ir6 THE STORY OF GERM LIFE. 



dients has been more or less of a puzzle. Fertile 

 soil everywhere contains nitrates and other nitro- 

 gen compounds, and in certain parts of the world 

 there are large accumulations of these compounds, 

 like the nitrate beds of Chili. That they have 

 come ultimately from the free atmospheric nitro- 

 gen seems certain, and various attempts have been 

 made to explain a method of this nitrogen fixa- 

 tion. It has been suggested that electrical dis- 

 charges in the air may form nitric acid, which 

 would readily then unite with soil ingredients to 

 form nitrates. There is little reason, however, 

 for believing this to be a very important factor. 

 But in the soil bacteria we find undoubtedly an 

 efficient agency in this nitrogen fixation. As al- 

 ready seen, the bacteria are able to seize the free 

 atmospheric nitrogen, converting it into nitrites 

 and nitrates. We have also learned that they 

 can act in connection with legumes and some 

 other plants, enabling them to fix atmospheric ni- 

 trogen and store it in their roots. By these two 

 means the nitrogen ingredient in the soil is pre- 

 vented from becoming exhausted by the processes 

 of dissipation constantly going on. Further, by 

 some such agency must we imagine the original 

 nitrogen soil ingredient to have been derived. 

 Such an organic agency is the only one yet dis- 

 cerned which appears to have been efficient in 

 furnishing virgin soil with its nitrates, and we 

 must therefore look upon bacteria as essential to 

 the original fertility of the soil. 



But in another direction still does the farmer 

 depend directly upon bacteria. The most impor- 

 tant factor in the fertility of the soil is the part 

 of it called humus. This humus is very complex, 

 and never alike in different soils. It contains ni- 



