53 
" precise proportion of these is, which is most favourable 
" to fertility, has not yet been determined." Strictly 
speaking, the soil itself merely affords a mechanical 
support for the plants which grow in it, and contains their 
food, but is not itself, except to a small extent (y'o of 
the plant) their food, and therefore, ceteris paribus, 
those soils which hold the greatest quantity of this 
nutrition are the most fertile. 
But in what state does this nutrition exist in the soil 1 
Now the food of plants consists of decomposed animal 
and vegetable matters reduced by the process of 
putrefaction so far as to be soluble in water, but 
not to the ultimate elements of oxygen, hydrogen, 
carbon, and nitrogen, and not always to binary com- 
binations; for the compounds of vegetable matter 
are ternary or quaternary, and the tendency by 
decomposition of these elements is to form binary com- 
pounds among themselves, and with other simple 
substances in the atmosphere around them. Sir 
H. Davy endeavoured to ascertain whether soluble 
vegetable substances passed in an unchanged state into 
the roots of plants by comparing the products of plants 
that had grown, some in common water, and some in a 
solution of sugar, and the results seemed to prove that 
soluble matters do pass unaltered into the roots of plants ; 
but he also found " that solutions of sugar, mucilage, 
tannin principle, and jelly, require to be diluted with 
200 times their weight of water. And that although 
mucilaginous, gelatinous, saccharine, oily, and extractive 
fluids, and solution of carbonic acid in water, are 
substances that in their unchanged states contain almost 
all the principles necessary for the life of plants, yet 
there are few cases in which they can be applied as 
manures in their pure forms, for vegetable manures in 
general contain a great excess of fibrous and insoluble 
matt^' which must undergo fermentation before they 
