Radish — Raphanus sativus L. — which is in the Brassicacea family, often referred to as brassicas, is a popular cover crop that can be planted in summer months. Radish sold as a cover crop are usually daikon-type oilseed or forage radishes. Daikon cultivars are typically larger-rooted than standard oilseed cultivars. Radish is often sold under various trademarked names or may be sold as forage radish when promoted for grazing.
In the article, cover crop radish is used as an all-encompassing term to represent all radish used in that capacity. Radish cover crops are promoted to have multiple benefits. It is considered to be an excellent nitrogen scavenger and can provide great coverage for erosion control; although the overall benefits to crop production have not been widely tested. The objective of the publication is to provide an overview of management recommendations for radish cover crops and to recap research that demonstrates potential benefits.
Radish grows the best when planted following harvest of short-season crops such as winter wheat, snap beans, peas or other vegetables. Planting should occur by Aug. 20 in southern Wisconsin and Aug. 15 in northern Wisconsin to ensure good establishment. Planting after Sept. 1 usually does not result in sufficient plant biomass. Radish can tolerate temperatures as low as 20 degrees Fahrenheit for several days, but it will ultimately die off during winter in Wisconsin. Most of the radish sold as cover crops have been selected for large taproot growth, which can grow two or more inches in diameter and more than one foot in length with adequate growing degree-days and soil fertility.
Radish should be drilled to a depth of one-quarter to one-half inch, or broadcast and incorporated with light tillage. However it is recommended to be planted in a no-till system. Seeding rate for drill seeding is 10 to 12 pounds per acre and should be increased by 10 percent when broadcast seeding. Limited work with seeding density of radish has occurred; although research conducted in Michigan found that seeding rates of 10, 15 and 20 pounds per acre all produced similar amounts of biomass, but lower seeding rates resulted in larger individual plants producing larger tap roots.
The radish seed is relatively small with 34,000 seeds in one pound of seed. Radish is often established with the addition of supplemental nitrogen. While growth of radish can be enhanced with additional nitrogen or manure applied at a rate of 30 to 50 pounds per acre that management practice is not recommended, unless nitrogen is known to be severely deficient.
Understand growth, development
Previous research in the Midwest has shown radish growth and nitrogen uptake can be quite large. In on-farm research trials in Wisconsin, radish growth and nitrogen uptake ranged from large amounts to small amounts based on the year. The low biomass affects can be attributed to poor soil moisture and low soil fertility. Radish is sensitive to dry conditions. About 50 percent of the total biomass of the radish is the large tap root and about 40 percent of the total nitrogen in the plant is stored in the tap root. The carbon-to-nitrogen ratio of the total plant are at levels that would indicate net mineralization of nitrogen would occur — there would be release of nitrogen into the soil during decomposition. Although the carbon-to-nitrogen ratio of the aboveground biomass is less than that of the tap root.
Radish as a cover crop has been well-studied with respect to how it affects the next crop in rotation, with positive, negative and neutral effects being reported. In the study, corn yields were measured following either radish as a cover crop or no cover crop, where the previous crop was winter wheat. Corn was no-till planted across both radish and no radish plots at Rock and Washington counties, but the no-radish plots were chisel plowed prior to corn planting at Sheboygan County. Six to eight nitrogen rates were applied after corn was planted. There were two instances where corn yields were decreased following radish and two instances where corn yields were increased, with the remaining site-years having little to no effect. However, both instances of yield reduction occurred at Sheboygan, which could be attributed to the fact that corn following radish was no-tilled and corn following no cover crop was chisel plowed. That would suggest that radish does not effectively simulate tillage on heavier textured soils.
A similar experiment in Michigan also determined neutral to negative effects of radish on corn yield. There does not appear to be much, if any, of a corn yield benefit to using radish as a solo cover crop in the Midwest.
Across all nine site-years of research in Wisconsin, the majority of the studies showed no effect at all on the optimal nitrogen requirement. Because a substantial amount of nitrogen exists in the radish plant immediately before winter kill — 40 to 120 pounds per acre — that would indicate that the nitrogen is not being provided to the corn crop. In those cases, it would indicate that the nitrogen that was taken up was either immobilized or more likely, leached out of the root zone prior to peak nitrogen uptake period of the corn.
Based on the carbon-to-nitrogen ratio of the radish, net immobilization is not likely. What is more likely, is that because radish is killed with freezing temperatures in the later fall, and has a relatively low carbon-to-nitrogen ratio, it will start to decompose quickly in the spring after snowmelt. There is a fair amount of nitrogen stored in the roots, and upon decomposition, the nitrogen may get transported quickly down the “hole” left by the large radish tap root. That would lead to nitrogen being moved quickly out of the root zone early in the growing season. That theory is also supported by the lack of detection of an increase in soil nitrate following radish as a cover crop. In the Wisconsin study, only one of the nine site-years showed an increase in soil nitrate in the upper 12 inches of soil at the time of planting or the time of sidedress application. However, in nearly all cases, growing a radish cover crop resulted in less plant available nitrogen in the late fall, which would suggest it functions well as a trap crop for nitrate.
Unfortunately, if the nitrogen isn’t conserved in the soil system, it may still be leached out just at a different time. For that reason, it is not suggested to grow radish as a stand-alone cover crop but instead as a mixture with a grass cover crop. That may help retain some of the nitrogen in the radish biomass in the spring with a grass that survives the winter; although some legumes that survive the winter may also be helpful. In addition, use of radish is also not recommended on sandy soils because it has been noted that there is even less ability to retain the nitrogen early in the spring.
In studies outside the Midwest where corn yields increased following cover crops, the nature of the benefit has been attributed to changes in the physical condition of the soil and the potential for nitrogen supply.
However, there appears to be little, if any, evidence in the study that the nitrogen stored in radish biomass gets contributed to the next corn crop. In the Wisconsin study, when corn yield increases were observed following radish cover crop, more nitrogen fertilizer was required to achieve those higher yields. That would suggest that the benefit of radish came from something other than supplying nitrogen.
A perceived benefit from the tap root of radish is compaction alleviation. Certainly, radish can be more deeply rooted than other cover crops, with roots being easily grown down to three feet in typical growing seasons and have been detected down below five feet in extreme cases. Research from Maryland did demonstrate that radish’s tap root can penetrate through compacted soil layers. The radish rooting structure may provide more benefits in the vertical direction rather than the horizonal direction, creating channels in the soil but not influencing the bulk soil as a fibrous root system would.
Cover crops with a dense and fibrous rooting structure, such as annual ryegrass and winter rye, will help hold soil better than deep tap-rooted covers. That, in-turn, may help reduce erosion losses. Radish as a stand-alone cover crop on highly erodible land is not recommended because there have been observations of enhanced erosion risk down the radish rows in the spring.
Weed suppression by a radish cover crop has been observed in field experiments as well as in farm fields. Research from Maryland has concluded that, that effect is from rapid germination by the radish, which out competes and smothers the weeds, and not allelopathy — chemical inhibition.
The canopy of a drill-seeded radish field can clearly smother out weeds when they are small. However, in the Wisconsin experiments, conducted under no-till conditions, there was plenty of regrowth of winter wheat, albeit less than the no cover crop areas.
Thus, radish alone does not necessarily alleviate the need for spraying fields for weed control. Soil health has not been widely tested with radish directly, but long-term use of cover crops can lead to increases in soil organic matter and soil health. Radish is a non-arbuscular mychorizal fungi associating plant. Radish will not help enhance colonization of the fungi in the subsequent crop, nor will they suppress the fungi growth. Radish produces glycosinolate compounds, which breakdown into chemicals found in commercial fumigants. While there is potential for radish to function as a biofumigant, field trials are lacking to evaluate its overall effectiveness. In addition, some radish cultivars can be a host for root knot nematode, while others may help control those nematodes.
Perhaps the biggest drawback to radish as a cover crop is the rotten egg odor that can occur during decomposition.
The results of the research show that a radish cover crop is a good scavenger of fall soil nitrogen. From an environmental perspective, the uptake of nitrogen in the fall by radish reduces the potential for nitrate nitrogen to make its way to the groundwater. However, the release of nitrogen from decomposing radish biomass was not available to a subsequent corn crop as indicated by the corn yield.
The study found that radish did not supply nitrogen to the next season’s crop as determined through nitrogen response curves. Thus, the ultimate fate of the nitrogen that was taken up by the radish remains unknown. For that reason, it is recommended to use radish in a dual crop or mixture. It is not recommended to fertilize the radish cover crop because the nitrogen in the biomass likely does not get retained in the soil system and thus the application will not be economical. Instead, consider planting radish with grass cover crops that survive the winter to help trap the nitrogen released from the radish biomass.
Adding radish to cover crop mixtures helps diversify cover crop systems beyond grass and legumes as well as diversifying the overall cropping system. If planted in a mixture and during the summer, it is recommended to plant radish at a low rate.
Visit learningstore.extension.wisc.edu/collections/farming/products/radish-as-a-cover-crop-in-wisconsin for more information.
