82 THE PRINCIPLES OF AGRICULTURE. 



and decaying vegetable matter, clay, and other soils. 

 Charcoal will absorb ninety times its own bulk of am- 

 monia gas. 



Ammonia is therefore widely diffused, though in small 

 quantities, in the atmosphere, in the land, and in the 

 water of the earth. It readily combines with acids, and in 

 the atmosphere is not generally found in a free state, but 

 combined with carbonic acid, forming carbonate of am- 

 monia. This carbonate is dissolved in the moisture of 

 the atmosphere, and brought to the earth, where, to- 

 gether with the ammonia already contained in the soil, 

 it adds to the supply of nitrogenous plant food. 



Only a part of the nitrogen required for producing 

 farm crops is furnished in the natural supply of nitric 

 acid and ammonia. The farmer adds to this supply by 

 furnishing fertilizers which either contain these sub- 

 stances or their compounds, or which will produce them 

 in the soil by decay and chemical changes. 



Nitrification. The vegetable matter which exists in 

 ordinary soil in the form of decaying grass, leaves, roots, 

 or stable manure, contains nitrogen combined with car- 

 bon. In this condition it is not available as plant food. 

 By a peculiar process called nitrification the nitrogen is 

 separated and converted into nitric acid. This acid again 

 combines with bases in the soil and forms nitrates, as 

 nitrate of lime, nitrate of soda, or nitrate of potash, and 

 these are easily dissolved and readily absorbed by the 

 roots of plants. They are also easily washed away from 

 the soil and lost when there are no plant roots ready to 

 take them up. 



The process of nitrification is brought about by the 

 growth of a minute plant. This plant thrives in warm, 

 moist soil, which is sufficiently porous to admit air, and 



