Clover is a great cover crop. It's a legume so it adds
nitrogen to the soil, has a massive root system which helps hold soil in place (from water or wind), and adds organic matter. It comes in annual and perennial types. The perennial ones can be invasive. The annual ones will often winter-kill if they haven't gone to seed before frost.

I don't know too much about turnips, but I do know they are used as a cover crop, and apparently produce some kind of control of the weed purple nutsedge.

But practically ANYTHING can be used as a cover crop, as long as you mow it or plow it up before it sets seed.

The paper catalog from Peaceful Valley Farm Supply is a good source of info on cover crops and other organic info.

Sue

Here is a link that might be useful: Peaceful Valley Farm Supply