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321. In soils containing a large proportion of clay oralumine, the decay of vegetable matter or humus pro-ceeds very slowly, because the close, dense nature of thesoil greatly retards the free access of air; though, inother respects, such soils are decidedly favourable todecay, inasmuch as they remain long moist, and a cer-tain degree of moisture is essential to decay.
322. It is necessary to acquire a clear conception ofthe nature of what is called humus, and the office whichit performs in the nutrition of plants. It was formerlysupposed that plants derived their carbon from the or-ganic matter of the soil,—that the brown decaying sub-stances called humus were directly absorbed by plants,and their carbon appropriated: it is now known thatthis is not the case. The real use of humus is, that itconstantly generates carbonic acid.
323. When water is poured on humus or decayingvegetable matter, very little of it is dissolved, and thewater acquires a pale yellow colour: if, however, thewater contains potash or soda, either caustic or in thestate of carbonate, it dissolves far more humus, andacquires a deep brown colour: the cause of this is, thatthe alkalies facilitate the decay of that substance. Underthe influence of the alkalies, a peculiar acid is formed,which has accordingly been termed the humic. Thebrown solution obtained under these circumstancescontains humic acid in combination with potash or soda.
324. It has been supposed that this acid might beformed by the action of alkalies on the humus or decay-