122 SOILS 



The evil in bacteria. So far, we have seen good only in 

 bacteria : they destroy ten thousand useless things that, 

 otherwise, would trouble man and load him continuously 

 with burdens and difficulties. It is impossible to estimate 

 the value or the extent of this useful work. But for all 

 the good they do, bacteria have an evil side : they send 

 nitrogen away from the soil. This must be said, how- 

 ever : that while decay and putrefaction bacteria have the 

 power of freeing nitrogen from its compounds in the soil, 

 they do so only to a limited extent, and only very 

 slightly, indeed, where the tiller of the soil properly co- 

 operates with them. 



There are some forms of bacteria in the soil that make 

 it their chief business to free nitrogen. They do this not 

 because they have any spite against plants or animals, 

 but simply in order that they may live. Here is the rea- 

 son : they need some carbohydrate a carbon compound 

 for food ; this they get from the organic substances that 

 have been sent to the soil. But they need, also, some 

 oxygen just as the higher plants. If air is not present in 

 the soil it is excluded often by water or bad texture 

 oxygen becomes in demand. But from whence may it 

 be secured? These bacteria have found a way through 

 the long, long line of their antecedents : they simply seek 

 out nitrogen compounds compounds that contain both 

 nitrogen and oxygen and extract from them the oxygen 

 they need, at the same time rejecting any nitrogen asso- 

 ciated with it there. This nitrogen, now released, escapes 

 its prison, rises into air, sails away, and becomes lost to 

 the soil until trapped again by other bacteria the good 

 fairies that do this philanthropic act. 



Denitrification : the nitrogen-freeing process. The 

 first effort in freeing nitrogen is that of changing the 

 nitrates into the next simpler form, the nitrites. Not just 



