32 AGRICULTURE. 



The ammonia (NH,) is oxidized to nitrous acid 

 (HNO/) by another species; this in turn is 

 changed into nitric acid (HNO,) by still another 

 species. In a similar manner the opposite pro- 

 cess of dcnitrijication goes on. First, the nitric 

 acid is reduced to nitrous acid, this to ammonia, 

 and then to free nitrogen, each step being per- 

 formed, respectively, by a distinct species of 

 bacteria. 



Both of these processes may take place in the 

 soil, their extent depending largely upon the 

 oxygen supply. In a well-aerated soil nitrifica- 

 tion takes place, while in an undrained, poorly 

 ventilated soil denitrification occurs. 



It has been proven by modern science that 

 the nitrifying organism of the soil is able to sub- 

 sist in a purely mineral environment. Now 

 certain bacteria, or soil ferments, are found in 

 great numbers about plant rootlets — in fact, liv- 

 ing in mutual relationship with them. It is, 

 therefore, thought probable that the action of 

 bacteria has an effect upon the mineral particles 

 of the soil which renders them solvent and pre- 

 pares them for absorption by plants as food.'^* 

 (Year-book, 1895.) 



Although these bacteria can subsist upon 



* The value of leguminous plants for worn out or poor soils 

 has long been realized, but not until iSSS, when Helriegel pub- 

 lished the results of his investigations, was the r^a,l source of 

 their fertilizing power known. 



