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Soil crust forms when fields begin to dry after heavy rainfall or flooding. When soil aggregates break down at the surface, the impact of raindrops can create a layer of soil that is without structure and turns into a cement-like crust when dry. Crusting tends to be a problem in fields that have weak soil structure. This includes fine textured soils and soils with low organic matter content. Conventionally tilled fields and those lacking adequate residue cover are also at a greater risk for crusting. Residue can help prevent soil crusting by helping to absorb the impact of rain which prevents the rain from damaging soil structure at the surface. Once soil structure is broken down, corn and soybean emergence can be difficult.
When dense surface crust forms emergence can be inhibited. Soybean emergence is prevented when the hypocotyl is broken as it pushes against the hard crust. If soybeans are able to penetrate the crust or emerge through a crack in the crust, cotyledons may be lost as the seedling tries to pull them through the hard soil surface (Figure 1). These seedlings may survive; however, development may be slowed due to the loss of energy reserves found in the cotyledons.
While corn is generally better able to tolerate soil crusting than soybean, crusting can lead to uneven corn emergence. When a corn seedling is trapped beneath soil crust, the coleoptile may be unable to penetrate the hard layer of soil. This results in the formation of a corkscrewed mesocotyl during elongation and eventually to leafing out underground if emergence is delayed long enough (Figure 1).
A rotary hoe can be used to break up soil crust and allow plants to emerge. When deciding whether or not to use a rotary hoe, consider that emerged plants could be damaged. One method to evaluate emerged plants versus those under the crusted soil is to set flags in an area of the field. One color should be used to designate plants that have emerged; a second color should be used to designate plants that are still underground.1 Run the hoe through the area and evaluate the plants. With soybeans, make sure that hypocotyls are not broken and that cotyledons are intact. Soybeans are particularly sensitive to damage as the crook emerges.2 Expect approximately a 1 to 2% stand loss from using a rotary hoe in corn. If corn is truly having difficulty emerging, this loss should be minor in comparison.3
When using a rotary hoe, run the tractor at high field speeds of 8 to 10 miles per hour. Work the soil just deep enough to break the crust. Get off the tractor periodically to evaluate crop damage and check for stand loss. If stand loss is greater than 3 to 5%, consider slowing travel speed. To reduce crop damage and stand loss, operate rotary hoes when soil surface moisture is slightly above field capacity. This can also decrease the risk of soil compaction.3 At field capacity, a handful of soil will crumple easily in your hand under minimum pressure and leave a small amount of moisture. Avoid running a rotary hoe in the morning when plants can be brittle. Wait until later in the day when plants are more pliable.2
A rotary hoe is an effective tool for breaking up soil crust and rescuing plants. Timing is critical to avoid excessive seedling damage and to achieve the intended result. Monitor fields that are susceptible to crusting and keep track of soil moisture levels to determine if and when use of a rotary hoe is warranted.