40 SOILS 



the plants to take some nitrogen from the air. 

 Only weak currents can be used; a strong current 

 kills the plants. It is quite doubtful whether 

 the benefit derived from the use of a weak current 

 will make it profitable to use electricity in general 

 crop production, for the expense of wiring a field 

 is large; but it may be useful in greenhouses. 



GERM LIFE IN THE SOIL 



No soil has exactly the same composition from 

 year to year, or even from month to month. It 

 is constantly receiving additions of new soil from 

 the weathering of rocks, from the decay of plants, 

 the deposits of winds and other sources. It is 

 constantly losing by leaching, by erosion and by 

 the demands of plant growth. It also has numer- 

 ous activities within itself that exert a most potent 

 influence on its fertility. Some of these activities 

 are physical, some are chemical and some are due 

 to germ life. A few are already known and under- 

 stood, but only the merest beginning has been 

 made in the study of soil life. 



Nitrogen-Fixing Germs. One of the most 

 interesting phases of soil life is the process called 

 "nitrification," due to the activity of very minute 

 germs or bacteria, and sometimes called the 

 "nitric acid ferment." This is somewhat like 

 the ferment that sours milk, and the bacteria in 

 yeast that raise bread by their growth. Although 

 the air contains vast amounts of nitrogen, this is 

 not used by any plants, so far as is known, except 

 to some extent oy the "legumes," of which clovers, 

 alfalfa and vetch are examples. (See Chapter 

 XII.) Most farm crops get their nitrogen, which 

 they need ki considerable quantities, solely from 



