40 WALL'S MANUAL 



exhausted soil, they do sometimes fail, owing to the 

 want of mold to retain moisture requisite to healthy 

 vegetation, as also to the production by slow decay of 

 carbonic acid gas required by the roots of the plant. 

 It is proper, on poor soils, to mix the salts with vegetable 

 composts. 



When vegetable matters are allowed to undergo 

 changes in a moist soil, there are various products 

 formed during their fermentation, putrefaction, or 

 slow combustion. The first changes which take place 

 are, properly, those of fermentation, during which 

 soluble vegetable acids are formed, amongst them 

 acetic acid or vinegar, when the plants are rich in sugar. 

 During this stage of fermentation vegetable matters 

 act injuriously to plants, the soluble vegetable acids 

 being to most of them poisonous. If lime, potash or 

 soda be present in the soil, the acids unite with them, 

 and are neutralized. Hence the importance of using 

 these alkalies. 



The next change is one of a different nature, in 

 which the fibre of the wood becomes brown and 

 rotten. Ulmic acid is now formed, and next humic. 

 It will thus be seen that all the processes of decay of 

 vegetable matter result in the formation of vegetable 

 acids and humus or mold, and by the action of the 

 air upon them, produces carbonic acid, which, in its 

 turn, acts upon the silicates and salts in the soil, or is 

 evolved as carbonic acid gas, which is absorbed by 

 the leaves of plants. 



It is a well known fact that acid matters of all kinds 

 tend to decompose the minerals in the soil. Ulmic, 

 humic, and crenic acids readily act on rocks. This 

 has been proven by placing a piece of granite, lime- 



