VEGETABLE MANURES. 313 



(667). A moderate quantity is very desirable, but 

 too much is decidedly hurtful. 



841. A soil containing a very large quantity of 

 humus or vegetable matter in a state of decay is 

 always full of carbonic acid ; seeds do not germinate 

 well in such a soil ; and the excess of carbonic acid 

 is even hurtful to plants themselves (745). 



842. The principal vegetable substances employed 

 as manure are straw of all kinds, leaves, sawdust, 

 bran, oilcake, seaweed, and green manures, or crops 

 which are merely sown to be ploughed in, and thus 

 afford food to a second crop, of some more valuable 

 plant. 



843. All these manures when mixed with soil 

 slowly decay, and yield carbonic acid and small 

 quantities of saline and earthy matters. They are 

 most advantageously used when employed in combi- 

 nation with some kind of animal manure ; this is the 

 case in farm-yard manure. 



844. Straw alone decays but slowly, but when 

 mixed with the dung and urine of cattle, it soon 

 begins to change, and in a short time the whole is 

 brought into a state of decomposition (802). 



845. In this case, a sort of putrid fermentation is 

 caused ; the animal manure decomposes rapidly, and 

 causes a similar change to take place in the vegetable 

 substances with which it is mixed (766); decomposi- 

 tion proceeds rapidly, heat is evolved, and if the 

 bulk of the mixture is large, this action becomes so 



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