MAifURES, 175 



Chemical manures are those wliicli enter into the 

 construction of plants, or produce such chemical 

 effects on matters ah-eady contained in the soil aa 

 ehall prepare them for use. 



Ilechanical manures are those which improve the 

 mechanical condition of the soil, such as loosening 

 stiff clays, compacting light sands, pulverizing largo 

 particles, etc. Many manures act both chemically 

 and mechanically. 



Manures may be classified under three distinct 

 heads, namely. Organic, miner al, and atmospheric. 



Organic manures comprise all vegetable and ani- 

 mal matters (except ashes) which are used to fer- 

 tilize the soil. Vegetable manures supply carbonic 

 acid, some ammonia, and earthy matter to plants. 

 Animal manures supply the same substances and 

 much more ammonia. 



Mineral manures comprise ashes, salt, phosphate 

 of lime, plaster, etc. They supply plants with earthy 

 matter. Their usefulness depends in great degree on 

 their solubility. 



Many of the organic and mineral manures have 

 the power of absorbing ammonia arising from the de- 

 composition of animal manures, as well as that which 

 is brought to the soil by rains — these are called ab- 

 sorbents. 



Atmospheric manures consist of ammonia, car- 

 bonic acid, oxygen and water. Their greatest use- 

 fulness requires the soil to allow the water of rains to 

 pass through it, to admit of a free circulation of air 

 among its particles, and to contain a sufficient 



