CHAPTER XVI 

 THE RELATIONS OF BACTERIA AND HUMUS 



HUMUS is defined as " decaying organic matter in 

 the soil." It is the seat of all the important bacteriologi- 

 cal activities for the very reason that it furnishes food 

 and energy to the microorganisms. The bacteria are 

 unlike the green plants in that they do not depend for 

 their food on the roots, stubble and other remains of 

 plants and animals. These organic remains are capable 

 of furnishing nourishment and energy to the bacteria. 

 In other words, they possess potential energy. It follows, 

 therefore, that, everything else being equal, the greater 

 the amount of humus in the soil, the greater the num- 

 ber of its bacteria. 



The quantity of humus as affecting number of bacteria. 

 Besides furnishing food to the bacteria, the soil-humus 

 also favors their development by providing better 

 moisture and temperature conditions. Because of its 

 great watei -holding power, the humus enables the soil 

 to retain greater quantities of moisture. The latter, in 

 turn, provides for a more un'form temperature. It has 

 been demonstrated by experiment that soils well pro- 

 vided with humus do not become warm, or cool as quickly 

 as do similar soils poor in humus. 



The application of barnyard manure multiplies the 



(144) 



