653 
Extracts from British and Foreign Journals. 
THE FOOD OF PLANTS. 
{Continued from page 5 90.) 
In the present generally received opinion of the action of 
manures, it is supposed they exert an influence in several 
ways—in imparting the matters they contain that are favour¬ 
able to the growth of plants, and also by adding, by future 
decomposition, to the quantity of vegetable matter already in 
the soil, and thereby increasing the fertility by operating 
upon other matters in the soil, breaking the texture, and 
setting at liberty other ingredients, and forming new com¬ 
binations favourable to the growth of vegetable life, and by 
producing changes and alterations in the constitution of the 
soil, and bringing it into a more fit condition for yielding 
food to plants, and also by acting partly in all the different 
ways now mentioned. Thus, it may be said that some 
manures afford nourishment only ; others yield nourishment, 
and, by leaving an earthy residue, add to the bulk of the soil; 
and others, again, do not nourish in their own nature, but 
exert an agency on other substances, and convert them into 
food for plants; and there is probably no manure that does 
not operate in more ways than one. These supposed modes 
of operation are resolved into the mechanical and chemical 
agency of manures : the first, by reducing the texture, dividing 
the earthy particles, and rendering the soil more open and 
porous, and in other cases more firm and compact, and adding 
to it by by decomposition ; and the second operates by the 
chemical attraction and affinities which different ingredients 
brought into contact exert on each other and on the soil 
forming new combinations, and producing aeriform or elastic 
fluids by means of the heat generated in the soil during the 
operation of the chemical combinations—thus joining with 
the mechanical agency in producing a state of action highly 
conducive to the growth of plants. 
The decrease in the quantity of produce on land by the 
ceasing of the action of manures may be accounted for by the 
chemical affinities having exhausted their activity, and every 
particle of the earth being at rest, and no more heat being 
generated to produce the elastic fluids. And as experience 
has ever shown that the benefits derived from the use of 
86 
XXXII. 
