51 
FOOD OF PLANTS, AND HOW THEY TAKE IT. 
other elements are found in water, which is composed of 
oxygen and hydrogen. These four elements, therefore, are of 
the greatest importance to plants, but of them the former, 
carbon and nitrogen, must claim our first and chief attention 
in this inquiry. It has been generally supposed that these 
substances were provided for plants chiefly by manures, 
and by the humus, or decaying vegetable matter of the 
soil. 
The last result of the decomposition of animal and vegetable 
matters, after the escape of the gaseous compounds, is a dark 
substance called humus. It abounds in the prairie soil, in 
most rich soils, in manured soils; further, humus is rich in 
carbonic acid and ammonia ; hence, when we speak of a soil 
being rich in humus, we mean that it is well furnished with 
these elements of plant-food—in other words, a rich 
soil. 
That ammonia and carbonic acid are the sources of nitrogen 
and carbon, for the supply of plants, there can be no doubt; 
and that these substances are produced in immense quantities 
on the globe, and diffused through the atmosphere, we are 
equally assured ; but we find the results of respiration, and of 
combustion, the escape of carbonic acid and ammonia from 
the craters of the volcanoes, and all our fires, and smoke 
stacks, do not increase the quantity in the atmosphere ap¬ 
preciably, for there is but a small, uncertain quantity of 
ammonia, and a definite, small proportion of carbonic acid. 
These substances are drained out of the atmosphere, and have 
not increased in it. Geologists tell us that the immense 
quantities of the coal on the earth, must have been derived 
from a rank vegetation existing at a former period, when it is 
probable that our atmosphere was very highly charged with 
carbonic acid gas. 
How do the grasses and weeds of these great prairies and 
the trees of our wide and noble forests obtain their carbon, if 
not from the atmosphere ? and yet we make no return of carbon 
for all that we take from them. Successive crops of charcoal 
have been burnt and removed from the mountain regions, 
where the trees can find little humus, and can return only to 
the soil their foliage to decay and make more; the cattle 
pasturing on the prairies must remove immense quantities of 
carbon in their tissues—the supply to the trees, grasses and 
weeds has been kept up from the atmosphere undoubt¬ 
edly. 
[To be continued.) 
