DISEASE-PRODUCING ORGANISMS IN THE SOIL 13 



ammonium tartrate increases at first in proportion to the concentrations 

 of these nutrients but later becomes independent of them. The onset of 

 the stationary phase may be due to several factors: exhaustion of sub- 

 stances necessary for growth, change in the reaction of the medium to 

 one unfavorable for further development, accumulation of toxic prod- 

 ucts. When the nutrients in the medium are exhausted, addition will 

 restore growth. When an unfavorable change in reaction has taken 

 place, the addition of acid or alkali will render the medium again favor- 

 able. The production of toxic substances in the medium can be counter- 

 acted usually by the use of heat or by treatment with charcoal, though 

 some of the injurious bodies may be heat-resistant. 



In the presence of other microorganisms, a certain organism may 

 show reactions markedly different from those obtained in pure culture: 

 it may produce substances that are either favorable or injurious to the 

 other cells, it may compete with the other organisms for the available 

 nutrients or it may render the medium more favorable for their de- 

 velopment. Some bacteria like Bacillus cereus can attack native proteins 

 but not amino acids, whereas others like Ps. jiuorescens can attack amino 

 acids but not proteins j when these two organisms are placed together in 

 the same medium, their activities supplement one another. Numerous 

 other instances are found in soil and water of an organism preparing 

 the substrate for another, ranging from distinct symbioticism, where 

 one organism depends absolutely for its living processes upon the ac- 

 tivities of another (symbiosis), to association, where one organism 

 merely is favored by the growth of another (metabiosis), to the injury 

 of one organism by another (antagonism), and finally, to the actual 

 destruction of one by another (parasitism). 



INTRODUCTION OF DISEASE- PRODUCI NG 

 ORGANISMS INTO THE SOIL 



Ever since higher forms of life first made their appearance on this 

 planet they have been subject to attack by microbes. These microscopic 

 organisms must have gained, at an early stage in the development of 

 the higher forms, the capacity of attacking them in one manner or an- 

 other. There is no plant or animal now living that is not subject to in- 



