ORGANIC MATTERS OF THE SOIL. 235 
(we use this word as a general collective term) to rain- 
water, and the richer the soil, as made so by manures and 
judged of by its productiveness, the larger the quantity, 
up to certain limits, of humus it contains. If, as we have 
seen, plants always contain silica, though this element be 
not essential to their development (H. C. G., p. 186), is it 
probable that they are able to reject humus so constantly 
presented to them under such a variety of forms? 
Liebig opposes the view that humus contributes directly 
to the nourishment of plants because it and its compounds 
are insoluble; in the same book, however, (Die Chemie 
in threr Anwendung auf Agricultur und Physiologie, 
7th Ed., 1862) he teaches the doctrine that all the food 
of the agricultural plant exists in the soil in an insoluble 
form. This old objection, still maintained, tallies poorly 
with his new doctrine. The old objection, furthermore, is 
baseless, for the humates are as soluble as phosphates, 
which are gathered by every plant and from all soils. 
It has been the habit of Liebig and his adherents to 
teach that the plant is nourished exclusively by the last 
products of the destruction of organic matter, viz., by car- 
bonic acid, ammonia, nitric acid, and water, together with 
the ingredients of ashes. While no one denies or doubts 
that these substances chiefly nourish agricultural plants, 
no one can deny that other bodies may and do take part 
in the process. It is well established that various organic 
substances of animal origin, viz., urea, uric acid, and gly- 
cocoll, are absorbed by, and nourish, agricultural plants ; 
while it is universally known that the principal food of 
multitudes of the lower orders of plants, the fungi, includ- 
ing yeast, mould, rust, brand, mushrooms, are fed entirely, 
so far as regards their carbon, on organic matters. Thus, 
yeast lives upon sugar, the vinegar plant on acetic acid, 
the Peronospora infestans on the juices of the potato, 
etc. There are many parasitic plants of a higher order 
common in our forests whose roots are fastened upon and 
