shown to have a
very striking influence on the temperature of a soil. This naturally is
best seen in climates where there is a good deal of sun. Dark-coloured
soils have a greater heat-absorbing capacity than light-coloured soils;
and experiments carried out for the purpose of determining the extent of
this influence have shown that under certain conditions the difference
between a soil covered with a black substance, and one covered with a
white substance, amounted to from 13 deg. to 14 deg. Fahr. Other things being
equal, a crop on a dark-coloured soil will be sooner ripened than one on
a light-coloured soil. A soil covered by a crop is cooler than one
without any crop.

_The Power Soils have for absorbing Gases._

We have just seen that one cause of the heat of soils is the oxidation
which is constantly going on in all soils, but more rapidly in soils
containing a large quantity of vegetable matter. This suggests a word or
two on the power soils have of absorbing gases.

The chief gases in the atmosphere are oxygen and nitrogen. Both these
gases are absorbed by soils, although not in similar proportions.[46]
With regard to the former, it is well known that a plentiful supply of
oxygen in the pores of the soil is a necessary condition of fertility.
This was long ago experimentally proved by de Saussure, who showed that
plants absorbed oxygen through their roots. At certain periods of their
growth this demand for oxygen on the part of the plant is greater than
at other times. For example, seeds in the process of germination require
to have free access to a plentiful supply of oxygen. This fact
emphasises the enormous importance of providing a good seed-bed, and of
seeing that the seed is not buried too deeply.

_Carbonic Acid and Ammonia._

In addition to oxygen and nitrogen, the air contains other gases which
are absorbed by the soil. Of these, carbonic acid is the most abundant.
By far the largest portion of the carbonic acid which the soil obtains
from the air, is washed down in solution in the rain.[47] Of the other
constituents of the atmosphere, the combined forms of nitrogen--viz.,
_ammonia_, _nitric_, and _nitrous acids_--are the most important. These
are all absorbed by the soil, but, like carbonic acid, they are chiefly
washed down by the rain. The amount of ammonia which may be absorbed by
a soil from the air, is very much greater than was formerly supposed.
Some recent experiments by Schloesing, re