INORGANIC MANURES.

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438. The mode in which inorganic matters act is veryimperfectly understood, and must of necessity remain sountil the office performed by saline and earthy substancesin the nutrition of plants is explained. It is, however,certain that the addition of an earthy substance to thesoil is of no use, if the soil already contains abundanceof that substance; and consequently that the applicabilityof any such manure is wholly dependent on the natureand composition of the soil.

439. When reading accounts of experiments withvarious inorganic manures, it must always be borne inmind that on one soil salts of potash, on another nitrateof soda, and on a third phosphate of lime, may be thebest manure, because the soils may happen to be deficientin those substances.

440. Many lands are found to be greatly improvedby the addition of a quantity of sand, clay, marl or cal-careous clay, and other substances of a similar nature ;part of the effect produced is of a mechanical nature,and part chemical. The substance added containssomething which the soil is deficient in, and which plantsrequire.

441. Plants almost invariably contain salts of thealkalies, and lime, or magnesia; sometimes combinedwith organic acids, sometimes with sulphuric, muriatic,or phosphoric acid. Phosphates of lime and magnesia,in particular, are very commonly met with in plants.It will be proper to briefly enumerate the artificial sourcesof these substances.