Tillage Radishes
May 10, 2010 White radishes are quickly becoming a popular cover crop in the Midwestern and Mid-Atlantic states. The unique forage or tillage radish is relatively new to both regions and growers planting them are seeing benefits beyond commonly used cover crops, such as rye and clover. Forage radishes can improve the soil and environment while helping growers avoid many problems traditionally associated with cover crops.
Soil Compaction
With taproots that can grow several feet deep, forage radishes bore holes into the ground, loosening the soil, hence giving them the nickname
“tillage radishes.”
“The deep roots penetrate many layers of compacted soil, with the thin lower part of the taproot reaching 6 feet or more during the fall,” says Ray Weil, professor of soil science at the University of Maryland. “The thick, fleshy upper part grows 12 to 20 inches long, including 2 to 6 inches above ground, creating vertical holes and zones of weakness that break up surface soil compaction and improve soil tilth. After the cover crop dies in the winter and roots decompose, open root channels can be used by subsequent crop roots to grow through compacted soil layers.”
The process, called “bio-drilling,” improves root access to water in the subsoil and makes crops more resilient under drought conditions.
Weed Suppression
Forage radishes planted by Sept. 15 produce a dense canopy that nearly eliminates weed emergence in the fall and winter, producing a largely weed-free seed bed in early spring; however, weed suppression does not extend into the summer cropping season.
“The low amount of field residue and a weed-free seedbed in early spring allows for easy summer crop planting without tilling or application of a burn-down herbicide, which may be of particular interest to organic farmers,” says Weil. Forage radishes are also found to suppress certain types of weeds, such as horseweed and marestail.
“Forage radishes winter-kill and the residues break down quickly so that the residue is very easy to incorporate or plant directly through without tillage in the spring,” says Joel Gruver, assistant professor of soil science at Western Illinois University. “They establish very rapidly and are strongly allelopathic, suppressing many weeds, even after they have winter-killed.”
Gruver says many growers who have had problems managing traditional cover crop residues are looking to white radishes, as they are more easily managed, especially after they have winter-killed.
“Farmers who are committed to 100% no-till systems are having good success no-tilling corn and other crops into winter-killed forage radish residues,” he adds.
In addition, because forage radish leaves the soil surface weed free, punctured by large root holes and covered by sparse residue, the seedbed soil warms up and dries out faster in early spring than do soils covered by winter weeds or a growing cover crop, according to Weil.
“The warmer, drier soil and elimination of the need for tillage can allow earlier spring planting, which may be more effective for utilizing nitrogen (N) released by forage radish residue in spring,” Weil says.
Topsoil Fertility
Forage radishes capture, store, and then release nutrients back into the soil, reducing the need for fertilizer applications to spring crops. In addition, forage radish residue decomposes rapidly and releases N early, making it important in sandy soils to plant as early as possible following forage radish cover crops to take advantage of the extra N before it leaches out of the root zone. Forage radishes recycle large amounts of N taken up from the soil in fall and can reduce the need for N fertilizer in spring, according to Weil.
“Because of their exceptionally deep root system, rapid growth, and heavy N feeding, forage radish cover crops can clean up most of the soluble N left in soil after summer crops have ceased uptake, preventing excess N from leaching into groundwater during the off-season,” Weil says. “Forage radish takes up N both from the topsoil and from deep soil layers, storing the N in tissues near the soil surface for use by the next crop.”
Reducing Erosion And Runoff
Ideally, the canopy of forage radish intercepts rain, preventing soil erosion during spring and summer. “Even after it is killed by a hard frost, its decomposing residue protects the soil from eroding,” Weil says. In spring, after surface residues have fully decomposed, runoff and erosion are reduced because of the many holes left behind from the large taproots. Rainwater goes into the holes, eliminating runoff and trapping sediment before it leaves the field.”
In addition, a good forage radish cover crop will add significant quantities of organic material to the soil, Weil says. “Microbially active soil organic matter and aggregation have been observed to increase after using forage radish for several years.”
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