By Roger Elmore and Mahdi Al-Kaisi, Department of Agriculture, and Mark Hanna, Department of Agricultural and Biosystems Engineering
Dry soils persist in many parts of Iowa with soil moisture conditions worse than last year. This contrasts with what we normally experience: Iowa soil profiles invariably are filled to field capacity at planting time and tile lines flow. That may not be the case in 2013. In earlier ICM News articles, Elmore addressed how dry soils at planting might affect seeding rates and hybrid selection. Seeding depth is another factor we can control – at least to some extent. With soil moisture at field capacity at planting, standard guidelines for seeding depth apply. However, how do we think about seeding depths if soils are dry at planting?
To address this, let’s first look at the basics of seeding depth and some important facts on root development that interact with seeding depth. Three factors affect corn responses to seeding depth: soil texture, soil moisture and soil temperature. Al-Kaisi addressed the first two factors in a recent ICM News article. He provided excellent background on how these two factors interact and how water moves in a soil profile. It is clear that tillage system dramatically affects all three of these factors.
The main objective at planting is to provide the seed an environment suitable for rapid germination and vigorous early growth. This not only requires the right depth for the three interacting factors, texture, moisture and temperature, but also excellent seed-soil contact that provides uniform emergence and vigor of all seedlings. How can we achieve this? If soil moisture at planting is not a limiting factor – either too much or too little - optimum seeding depth recommendations are fairly simple.
Seeding depth recommendations with ‘adequate’ soil moisture
Corn needs water, aeration and temperature – all in the right proportions - and contact with soil to germinate and emerge. Standard recommendations for corn seeding depth vary little across the Corn Belt: 1.5 to 2 inches. This is a good compromise for seeding depth because it allows for quick and maximum emergence rates as well as proper root system development. Corn seeds must absorb about 30 percent of their weight in water to begin the germination process. In most soils and most tillage systems, a 1.5 to 2 inch seeding depth is sufficient to reach soil moisture adequate enough to germinate seeds but not excessive to the point where it reduces soil aeration.
Soil moisture and root development
‘Adequate’ soil moisture - We put adequate in single quotes because of its subjectivity; what is adequate? That is similar to saying, “…plant to moisture!” Al-Kaisi addressed that in his earlier ICM News article: adequate soil moisture for germination occurs at field capacity. As mentioned above, seeding depth also directly affects root system development. Understanding this will help us understand the importance of proper seeding depth.
Root system development - Proper root system development depends on correct seeding depth. Figure 1 illustrates how the mesocotyl length compensates for seeding depth. With correct seeding depth and soil conditions, the nodal root system forms ½ to ¾ inch below the soil surface. The mesocotyl - the first internode – elongates out of the seed to the extent necessary to position the nodal root system at that ‘perfect’ place near the soil surface. Deeper planting results in a longer mesocotyl; shallow planting results in a shorter mesocotyl. With very shallow seeding, seminal root and nodal roots systems occur at the same depth near the soil surface (Figure 1). We discuss the problems of too deep and too shallow planting below. In the meantime, it is important to remember that the seminal root system supports the entire plant during the early vegetative stages; by V6, the nodal root system completely supports the plant. Both roots systems are essential for corn growth and development. See Corn Growth and Development for more detail on this important process.
Figure 1. Seminal and nodal root system development at different seeding depths from 2.5 to 0.5 inches (6.2 to 1.3 cm) left to right, respectively. Mesocotyl length varies due to different seeding depths. Identifying the nodal root system apart from the seminal root system in the right-handplant is not possible because of shallow seeding. From Corn Growth and Development, Iowa State University Extension and Outreach, PMR 1009.
Frequently asked questions
1) What if the soil is dry at planting time?
a) Should I place seed into dry soil? The old saying, “Plant in dust, your bins will bust!” is better suited perhaps for winter wheat country than for the central Corn Belt. But, should we adjust corn seeding depths if soils are dry at planting? If soils at 1.5 to 2 inches are drier than field capacity, three possible options exist: plant deeper, adjust planter row cleaners and/or down pressure, or plant at 1.5 to 2-inch depths and hope for rain. The first two options seem most viable – we’ll talk about them below. If you are certain of timely rain then the third option may work, albeit inherently with high risk. From our perspective, planting in ‘dust’ may rank higher in risk than leaving the seed in the bag awaiting better planting conditions.
Although either planting deeper or making planter adjustments are the best alternatives for dry soils, some may consider planting shallow if it’s dry. Let’s discuss that next.
b) Should I plant shallow: less than 1.5 inches? First, if soil and environmental conditions all season – not just from planting until emergence - are excellent, shallow planting should result in plants that emerge faster and yield similar to those planted deeper. However, shallow seeding depths usually results in shallow nodal root placement (see figure 1) which may compromise stands, uniformity of emergence and yield - if conditions aren’t perfect. In addition, ‘rootless corn’ results from shallow planting among other things. In some situations, shallow planting results in more bird damage. Planting shallower than an 1.5 inches is rarely justified.
c) Should I plant deeper than 2 inches? With drought conditions, it is often impossible to plant deep enough to reach moisture. Fortunately, corn has an astounding ability to emerge from deep planting. For example in the mid-twentieth century, researchers found that a Navajo corn variety had 11 percent emergence from a 1 foot seeding depth and 61 percent from a 10 inch seeding depth! A modern double cross hybrid at the time, US13, did not emerge at all from 10 inches or 1 foot, but at 8 inches, it did have 56 percent emergence. Leaves of both cultivars opened underground when planted at 1-foot depths. The Navajo corn variety was clearly adapted to deep planting in dry conditions. Modern hybrids emerge from depths greater than what most planters can plant.
d) How deep can I plant? Again, that depends on soil moisture, texture and type of tillage system. For example, if necessary, corn can be planted as deep as 3 to 3 1/2 inches on clay soils, 4 to 4 ½ 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. Remember, though, if soil moisture is adequate at recommended seeding depths there is no reason to plant deeper.
e) What are the potential problems with deep planting? Unfortunately, an array of problems accompanies unnecessarily deep planting. Slower and more variable emergence is among the biggest potential problems inherent with deep planting. For example, scientists in North Dakota found that an additional day for emergence is required for every 1-inch deeper seeding depth.
Coupled with the slower emergence, deeper seeding exposes seedlings to a greater chance for soil surface crusting and exposure to seedling diseases and insects – since the time between planting and emergence increases. Other issues may come up too; researchers in Virginia working on sandy soils found that deeper planting resulted in wetter grain at harvest – again probably associated with slower emergence.
As we discussed in the root development paragraph above and as shown in Figure 1, mesocotyls compensate for deep planting by elongating and positioning the nodal root system within a ½ to ¾ inch of the soil surface in optimum environments. Because of this, deep planting does not result in more nodes formed underground. In fact there is some older evidence that deeper seeding (4 inch vs. 1 inch) resulted in fewer roots above the coleoptile node; thus root numbers from other below – ground nodes were reduced. Deeper rooting does not result in more extensive root systems.
2) What other factors might affect seeding depth?
a) Planting date - Early planting? Most agronomists suggest planting shallow if planting early, but never shallower than 1.5 inches. Soil temperature overrides many factors with early planting. Corn seeds will absorb moisture – if it is available – when soil temperatures are less than 50° F., but they will neither germinate nor begin to grow. With normal seeding depths it takes 90 to 120 Growing Degree Days ° F (GDD) for corn to emerge. Planting too early in cool soils can take up to a month to emerge.
Late planting? The North Dakota researchers mentioned above found that as soil temperatures increase above 50° F., the impact of seeding depth on rate and timing of emergence is also reduced. So, if we experience delayed planting, planting deeper may not be as much of a concern if soil temperatures are greater than 50° F. But since delays are more likely from too much moisture rather than too little, deeper planting to reach moisture likely won’t be necessary anyway. Air temperatures around 86° F are optimum for root and shoot growth. Thus, later planting because of warmer temperatures, usually results in faster emergence, development, and taller plants.
b) Variability within fields - Variable seeding depth? We've seen fields where one plant out of five within a row was two to three growth stages behind their bigger, competing neighbors. The slower plants were planted ½ inch shallower than the others…planter speed or wet soil conditions at planting were likely culprits. No doubt, this differential would reduce yields – especially in a good year or when combined with other stress factors.
Normally, we think uniform seeding depth both within a row and across a field as our goal. With new – and perhaps some ‘on the drawing or dream board’ - technology, we should be able to characterize soil variables important to germination and growth. Technology and the planters with on-the go seeding depth adjustment could possibly improve emergence and early-season growth. In theory, within fields having inherent soil variability, optimum seeding depths should vary. However, no Corn Belt research results testing this hypothesis are available.
Should I use planter row cleaners or adjust down pressure?
Another possibility is adjusting planters differently in tilled fields. One adjustment might entail removing dry surface soil and residue over the seed furrow with row cleaners or some other device. For instance, if soil is dry at the surface and at field capacity at 4 inches, the top 1.5 to 2 inches of dry soil could be moved to the side and seed planted around 2 inches below the surface in a shallow furrow. Soil conditions would need to be good and row cleaners carefully adjusted and monitored for this to work well. This could allow for more rapid and uniform emergence than planting at a 4-inch depth without removing drier soil. However, if row cleaners are used to remove soil over the row and subsequent heavy rainfall occurs, soil can erode on sloping rows or rows can ‘puddle’ with silt and create a surface crust. This could reduce emergence and handicap early-season growth.
Another adjustment is to set the press wheels with sufficient down pressure to insure better seed-soil contact. This will increase soil capillary action to the seed furrow.
No-till fields should retain adequate soil moisture at a reasonable planting in most years, even with drought. If not, follow the guidelines above.
The bottom line on corn seeding depth is if plants emerge at about the same time and if optimum plant populations are achieved, yields won’t be affected. In dry conditions, seeding depth can and should differ depending on the soil texture, tillage system and residue cover. Knowing the texture of the soil and its management requirements, will dictate how deep seeds should be placed to have adequate available moisture for successful germination especially in dry conditions.
Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by e-mail at firstname.lastname@example.org or 515-294-6655. Mahdi Al-Kaisi is a professor in agronomy with research and extension responsibilities in soil management and environmental soil science. He can be reached at email@example.com or 515-294-8304. Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery. Hanna can be reached at firstname.lastname@example.org or (515) 294-0468.