Silica (or sand) always exists in the soil in sufficient quantities for the supply of food for plants; but, as has been often stated in the preceding pages, not always in the proper condition. This subject has been so often explained to the student of this book, that it is only necessary to repeat here, that when the weakness of the straw or stalk of plants grown on any soil indicates an inability in that soil to supply the silicates required for strength, not more sand should be added, but alkalies, to combine with the sand already contained in it, and make soluble silicates which are available to roots.
Sand is often necessary to stiff clays, as a mechanical manure, to loosen their texture and render them easier of cultivation, and more favorable to the distribution of roots, and to the circulation of air and water.
CHLORINE.
How may chlorine be applied?
Chlorine, a necessary constituent of plants, and often deficient in the soil (as indicated by analysis), may be applied in the form of salt (chloride of sodium), or chloride of lime. The former may be dissolved in the water used to slake lime, and the latter may, with much advantage, be sprinkled around stables and other places where fertilizing gases are escaping, and, after being saturated with ammonia, applied to the soil, thus serving a double purpose.
OXIDE OF IRON.
How may the protoxide of iron be changed to peroxide?
Nearly all soils contain sufficient quantities of oxide of iron, or iron rust, so that this substance can hardly be required as a manure.
Some soils, however, contain the protoxide of iron in such quantities as to be injurious to plants,—see page [86]. When this is the case, it is necessary to plow the soil thoroughly, and use such other mechanical means as shall render it open to the admission of air. The protoxide of iron will then take up more oxygen, and become the peroxide—which is not only inoffensive, but is absolutely necessary to fertility.