274 FIRESIDE SCIENCE. 



and ultimately die; -and hence we must supply it 

 in some form, either as it exists in manure, or in 

 commercial substances. The soil does not furnish 

 it in sufficient abundance, neither does the atmos- 

 phere, in any available f<^rm. There is always a 

 little ammonia in moist air, which comes from 

 decaying animal or vegetable matter, and also 

 there are traces of nitric and nitrous acids in rain 

 water, but these sources of supply are wholly in- 

 adequate to the wants of plants upon most fields. 



An acre of wheat yielding twenty-five bushels 

 requires, in straw and grain, forty-five pounds of 

 ammonia. The results of careful experiments show 

 that under the most favorable circumstances no 

 more than ten pounds of ammonia is ever supplied 

 to an acre of soil by rain-water; so if all the ammo- 

 nia of the rain-fall is assimilated, thirty-five pounds 

 in addition would have to be supplied, to meet the 

 wants of the wheat field. 



Carbon, the agent so largely consumed by plants, 

 fortunately costs us nothing. The farmer need 

 not trouble himself concerning this important ele- 

 ment in plant food, for the atmosphere furnishes 

 an abundant supply for all our wants. It is sup- 

 plied in the form of carbonic acid, and we do not 

 know that it can be assimilated through any other 

 carbon compound. A carbonate, unless it be of 

 potash or soda, is practically valueless to the farmer, 



