ORGANISMS IN THE SOIL 425 



347. Plant microorganisms. — The microscopic plants 

 of the soil may be classed as slime molds, bacteria, fungi, 

 and algae. 



348. Plant microorganisms injurious to higher plants. 

 — Injurious plant microorganisms are confined mostly 

 to fungi and bacteria. They may be entirely parasitic in 

 their habits, or only partially so. They injure plants by 

 attacking the roots. Those that attack other parts of 

 plants may live in the soil during their spore stage, but 

 they are not strictly microorganisms of the soil. Some 

 of the more common diseases produced by soil organisms 

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

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

 ber of plants ; root-rot ; galls. 



These fungi or bacteria may live for long periods, prob- 

 ably 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 on 

 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 indefi- 

 nitely. Infection is easily carried. Soil from infected 

 fields may be carried on implements, plants, or rubbish 

 of any kind, in soil used for inoculation of leguminous 

 crops, or even in stable manure containing infected plants 

 or in the feces resulting from the feeding of infected plants. 

 Flooding of land by which soil is washed from one field 

 to another may be a means of infection. 



Prevention is the best defense from diseases produced 

 by these soil organisms. Once disease has procured a 

 foothold, it is practically impossible to eradicate all its 

 organisms. Rotation of crops is effective for some dis- 

 eases, but entire absence of the host crop is oftener neces- 



