SOIL Soil is generally used in compost heaps . Fertile soil contributes a large population of organisms which help decompose the compost materials , as well as minerals which the organisms need to live. Soil also helps prevent the loss of nitrogen to the air in the form of ammonia gas. Use very thin layers of soil —perhaps 1/8 inch—in the compost heap . It is important not to use too much soil , because it may add too much weight to the heap and compress t