182 HIGHER PLANTS AND SOIL MICROORGANISMS 



difficult to differentiate between those activities of plant growth 

 which do or do not affect microbial development, directly or indi- 

 rectly. 



The plant withdraws from the soil through its root system a 

 considerable amount of substances, principally inorganic; it con- 

 sumes the supply of carbon dioxide which is continually evolving 

 from the soil; it renders the soil more porous by the penetration 

 of its roots; it exerts a solvent effect by the excretion of carbon 



£2 



Beans 



Cucumbers 



Tomatoes, 



Vegetative 



Growth 



0.02 0.04 0.06 0.08 



Per Cent Carbon Dioxide in the Atmosphere 



0.1 



Fig. 69. — Influence of concentration of carbon dioxide in the atmosphere on 

 growth of plants (after Lundegardh). 



dioxide; eventually the plant itself returns to the soil and under- 

 goes disintegration through the agency of the microorganisms. 

 The activities associated with the complex transformations of 

 organic and inorganic substances in soils are largely regulated by 

 microbial development. While practically all organic compounds 

 disappear under favorable conditions in soils, the course and speed 

 of their transformation, the agents involved in the change, and the 

 immediate and ultimate results on plant growth vary greatly. 

 There may be an immediate or delayed benefit to plant growth 

 determined by the nitrogen content of the substances concerned. 



