Influence of Plants upon Microorganisms 



255 



potassium salts. This results in a change in the composition of the 

 soil solution and in a modification of the activities of microorganisms. 

 4. Plants excrete considerable COj into the soil. This tends to 

 change the reaction of the soil. It also increases the solubility of 

 certain inorganic soil constituents and changes the composition of 

 the soil atmosphere. 



Fig. 102. Influence ot partial sterilization by heat upon growth of tomatoes. 



The soils in the pots from left to right were heated as follows before planting: 



30°C, 60°C, 80°C, 100°C, 12.5°C, 150°C for 2 hours (from Pickering). 



5. Plants remove from the soil considerable moisture, thus exerting 

 an injurious influence upon the growth of microorganisms. 



6. Plants modif)- the structure of the soil and thereby produce a 

 medium that is more favorable for the development of micro- 

 organisms. 



7. Plants remove the nitrates from the soil, leaving the bases be- 

 hind and thereby affecting the reaction of the soil. 



To measure the influence of plants upon microbiological activities 

 in the soil, certain well-defined procedures may be utilized. These 

 comprise some of the common measures of the microbiological state 

 of the soil, such as (a) the numbers of microorganisms, (b) nitrate 

 accumulation or nitrifying capacity of the soil, (c) oxidizing power 

 of the soil as expressed in terms of oxygen absorption or COo pro- 

 duction, (d) a. variety of other biological activities. It is often diffi- 

 cult to differentiate between the direct influence of the growing 

 plant and the influence of the plant products. 



Among the bacteria which are particular!)' influenced by growing 

 plants, the Radiobacter group occupies a prominent place. A crystal 



