SYMBIOTIC RELATIONSHIPS 



41 



Fig. 33. — Soil amoeba, Vahlkampfia soli 

 (from Martin and Lewin). 



microbes are considered as taking part in the various transforma- 

 tions in the soil which result in the liberation of nutrients neces- 

 sary for the growth of plants, which in their turn supply the food 

 for domestic animals and man. The injurious organisms either 

 attack the plants and animals directly, by causing various diseases, 

 or injure them indirectly by bringing about transformations in 

 the soil which are unfavorable to plant growth. 



Such a strict classification of microorganisms as either bene- 

 ficial or injurious forms is too arbitrary. Some organisms may 

 be beneficial at one time and injurious at another, depending 

 upon the soil conditions and 

 plant growth. Even certain 

 organisms, as various Fusaria, 

 Pythia, and Rhizoctonia, which 

 cause plant diseases, grow ex- 

 tensively in soil, leading a 

 saprophytic existence and tak- 

 ing a part in the decomposi- 

 tion of organic matter. Other common soil saprophytes may 

 cause nitrogen starvation of plants at one time and produce an 

 abundance of nitrogen available for plant consumption at another 

 time, depending upon the cultural treatment of the soil. 



Symbiotic Relationships. — Certain soil microorganisms live 

 in definite symbiosis with plants. Included in this group are 

 various fungi that form mycorrhiza with forest trees and other 

 plants, such as orchids. The activities of these fungi are of 



decided importance in the nutrition 

 of the specific plants; some of the 

 higher plants may actually be 

 parasitic upon the fungi which 

 supply them with nutrients from 

 the soil, or play another important 

 part in their metabohsm. In the 

 case of legume bacteria, which form 

 nodules on the roots of leguminous plants, the symbiosis appears 

 to benefit both the plants and the bacteria. 



Summary: The Complex Soil Poplxation. — All the micro- 

 organisms living in the soil and included in the numerous groups 

 of bacteria, actinomyces, fungi, protozoa, algae, and small inverte- 

 brate animals make up the soil population. They do not live in 



Fig. 34. — Parasitic nematode 

 (from Cobb). 



