REID — AGRICULTURE ALLIED TO CHEMISTRY. 405 



The old idea, advocated by Sir H. Davy, was that plants derived 

 their gaseous nutrition (carbon, hydrogen, nitrogen, and oxygen) 

 from humus, a constituent of all productive soils. Boussingault 

 taught that plants obtained these elements both from the air and 

 soil, but could not solely depend on either source for their require- 

 ments. That notably the nitrogen and ammonia in the air had to 

 be supplemented by these substances if not existent in the ?oil. 



Liebig taught that the food of the chief mass of the plant 

 (carbon, hydrogen, oxygen, nitrogen) consisted solely of carbonic 

 acid, water and ammonia. That these were alto2;ether obtained 

 from the atmosphere, which was abundantly supplied by the decay 

 of anim.als and vegetables, their decomposition giving off these 

 substances to the air. That thus is produced much more oxygen 

 than plants can use, and hence this gas so absolutely necessary for 

 the maintenance of life had its supply kept up by plant growth. 

 The decomposition of carbonic acid depositing carbon in the tissue 

 of the plant and giving off oxygen to the air. That the only 

 substances furnished by the soil were the * ' ash constituents " of the 

 vegetable, or the mineral matter it contained. That these alone 

 were all that were necessary to be supplied to the land, as they were 

 ail that were taken from it. That manures were only of value in 

 proportion as they contained the mineral or ash constituents of the 

 crops they were intended to nourish. 



All of these ideas of Liebig are yet believed to be and are cor- 

 rect, the only error being thtit they were made too exclusive. 

 Plants do absorb and assimilate carbonic acid, water and ammonia 

 from the air, but they require a portion from the soil as well, and 

 hence manures containing these, or equivalents, are demanded. 



The ashes of the plants or mineral constituents are derived from 

 the soil, in which they must exist in a state capable of being dis- 

 solved in water, and there is need for their return in this form to 

 keep up productiveness. In this particular a good soil is an 

 extensive deposit that may be drawn on for many, many years, 

 without showing very marked deterioration. For good husbandry 

 exposing it to the air, causes the insoluble salts of silica, potassa, 

 lime, phosphates, &c., to be decomposed, and in addition much 

 ammonia is absorbed from the air and retained, this being a property 



