142

USE OP LIME.

442. Carbonate of lime is a very common ingredient !in plants, and is also found in almost all fertile soils; jthe addition of lime or calcareous matter, therefore, tosoils which are destitute of it, or contain but very little, jinvariably improves them. Besides bones, and theorganic substances already mentioned which containlime, a good many might be mentioned which are used

as manure. Lime is added, in the forms of quick lime,chalk, marl, shell-sand, &c.

443. When quick lime is spread over the surface ofland, other eifects are produced besides the mere additionof carbonate of lime. The quick lime soon absorbsmoisture, and crumbles to powder, gradually absorbingcarbonic acid from the air, and returning to the state ofcarbonate. The chalk which is thus formed is in a stateof very fine powder, and becomes more thoroughly mixedwith the soil than it could be by ordinary means; but ;at the same time the quick lime has greatly assisted inpromoting the decay of organic matters in the soil, andcaused the formation of a considerable quantity of nitrate

of lime, a salt of some value as a manure.

444. There is great difference in the goodness of diffe-rent kinds of lime. All limestones which consist merelyof carbonate of lime yield pure caustic lime when burnt(163), but many limestones contain a portion of carbonateof magnesia, and these when burnt yield a mixture ofcaustic lime and magnesia. Such lime is in general \objectionable as manure. It is probable that the ten- ]dency which magnesia has to remain caustic for a long !