ELEMENTS OF AGRICULTURE. 
49 
and exposed to the influence of the air, have attained a fine 
development. But these are altogether exceptional cases; 
and it must always be admitted as a general rule, that a 
soil yields fine crops only when it contains a supply of 
humus in proportion to the wants of the plants. More¬ 
over, that a vegetable should be able to derive its food 
from the atmosphere, it must already have grown out of 
the earth; and it must consequently have received from 
the soil sufficient strength to perform this act. 
67. It is known that a good deal of carbon enters into 
the composition of all plants; but they do not absorb this 
substance in the state in which it is found in their organi¬ 
zation. It can only be absorbed by the roots and leaves in 
the form of gas, and united with oxygen that is in the state 
of carbonic acid. Plants exposed to the influence of the 
sun lose their oxygen, and the carbon combines with the 
other elements to furnish that infinite variety of products 
that vegetation displays. 
68. As to the physical properties of humus, it loosens 
the compact, and give consistency to light soils. By its 
color it imparts coolness to a dry soil, and can in other 
cases give warmth to the land. It renders stiff soils per¬ 
meable to air and other atmospheric influences, and per¬ 
mits the roots of plants to penetrate the arable surface 
with greater ease. 
69. In light land that is hot and burning, humus is de¬ 
composed with great rapidity, so that it requires but a 
slight quantity to produce an immediate effect; but in re¬ 
turn these soils are soon exhausted. Calcareous soils also 
decompose humus with great facility, and for this reason 
such lands should be manured with substances of difficult 
decomposition; for in a stiff soil they would remain inert 
for verv many years. 
6 
