THE SOIL.

97

296. The fact that some solid substances can be thuscarried along in the air, in a state of very fine powder, isimportant, and well worthy of being remembered, as itexplains many apparently mysterious phenomena. It isexceedingly difficult to detect the very minute quantityof solid matter contained in the air, but there is nodoubt that it often does exist, although we are notaware of its presence.

297. Setting aside the small quantity of earthymatters which plants may derive from the air, it isevident that the great source of the lime, potash, andother similar substances contained in plants, must bethe soil: hence the chemical composition of the soilexerts great influence on the plants which are cultivatedin it.

298. The soil or earth is essential to the growth ofplants in several distinct ways. It enables them to fixthemselves firmly, as from its open, porous nature itallows the roots to extend, in various directions, andobtain a secure hold, so that the plant can grow erectinto the air without danger of being blown away by thewinds. The soil likewise supplies them with substancesessential to their growth: such as carbonic acid andammonia, either generated by the decay of substanceswhich the soil naturally contains, or absorbed from theair; and also yields them the earthy and alkaline saltswhich they require.

299. The soil is, as may be supposed, very variablein composition; its nature is generally a good deal

H