188 HIGHER PLANTS AND SOIL MICROORGANISMS 



nutrients. In addition to that, the production of relatively large 

 amounts of carbon dioxide in soil is very important in determining 

 the ability of plants to absorb nutrients from relatively insoluble 

 compounds. Organic acids are eliminated by roots in only 

 limited amounts under conditions favorable to plant growth 

 and, consequently, the solvent action is almost entirely exerted 

 by the carbon dioxide which is evolved. 



The extent of the increase in solvent effects due to micro- 



FiG. 72. — Etching of marble as a result of root development. No microbes 



were present on the roots which etched the marble on the left. The greater 



etching of the slab on the right was due to acids produced by microorganisms 



developing on the roots (from Fred and Haas). 



organisms is suggested by Fig. 72. Plants were grown both under 

 sterile conditions and in the presence of microorganisms, in such a 

 manner that the roots passed over marble slabs. Where the roots 

 were free of microbes there was much less etching of the marble 

 than where they were present. These effects are largely asso- 

 ciated with differences in amounts of carbonic acid produced in the 

 absence and presence of microbes. 



The factors responsible for the various changes in microbial 



