126 SOILS AND FERTILIZERS 



germs are : wilt of cotton, cowpeas, watermelon, flax, tobacco, 

 tomatoes, and other plants ; damping-off of a large number 

 of plants, root-rot and galls. 



Some of the germs causing these diseases may live in the 

 soil for many years. Some of them will die within a few 

 years if the plants on whose roots they live are not grown on 

 the soil, but others are able to maintain existence on almost 

 any organic substance. Infection is carried in the soil, or by 

 the roots of the plants themselves, consequently farm imple- 

 ments or manure may often be a means of spreading the germs. 



For combating the difficulties caused by the germs, many 

 methods have been tried with more or less success. Rota- 

 tion of crops is successful in some cases, but in others entire 

 discontinuance is the only remedy. The use of lime has been 

 beneficial in the case of some diseases. Steam sterilization 

 for greenhouse soils will hold in check a considerable number 

 of diseases. Strains of cowpeas and cotton plants have been 

 bred that are immune to the effects produced by some germs. 



155. Germs not directly injurious to crops. — The part 

 played by the microorganisms that affect the growth of 

 crops may be roughly listed as follows : (1) action on mineral 

 matter; (2) decomposition of non-nitrogenous organic 

 matter; (3) decomposition of nitrogenous organic matter; 

 (4) fixation of nitrogen from the air and its incorporation 

 in the soil. Most of the processes involved in these trans- 

 formations bring about conditions favorable to crop growth, 

 but some of them are injurious, as, for instance, the forma- 

 tion of substances poisonous to plants and the liberation 

 of nitrogen which escapes into the air. These injuries are, 

 however, not direct effects of the germs on the crops, but 

 indirect ones caused by the products of the organisms. 



Bacteria, fungi, algae and certain molds all play a part 

 in these processes, but none of them so actively as do the 

 bacteria. On account of the dominant part that bacteria 



