stituents, not
in solution, but in such a state that they may be readily made soluble
by the plants. Many of the minerals from which fertile soils are formed
are probably not attackable by plants when in their natural condition,
and even after disintegration the quantity of the essential elements of
their food, which are present in an easily assimilable state, is at no
one time very large. But this is of comparatively little importance, for
the soil is not an inert unchangeable substance; it is the theatre of an
important series of chemical changes effected by the action of air and
moisture, and producing a continued liberation of its constituents. This
decomposition is effected partly by the carbonic acid of the atmosphere,
but to a much larger extent by its oxygen acting upon the organic
matters of the soil, and causing a constant though slow evolution of
that acid, which in its turn attacks the mineral matters. Boussingault
and Levy have illustrated the extent of this action by examining the
composition of the air contained in the pores of different soils, and
have obtained the following results:--

+--------------------+------------+---------------+---------------------------+
| Nature of | Crop. | No. of cubic |100 VOLUMES OF AIR CONTAIN |
| Soil. | | inches of air +--------+--------+---------+
| | | in 34 cubic |Carbonic| Oxygen.|Nitrogen.|
| | | inches of soil| acid. | | |
+--------------------+------------+---------------+--------+--------+---------+
|Light sandy soil, | | | | | |
| newly manured | ... | 8.0 | 2.17 | ... | ... |
| Do. manured | | | | | |
| 8 days before | ... | ... | 1.54 | 18.80| 79.66 |
| Do. long |Yellow | | | | |
| after manuring | turnip | 7.9 | 0.93 | 19.50| 79.57 |
|Very sandy |Vineyard | 9.6 | 1.06 | 19.72| 79.22 |
|Sandy, with many | | | | | |
| stones |Forest | 4.0 | 0.87 | 19.61| 79.52 |
|Loamy | ... | 2.4 | 0.46 | ... | ... |
|Sandy, subsoil | | | |