Kinds of Humus and Bacteria 145 



soil bacteria since it not only adds directly to their 

 number, but also furnishes food for their further devel- 

 opment. On the other hand, green-manures and organic 

 fertilizers, like cottonseed meal, castor- 

 pomace, or tankage, do not add as great 

 numbers of bacteria to the soil, and are 

 chiefly valuable for the food they furnish to 

 those already in the soil. 



Arid and semi-arid soils contain smaller 

 proportions of humus than those found in 

 humid soils. It would seem, therefore, 

 that aside from the moisture conditions in ' ' 



the former, the two classes of soils must bacillus/, ^ 

 show very considerable bacteriological dif- sou" 1 ? slooo 

 ferences. 



Quality of humus as affecting number of bacteria. 

 The influence of the quality of humus on the soil bacteria 

 is also important. The so-called mild humus, or mull, 

 of arable soils, or of woodland, is different in its compo- 

 sition from the raw humus of heaths, meadows, and 

 swamps. It influences in an entirely different way the 

 numbers and character of the bacteria. The differences 

 observed are due largely to the origin and mode of 

 formation of the two classes of humus substances. The 

 mild humus is formed under conditions admitting the 

 free access of air and through the activities largely of 

 aerobic organisms. It is either neutral or alkaline in 

 reaction. The raw humus is formed through the process 

 of putrefaction rather than that of decay. It is acid in 

 reaction, and is not a suitable medium for the develop- 

 ment of most bacteria. It has been shown that peat 



