134

FIXING AMMONIA.

or its carbonate, it is decomposed, sulphate of am rmonia is formed (154), and carbonate of lime or oxideof iron is left, as an insoluble powder. For severalreasons acid is preferable to fix ammonia; it is moreeasily used, and requires far less care and attention inmixing with the solution containing ammonia.

415. Animal substances must have undergone decom-position before they can serve as the food of plants. Ithas already been stated that plants appear only ableto absorb nitrogen in a state of combination (291) ; noform of organic matter is suitable to the nourishment ofa healthy plant. It is the office of the leaves to convertcarbonic acid, ammonia, and water, into organic matter,and hence organic matter must be resolved by decom-position into these substances, before it can be absorbedby plants.

416. There are some plants, however, which seem tobe exceptions to this rule, and which appear able to feedOn organised matter. Certain fungi and parasiticalplants, or those which grow upon others, probably feedby directly absorbing organic matter. The small fungiwhich constitute the various forms of mouldiness are ofthis description; they flourish in the dark, and grow onany kind of dead organic matter. All plants whichhave leaves and require the influence of light, feed ongaseous matter, and never on organic compounds.

417. Amongst the principal animal substances em-ployed as manures are urine, and dung of all kinds, theflesh and blood of dead animals, fat and oily matters,