Sheet composting is the process of composting organic matter directly onto the soil 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 lupine, 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.
Proponents of this system argue that sheet composting causes fewer nutrients to be lost through leaching than heap methods, also that fresh organic matter provides a slower release of minerals when applied than when decayed. It is also said that, in the long term, sheet composting leads to higher nitrogen levels in the soil, as much is lost by vaporisation when a traditional heap heats up.