy or rootless corn syndrome.
For more information on proper seeding depth:
http://www.extension.iastate.edu/…
Pamela Smith can be reached at Pamela.Smith@telventdtn.com
(AG/CZ)
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