Numberless Soil Organisms 137 



world of the living and the world of the dead. They are 

 the great scavengers intrusted with restoring to circu- 

 lation the carbon, nitrogen, hydrogen, sulfur, and other 

 elements held fast in the dead bodies of plants and 

 animals. Without them, dead bodies would accumulate, 

 and the kingdom of the Hving would be replaced by 

 the kingdom of the dead. And yet the soil bacteria are 

 not mere destroyers, for there are among them species 

 that do constructive work, also indispensable. 



Number of bacteria in soil. — The number of bacteria 

 in arable soils is large, ranging from several hundreds 

 of thousands to several millions per gram of soil (about 

 2V of an ounce) . In dry, sandy soils, very poor in humus, 

 their numbers may be low, scarcely more than a few 

 thousands per gram. In rich loam soils, they may reach 

 the enormous total of fifteen to twenty millions per gram. 

 In soils polluted with sewage, the number of bacteria 

 may, at times, exceed one hundred millions per gram. 



There is a very intimate relation between the mois- 

 ture content of the soil and the number of its bacterial 

 inhabitants. Periods of rainfall are followed by a very 

 marked increase in the number of soil bacteria. Periods 

 of drought are • inimical to their development. The 

 temperature of the soil has also a very direct relation 

 to the number of its bacteria. Like higher plants, they 

 need a certain degree of warmth for the manifestation 

 of their vital activities. They cannot grow in the frozen 

 earth, and they must wait for moisture and warmth to 

 wake them up and to stir them into activity. 



With the coming of spring and of the longer hours 

 of sunshine, the water moves more fteely through the 



