By Dean Baas and Elizabeth Schultheis
If you’re exploring your options for different cover crops, consider the Brassicas. The Brassicas are a plant family that includes radish, turnips, rapeseed and mustards. The two main benefits from this group of cover crops is weed suppression and breaking up soil compaction.
Brassicas are leafy and grow very rapidly. Because of this rapid growth, you get a lot of shading of the soil very quickly, which helps prevent the growth of weed species. Unique to the Brassica family is the production of glucosinolates, or chemical compounds that are natural inhibitors of weed species. Because of these chemicals and their fast growth, you will see very few weeds growing in plots containing Brassica cover crops.
Depending on which Brassica you select, they have different root systems that can break up different forms of soil compaction. Some of the Brassicas, like turnips, grow a large bulb just below ground, which is good for reducing compaction in the topsoil layers. Others, like radishes, make a longer tuber, which does more for deep soil compaction. Finally, others like rapeseed and mustards make fibrous roots that are also great at breaking up deeper compaction.
Most Brassica species, with the exception of some rapeseeds, will winterkill and decay rapidly due to their high water content. This means you might need to worry that the nutrients from their tissues will be lost from the system. Therefore when using Brassicas as a cover crop, plant them in a mix with other species like grasses.Source : msu.edu