32 THE CULTURE OF FARM CROPS. 



"spirits of hartshorn." It is largely contained in decom- 

 posing urine and causes the pungent odor of stables in 

 which horses are kept. Being volatile, it escapes into the 

 air with great ease, and unless combined with some acid 

 into a permanent form it is disengaged readily from decom- 

 posing manure by fermentation and heat, and is lost to the 

 farmer. 



It has been supposed heretofore, that this gas was the 

 source from which plants derived their nitrogen, but re- 

 cent investigations go to prove that ammonia is oxidized in 

 the soil and changed into nitric acid before it can become 

 available for the nutriment and support of plants. 



Ammonia combines freely with acids. With sulphuric 

 acid it forms a stable compound, sulphate of ammonia; 

 hence it is useful to employ solutions of sulphate of iron,, 

 (copperas) or sulphate of lime, (gypsum or plaster) to fix 

 any escaping ammonia which may be in danger of loss from 

 stables and manure yards. The employment of plaster in 

 this way and for this purpose is quite common among care- 

 ful and economical farmers, who scatter it liberally about 

 the stables and yards, and so deodorize and purify them ; 

 make them more agreeable and healthful, and save all this 

 exceedingly valuable fertilizing agent. The ammonia hav- 

 ing an exceedingly strong affinity for sulphuric acid takes 

 this from the sulphate of iron cr lime and combines with it 

 leaving the iron in the form of an oxide, or the lime in the 

 form of a carbonate. 



