Humus is the organic component of soil, decomposed plant and animal material. Dark brown to black, humus is intimately mixed with the inorganic soil particles. Mainly for its good carbon, nitrogen, phosphorus, and sulfur content, it is of great agricultural importance because in soil it retains moisture, aerates, and increases mineral content, and bacterial activity. Humus can be made in a compost heap; types of natural humus include peat moss and leaf mold.