88 THE CULTURE OF FARM CROPS. 



cayed matter which remained. This process continually 

 repeated, laid a foundation for a higher character of vege- 

 table growth ; until in time the soil became well supplied 

 with organic matter; and fitted for the occupation of man, 

 who afterwards appeared upon the scene, and entered into 

 possession of a soil abounding in accumulated fertility. 

 We know of our own knowledge that the soil we cultivate, 

 however rich it may be at the first, is very quickly ex- 

 hausted of nitrogen, and that a renewed supply is indispen- 

 sable to the growth of crops. This exhaustion is so rapid 

 that there connot be any material addition to the supply, 

 from the atmosphere. 



The most important combination of nitrogen is that with 

 hydrogen, known as ammonia; and that this gas enters into 

 the circulation of plants is rendered probable by a variety 

 of circumstances. 



It is known that ammonia exists in the sap of many 

 plants ; as in the beet, birch, and maple, in which it is asso- 

 ciated with cane sugar ; in the leaves of tobacco, in elder 

 flowers, in various fungi and in other plants. A species of 

 chenopodium actually exhales ammonia from its leaves ; it 

 also appears in the odorous exhalations of many other 

 plants and flowers. 



Ammonia can be procured from nearly all vegetable sub- 

 stances by distillation ; and many vegetable extracts are 

 found to contain it. When wood is distilled in retorts for 

 the manufacture of acid, ammonia is produced. 



These and other facts of similar bearing are in no wise 

 proofs that ammonia is the form in which nitrogen enters 

 into the substance of plants ; either through the roots or the 

 leaves ; because there are ways in which it could be pro- 

 duced in the plant by the same converting power which 

 produces sugar and starch in the interior of the plant from 

 carbonic acid and water ; and while ammonia is easily pro- 

 duced from coal and wood, yet we know that it does not 

 actually exist in these substances in their natural condition. 

 In the case of tobacco, the production of ammonia by means 

 of a high temperature may be illustrated by a simple ex- 



