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Tillage radishes

The taproots of a tillage radish compared to the fibrous roots of an oilseed radish. The decomposed radish taproots provide great water infiltration and soil aeration benefits. Photos courtesy Dr. Robert Coffin.

Like other cover crops, tillage radishes can offer many important benefits, especially to soil health. However, how this crop is used is critical to maximizing the benefits it can potentially provide to potato farmers.  

Tillage radishes have been found to be effective in controlling winter annual weeds and capturing nitrogen. When they decompose quickly in the spring, the roots release a great deal of nutrients (especially nitrogen) into the upper portion of the soil, which then may be available to the next crop (however, the foliage can disintegrate quickly in the late fall after several hard frosts).

Further, the holes left by the decomposed radish taproots provide excellent water infiltration and soil aeration benefits. The decomposition also obviously increases soil organic matter (OM), boosting yield to crops that follow in the long-term with repeated use. The taproots have a potential depth of three feet and size of several inches across at the top. The plants keep growing until temperatures of -10 C or colder are reached for three or four days in a row.

The radishes must be planted in August so that they can grow to a large size and good depth, therefore growing them after main-crop potatoes are harvested is perhaps not the best option. Planting them in late-summer, after another crop in the rotation and before potatoes, may provide growers with the best benefits – most importantly, better potato yields in the long-term due to better soil health.

The need to plant them in mid-August in Canada is confirmed by recent studies done by Dr. Robert Coffin and his colleagues. Coffin, a P.E.I.-based potato industry consultant, worked with Erica MacDonald (Paradigm Precision/A&L Canada Laboratories in P.E.I.), Jennifer Roper (Newfoundland and Labrador Department of Natural Resources) and Brian Beaton (P.E.I. Department of Agriculture). In mid-March, Coffin presented the group’s results on tillage radish growth – as well as how it and two other Brassica cover crops could play a role in soil nitrogen management in potato rotations – at the 2014 North East Potato Technology Forum in Fredericton, N.B.  

The team planted Indian hot mustard, oilseed radish and tillage radish (using branded variety Tillage Radish) at four different times (August 14, August 21, September 13 and October 12) in Green Bay, P.E.I., in 2011. “Planting at the two later dates gave very limited plant and root growth for all three Brassica crops,” Coffin notes. “Large tap roots of six to 10 inches in the tillage radish were only formed from August seeding.”

Benefits
How might planting tillage radishes following another crop in the rotation boost potato yields? This is an important question. Coffin says that some growers in P.E.I. are currently not grossing enough pay yields to cover potato production and storage costs. Cover crops such as radishes may help mitigate some of the factors leading to declining yields, such as soil compaction, reduced organic matter content and crop pests. “Soil health is beginning to be a very important topic amongst famers,” Coffin notes.

Some P.E.I. potato fields have OM levels as low as one to two per cent. “To maximize yields, that should be doubled,” Coffin says. Soils with lower OM levels can limit root growth, and also have reduced water-holding capacity.” Tillage radishes have been shown to build up organic matter over the long term, reduce compaction and also may assist with crop pest populations. These pests include those that affect potatoes and other crops in the rotation, whether wheat, oats, barley, mixed grain, oilseeds or others.

“There are numerous claims that some Brassica crops (mustard, rapeseed and radish) can reduce populations of nematodes, wireworms, Verticillium fungi and some types of weeds,” Coffin explains. “Most Brassica crops contain glucosinolates in plant tissues, and when you chop the foliage and/or disk the plants into the soil, there is often a release of compounds (isothiocyanates) from the glucosinolates that can kill or inactivate some crop pests.”  

Tillage radishes, along with other cover crops, may also help with mitigating nutrient release into soil. Nutrient release – nitrogen in particular – is a big concern on P.E.I. as 100 per cent of drinking water is from wells. “There are increasing concentrations of nitrate in groundwater in areas of intensive agriculture,” Coffin says. “What’s more, the Russet Burbank variety accounts for approximately 60 per cent of our potato acreage and is usually fertilized with a higher rate of nitrogen fertilizer than other varieties, and some farmers have been trying additional fertilizer to boost yields.”

The use of cover crops has been suggested as a method to absorb and hold the residual nitrogen. While the late potato harvest (Russet Burbank in mid-October) does not allow for significant growth of some cover crops planted at that point in time, tillage radishes can still absorb at least some nitrogen. However, Coffin and his team found that radishes planted at any point can release nitrogen after a few hard frosts in the late-fall as the leaves and roots quickly disintegrate. “It’s been suggested that winter rye planted with them might pick up the nitrogen they release and carry it through to the spring, helping to reduce leaching,” he notes. At this stage, though, “more study is needed.”

Like Coffin, Anne Verhallen sees the potential in using tillage radishes with rye. “Radish leaves disappear to nothing as the spring progresses, so oat and rye will protect the soil a bit better in the freeze-thaw and rainstorms of that season,” says the soil management (horticultural crops) specialist at the Ontario Ministry of Agriculture and Food (OMAF) and Ministry of Rural Affairs. Indeed, Verhallen recommends mixing cover crops for a variety of reasons – not the least of which is because different plants in the mixture do better or poorer in different areas of the fields. “Mixtures are a good opportunity, and there are a lot of commercial mixtures available now,” she notes.

In terms of the specific benefits of tillage radish, Verhallen confirms that they are heavy nitrogen scavengers and excellent at weed control, but that growers should never expect spectacular results the year after, in terms of nitrogen available to the crop that follows. She notes that multiple-year studies conducted at the University of Guelph’s Ridgetown Campus, during OMAF field trials in the Woodstock, Ont., area and by Michigan State University have shown that nitrogen release by disintegrating radishes can come too early to benefit the next crop (in these studies, that was corn). “It’s long-term benefits that you are after with tillage radishes,” Verhallen notes.

“They must be used with care,” she adds. “Tillage radishes won’t do everything for you, but do provide improved water infiltration, and improved soil aggregation and soil organism activity around the root in the rhyzosphere and around the tap root.”

Coffin plans to continue research in 2014 with the Tillage Radish brand of radishes to document performance under P.E.I. conditions.

For more
Verhallen recommends checking out the Midwest Cover Crop Selector Tool (which includes Ontario) at www.mccc.msu.edu/selectorINTRO.html

There is another cover crop decision tool at http://covercrops.cals.cornell.edu/decision-tool.php and more good information here: http://plantcovercrops.com/

October 3, 2014  By Treena Hein