412 MICROORGANISMS 



the first place, the acids, being in a sense waste products of 

 the soil microorganisms, are injurious to the organisms which 

 produce them, and thus check their normal useful roles in the 

 soil. In the second place, most of the higher plants are 

 seriously injured directly by an excess of acid in the soil. 

 Consequently, it is important that the farmers have some way 

 of correcting the acidity of the soil. This is accomplished by 

 adding to the soil some of the materials which have the power 

 of combining with the acids. Finely ground limestone is 

 probably the most widely used substance for this purpose. 



BACTERIA AND DECAY 



470. Real Meaning of Decay. So far, we have considered 

 bacteria mainly from the standpoint of the service they render 

 us and the rest of the living world, by keeping up the fertility 

 of the soil and by helping to keep in circulation the chemical 

 elements which living organisms need. We shall now change 

 our point of view, for a time, and consider how they tend to 

 destroy many things which we wish to preserve. The dis- 

 integration of organic materials through the agency of bacteria 

 or other fungi is commonly spoken of as DECAY, ROTTING, 

 SPOILING, or PUTREFACTION. These different terms came into 

 use before the real nature of the process referred to was 

 recognized and they are still used to some extent, though more 

 or less vaguely, to represent different forms of the process. 

 Thus, for example, when disagreeable odors result from the 

 process, as often occurs in the disintegration of proteins, the 

 process is often spoken of as putrefaction. All of these terms 

 really mean the same thing; they are different names for a 

 single process by which the nature of matter is changed. They 

 are different terms applied to the chemical changes brought 

 about by living organisms. Hence, we shall use the single 

 word DECAY to represent the process in all its forms. 



Doubtless, you have already recognized the disintegration 

 of the soil humus as decay. In this instance, we are interested 



