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 | | | | | | | of the last | ... | 3.0 | 0.24 | ... | ... | |Sandy soil, long | | | | | | | after manuring |Trefoil | 7.6 | 0.74 | 19.02| 80.24 | | Do. Recently | | | | | | | manured | ... | ... | 0.85 | 19.41| 79.74 | | Do. manured 8 | | | | | | | days before | ... | ... | 1.54 | 18.80| 79.66 | |Heavy clay |Jerusalem | 7.0 | 0.66 | 19.99| 79.35 |