1155. Humus, or decayed woody fibre, with the addition of water, was formerly supposed to be all that was required for the nourishment of plants; and it was thought that the soil acted only in retaining the water necessary for dissolving the humus, and the decayed animal matter, on which the plants were to feed. Liebig, however, has proved that most plants contain considerably more carbon than could be supplied to them by the humus in the soil, and that they obtain this additional carbon from the atmosphere. He also asserts, 'that humus in the form in which it exists in the soil does not yield the smallest nourishment to plants.' (Liebig's Chemistry, &c., 4th edit., p. 7.) The reason of this is, that humus is insoluble in water, and, therefore, cannot be taken up by the roots of plants, unless it is first combined with potash, soda, or some other alkali, so as to change it into humic acid.