cover crop

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A crop, such as winter rye or clover, planted between periods of regular crop production to prevent soil erosion and provide humus or nitrogen.

from the GNU version of the Collaborative International Dictionary of English.

• A catch crop planted, esp. in orchards. as a protection to the soil in winter, as well as for the benefit of the soil when plowed under in spring.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several crops planted between others in order to prevent erosion and to improve soil quality (especially by adding nitrogen)

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun crop planted to prevent soil erosion and provide green manure