Sheet composting is the process of composting organic matter directly onto the soil as a mulch and letting it decay there, rather than in a heap. Most commonly, this is achieved by sowing a 'green manure' crop such as mustard, alfalfa, or buckwheat, which is then hoed in, preferably just before flowering. This practice can cause temporary nitrogen depletion, but this can be reduced by employing leguminous green manure crops such as lupin, winter tares, field beans, or clover, which are able to fix their own nitrogen supply in root nodules. The nitrogen is then released as the plants decay.