SOILS. 409 



What is said 1026. ARTIFICIAL AMMONIA. The COI1- 



of the artifi- stituents of the ammonia which we pur- 



cial produc- .. ~ 



tionofammo- chase, in the form of guano, at so great 

 expense and bring from distant regions of 

 the earth, exist in unbounded quantity at our very 

 doors. Four-fifths of the atmosphere are nitrogen gas, 

 and the ocean is an exhaustless reservoir of hydrogen. 

 But, strange to say, the chemist with all his skill, 

 cannot, except by circuitous and expensive methods, 

 effect their combination. The discovery of some cheap 

 and ready means of accomplishing this object, would 

 transform the face of the earth, by the unlimited quan- 

 tity of fertilizing material which it would supply. This 

 result may, perhaps, be reached by patient investiga- 

 tion. But no sudden triumph over nature need be 

 anticipated. Improvements in Agriculture will, as a 

 general thing, be only realized by the earnest co-opera- 

 tion of scientific and practical men, in laborious and 

 oft-repeated experiment. 



1027. EXHAUSTION OF SOILS. When 



What is said i * , 



of the exhaus- soils become exhausted 01 those substances 

 tion of soils? which form the mineral food of plants, the 

 growth of vegetation ceases. It is never absolute, 

 but consists in a great reduction of that portion of 

 their material which is in a condition to be appropri- 

 ated by the growing plant. Such soils are gradually 

 restored by rest. A gradual decomposition of their 

 insoluble material occurs by means of agencies which 

 have before been mentioned, and the soil is thus re- 

 stored to its original condition. These effects are very 

 much hastened by plowing in such a growth as can 



18 



