1156. As a proof that humus is not sufficient alone for the nourishment of plants, it is well known that peat or bog soil, which consists almost entirely of humus, is one of the most barren of all soils, and that the few plants which it produces are members of the rush and sedge families, and consequently are plants of scarcely any use to man. On the contrary, the quantity of humus in fertile soils, which produce the cereal grasses and other plants used for food, has never been known to exceed ten per cent., and occasionally it has been found as low as one per cent., or even less. Thus humus or vegetable mould only becomes a fertile soil when mixed with sand, lime, or some other earthy substance, which not only supplies the plants grown in it with the necessary salts, but also keeps the particles of mould open, so as to expose them to the atmospheric air, from which they extract the carbonic acid gas. This is the reason that peat soil becomes fertile by mixing it with sand, or in fact with any other kind of earth. Heath mould, so often recommended in the cultivation of the finer kinds of Australian plants, is, in fact, nothing else but peat earth rendered fertile by the admixture of sand.