392 THE PRINCIPLES OF SOIL MANAGEMENT 



to animal life. Of the latter, the only organisms of 

 economical importance are the nematodes, whose 

 injurious effect upon plant growth is accomplished 

 through the formation of galls on the 

 roots, in which the young are hatched 

 and live to sexual maturity. 



262. Plant micro-organisms. The 

 microscopic plants of the soil may be 

 classed as slime-molds, bacteria, fungi 

 and algae. 



263. Plant micro-organisms injurious 

 Nematodes enter- to higher plants. Injurious plant 



mgaroot. micro- organisms are confined mostly 

 to fungi and bacteria. They may be entirely para- 

 sitic in their habits, or only partially so. They 

 injure plants by attacking the roots. Those attacking 

 other portions of plants may live in the soil during 

 their spore stage, but these are not strictly micro- 

 organisms of the soil. Some of the more common dis- 

 eases produced by soil organisms are: Wilt of cotton, 

 cowpeas, watermelon, flax, tobacco, tomatoes, etc., 

 damping-off of a large number of plants, root-rot, 

 galls, etc. . 



These fungi or bacteria may live for long periods, 

 probably indefinitely, in the soil, if the conditions 

 necessary for their growth are maintained. Some of 

 them will die within a few years if their host plants 

 are not grown upon the soil, but others are able to 

 maintain existence on almost any organic substance. 

 Once a soil is infected, it is likely to remain so for a 

 long time, or indeed indefinitely. Infection is easily 



