138

VEGETABLE MANURES,

use causes the loss of a large quantity of ammonia, inconsequence of its power of decomposing all the salts ofammonia (95.97), combining with the acid which theycontain, and expelling the ammonia in the state of gas.The addition of lime greatly accelerates putrefaction, andrenders it a far less noisome process, as the lime absorbsthe sulphuretted hydrogen evolved (395), which wouldotherwise escape into the air.

427. Wood ashes mixed with putrefying animal mat-ters act in a similar manner to lime ; the alkali whichthey generally contain accelerates decomposition, butoccasions loss of ammonia.

428. A good deal of what has been said respectinganimal manures is equally applicable to those of vege-table origin; but in general the decay of vegetablemanures is far less rapid than that of animal substances,and they are chiefly of value for their mechanical effect,and the carbonic acid and saline matters which theysupply ; whilst, in addition to these, animal manuresyield ammonia, the substance which is indispensable tothe formation of gluten and albumen, two of the mostimportant of the constituents of vegetables (236).

429. The use of decaying vegetable matters hasalready been described whilst speaking of the nature ofhumus, and the organic constituents of the soil (322).A moderate quantity is very desirable, but too much isdecidedly hurtful. A soil containing a very large quan-tity of humus or decaying vegetable matter is alwaysfull of carbonic acid ; seeds cannot germinate in such a