18 MICROORGANISMS IN SOILS AND WATER BASINS 



pastures safe for use. The fact that most pathogenic organisms rapidly 

 disappear when added to the soil makes this problem rather simple j the 

 prevention of infectious diseases would have presented far more diffi- 

 cult problems were the infecting agents to remain indefinitely virulent 

 in the soil. The few disease-producing agents that are capable of per- 

 sisting, such as anthrax, blackleg, and coccidiosis, have been the cause, 

 however, of considerable damage to animals. 



Of greater economic importance than the survival in the soil of hu- 

 man and animal pathogenic agents is the fact that the soil harbors a 

 number of plant pathogens, including not only fungi, bacteria, and 

 actinomycetes, but also nematodes and insects. Fortunately, the con- 

 tinued development of these organisms in the soil also leads to the ac- 

 cumulation of saprophytic organisms destructive to them. 



The extent to which virus diseases persist in the soil is still a matter 

 for speculation. It has been demonstrated that the phage of legume 

 bacteria may persist and become responsible for a condition designated 

 as "alfalfa-sick soils" and "clover-sick soils" (169, 474). In order to 

 overcome this condition, the breeding of resistant varieties of plants has 

 been recommended. 



