152 THE PRINCIPLES OF 



The first reason no doubt is what has been already ad- 

 vanced the general composition of dung. A great many 

 science students stop here. When they are asked why farm- 

 yard manure is a more potent and more valuable manure 

 than many artificial fertilizers, they say it is because of its 

 general composition. But there are a good many other 

 reasons beside, one of which is no doubt its effect upon the 

 mechanical condition of the soil, a subject which we have 

 already had before us, and which it is therefore not necessary 

 to further enlarge upon. Then, in the third place, there is 

 the reaction of the carbonic acid gas which is evolved from 

 farmyard dung, upon the mineral matter in the soil. I do 

 not doubt in the least that it digests the soil. 



I do not doubt that Jethro Tull was perfectly right when 

 he said farmyard manure prepared plant food. No doubt it 

 does ; it is the source of carbonic acid gas-, and we know that 

 that gas in watery solution reacts on the mineral matter in 

 the soil with great effect. 



Now take another reason. Farmyard dung is rich in 

 nitrogen; that alone places it on a superior basis to most 

 artificial manures. It is rich in nitrogen in a state of organic 

 combination, from which it is liberated slowly by the process 

 of decay, that liberation of nitrogen being known as nitrifica- 

 tion. Performed under favourable temperatures, with access 

 of air, and no doubt also assisted by the agency of certain 

 bacteria which work in the soil and produce the peculiar 

 fermentation necessary. This nitrification of farmyard manure 

 in the soil is arrested at freezing-point. It proceeds very 

 slowly at low temperatures, and with accelerated speed at 

 higher temperatures. Especially does it take place freely 

 during the summer months, when vegetation is most 

 luxuriant. 



Hence farmyard manure subjected to gradual decay yields 

 up its materials, especially nitrogen oxydized into nitrates, at 



