Some areas of Michigan received pounding rain events soon after planting, and the weather-related planting delays will probably cause soybean producers to till or plant some of their fields when the soil is too wet. These situations weaken the stability of soil aggregates and increase the potential for a crust to develop at the soil surface.
A soil crust forms when raindrops strike soil aggregates, breaking them into individual sand, silt and clay particles. Once the particles are separated, they move with the water and pack together very tightly. As the soil dries, the closely-packed soil particles form a crust sealing the soil (Photo 1).
A soil crust is more likely to develop on fine-textured soils, soils low in organic matter and tilled fields where surface residue levels are not sufficient to protect the soil aggregates from raindrop impact. Soil crusting reduces soybean emergence, oxygen diffusion and water infiltration, and increases soil erosion.
Producers should identify which fields are most likely to form a crust and develop proactive strategies for improving soybean emergence in these fields. One possible option is to plant your smallest soybean seed in fields prone to crusting. The smaller seed produces smaller cotyledons (seed leaves) that are more easily lifted out of the soil than the cotyledons of large-seeded soybeans.
Another option is to plant these fields in 28- or 30-inch rows. The seeds are usually spaced closely enough within the row at these row widths to crack the crust and help each other emerge (Photo 2). Also, consider planting slightly shallower and lifting row cleaners to maintain residue cover over the row to improve emergence.
If heavy rainfall occurs within a few days after planting, look for signs of soil crusting after the soil dries and be prepared to use a rotary hoe to break the crust if rain is not predicted soon. If the crust is not broken up on a timely basis, the soybean seedlings will exhaust all of their energy reserves trapped beneath the crust, undergo mechanical damage such as broken hypocotyls and lost cotyledons while struggling to emerge or be invaded by plant pathogens.
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A rotary hoe operated timely and correctly will break up an existing soil crust and improve soybean emergence without causing significant stand loss or excessive damage to the seedlings. Stop periodically and check the seedlings for mechanical damage. Crop damage can be reduced by operating the rotary hoe in the same direction as the rows and during the heat of the day when plants are less brittle.
It may be necessary to reduce the operating speed if crop damage is unacceptable, and always avoid rotary hoeing during the vulnerable “crook stage.” Stand loss and crop injury can also be minimized by operating the rotary hoe just deep enough to break up the crust.
Some sugar beet producers have successfully used their planters to crack soil crusts and improve seedling emergence. This practice probably can only work using an RTK–based guidance system. When performing this operation, retrace each of the original planter passes as if planting the field for the first time, keeping the openers centered on the row.
The fertilizer opener may need to be removed. Set the seed openers as shallow as possible and experiment with the down pressure on the closing wheels when running two standard rubber wheels. All spoked closing wheels, even when there is only one per row, will be more aggressive, so operate these at the lightest down pressure setting possible.
Drive slowly, experiment with different settings and stop frequently to check your progress towards breaking the crust without damaging the seedlings. Soybean seedlings must be at least 0.5 inch below the soil surface to prevent excessive crop damage due to severed hypocotyls. The obvious downside of this practice is that the planter is tied up and not available for its intended use.
Some of the short-term effects of soil crusting can be reduced with proactive and remedial management practices this spring. The long-term solution is to increase soil organic matter levels and leave more crop residue on the soil surface.