There are several planter settings that must be checked when heading out to the field this spring. Your planter may have been perfectly adjusted for the last field planted in 2009, but likely is not well-adjusted to plant your first field this spring. Soil moisture in the top couple of inches can vary from saturated to moist or even dry depending on sun and wind. For those fortunate enough to be using a new planter, avoid assuming all adjustments are ‘correct’ because it just came from the dealer. Features such as adjustable closing wheel down pressure are on the planter so that the operator can make final adjustments subject to field conditions.
Planting depths of about 2 in. are appropriate for corn in Iowa. Given this, Iowa State University research shows that cornemerges and produces well from both much deeper and shallower planting depths – provided ideal conditions occur following planting. We can’t count on ideal conditions.
Corn plant crowns and the primary nodal root system form from 1 to 1½ in. below the surface provided soil conditions are good and seeding depth is below that point. Shallow seeding can result in rootless corn and may seriously impact crop standability and yields. Planting too deep can reduce stands and uniformity of stands. In some cases, inadequate planter adjustments coupled with fast planter speeds results in seeding depth variability within a row. Variable seeding depth translates into variable emergence rates, which results in reduced yields.
Remember that the planter row unit must carry enough weight so that the depth-gauge wheels are firmly on the soil surface. Otherwise the double-disc seed opener is holding the row-unit out of the ground and seed will be planted too shallow.
Closing wheel pressure
Virtually all planters feature adjustable down pressure by using a spring on the closing system. Most Deere, Kinze and White planters use a multi-position lever to adjust surface pressure exerted by two closing wheels. Case planters have an adjustment of spring pressure on two closing discs ahead of a single press wheel.
Research shows that increasing down spring pressure can increase soil strength and help bring capillary water to the seed if soil is dry. Higher contact pressure may be useful in dry soil conditions. Light or no down spring pressure should be used in moist or wet soils to avoid over compaction of soil around the seed. Using no spring pressure, sometimes called the float position, supplies the lightest pressure using only the weight of the wheels. In wet soil conditions, consider using a drag chain or tine to close the seed furrow instead of resorting to high spring down pressure that can compact soil. Although rubber-coated aluminum closing wheels are commonly used, some planter operators prefer finger- or spade-type closing wheels in wetter soil conditions.
Depth-gauge wheel pressure
Depth-gauge wheels on either side of the double-disc seed opener need to have enough contact pressure in order to be firmly on the soil surface (to gauge seed depth), but not so much contact pressure that the depth wheels overly compact soil adjacent to the seed zone. In practical terms, this means that when soil moisture is overly moist or wet, use only enough down spring pressure on parallel links attaching the row unit to the toolbar frame to maintain firm contact of the wheels on the soil surface. Some newer planters adjust depth-gauge wheel contact pressure with pneumatic diaphragms. If significant weight has been transferred by down pressure springs for drier soil and you have been rained out of the field, remember to lighten spring tension when going back in to wetter soil.
For further information see Don't Use More Pressure than Needed on Wet Soils.