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CHAPTER VI.

ARTIFICIAL SOURCES OP THE FOOD OF PLANTS.—ACTION OF MANURES.

372. "When we remember that, although plants derivea large portion of their nourishment from the air, yetat the same time they absorb considerable quantities ofsaline matters from the soil, it is evident that allplants must more or less impoverish the soil, by takingaway that which causes its fertility.

373. The natural vegetation of any country enrichesrather than deteriorates the soil, because nothing iscarried away from its surface ; the plants which growon it return to the soil, during their decay, all the earthyand saline substances which they had absorbed from itduring their growth, whilst they add to it a considerablequantity of the carbon they had collected from the air.

374. Very different, however, is the condition of cul-tivated lands ; on them large crops are raised year afteryear, which are removed and carried away to a distance,to form the food of men and animals. Such land israpidly impoverished, because with the crops a largequantity of inorganic matter, necessary to the fertilityof the land, is removed. It remains for us to inquire,what are the best means of restoring these matters to