80 AGRICULTURAL BACTERIOLOGY 



stated as a general truth that, in material consisting of a 

 mixture of sugars and proteins, the former will be the first 

 to be attacked in processes of decomposition, with the result 

 that an acid reaction will be established which will prevent 

 further decomposition, except as it may be occasioned by 

 such organisms as the molds which use acid as food. If 

 any condition, such as the absence of free oxygen, does not 

 permit these aerobic forms to grow, the partially decom- 

 posed material will not suffer further action. This fact is 

 of great practical importance in the soil. 



In the decomposition of organic matter under the action 

 of a sequence of forms, each of which obtains its building 

 material and its energy from the food consumed, the mater- 

 ial is gradually changed to a more stable form and its energy 

 content reduced until it reaches a stage in which the vari- 

 ous elements are available for the use of the green plant. 



Humus. The soil was originally formed from the disin- 

 tegration of the rocks, and at first contained no organic 

 matter. In the evolution of life, plants began to grow in 

 the soil. Following their death, decay occurred. If the 

 decomposition had been complete, the soil would have re- 

 mained purely inorganic matter, since the final products of 

 decomposition are readily soluble in water. If decompo- 

 sition is not complete, a residue of partially decomposed 

 organic matter remains in or on the ground. To this resi- 

 due the term humus is applied. The content of the soil in 

 humus varies from an almost complete absence to soils in 

 which it is the most prominent constituent. The accumu- 

 lation of humus is exceedingly slow. It has required many 

 thousand years to accumulate the amount found in rich 

 prairie soils. Under cultivation it is removed at an infin- 

 itely more rapid rate than it was formed. 



The rate and extent at which humus accumulates bears 



