192 Bacteria in Relation to Country Life 



gas for their growth. It seemed that the nitrogen must 

 be combined with other elements to form either 

 ammonia or nitrate before it could be employed by 

 plants. 



The ammonia theory. Toward the middle of the 

 nineteenth century the opinion came to prevail that 

 ammonia is the important source of nitrogen to plants. 

 Liebig, in Germany, known as the father of agricultural 

 chemistry, demonstrated that ammonia is a constant 

 constituent of the atmosphere. He maintained that the 

 natural supply of ammonia is usually sufficient for the 

 growth of crops. According to him, the exhaustion of 

 soils should be ascribed to their decreased content of 

 mineral ingredients rather than to decrease in nitrogen. 



When careful study of the composition of the atmos- 

 phere, conducted in France, England, and Germany, 

 had proved that the amount of ammonia brought down 

 to the earth by rain and snow scarcely exceeded a few 

 pounds per acre annually, Liebig maintained that 

 plants are capable of directly absorbing ammonia by 

 means of their leaves. He pointed out that the beneficial 

 effects of nitrogenous manures are most apparent in 

 the case of cereal crops with a comparatively short 

 vegetation period, and least apparent in the case of 

 leafy crops with a long vegetation period. Experience 

 had taught the farmer, he said, that it was useless to 

 apply nitrogenous materials to clover. No benefit at all, 

 or only slight benefit, was likely to result therefrom. 

 The long vegetation period of crops like clover allowed 

 the gradual utilization of the ammonia in the air, and no 

 artificial supply was necessary. On the other hand, 



