K CONTENTS 



:hapter page 



cellulose. Fungi decomposing cellulose. Bacteria decom- 

 posing cellulose. Influence of soil conditions upon cellulose 

 decomposition. Lignin and its decomposition. Decomposi- 

 tion of fats. Decomposition of other plant constituents. 

 Decomposition of the plant as a whole. Metabolism of 

 microbes and decomposition of organic matter. Summary — 

 Decomposition of organic matter in soil and formation of 

 soil humus or soil organic matter. 



V. Transformation of Nitrogen by Soil Microbes 102 



Sources of nitrogen in soil. Transformation of nitrogen in 

 soil. Fixation of atmospheric nitrogen. Non-symbiotic nitro- 

 gen-fixing bacteria. Importance of non-symbiotic nitrogen 

 fixation. Symbiotic nitrogen fixation. Nature of the bacteria 

 and classification of leguminous plants. Influence of nitrate 

 upon nitrogen fixation. Amounts of nitrogen fixed. Influence 

 of soil conditions upon nitrogen fixation. Decomposition of 

 proteins by microorganisms. Decomposition of nitrogenous 

 substances of a non-protein nature. Formation of ammonia 

 by soil microbes. 



VI. Transformation of Nitrogen by Soil Microbes (Continued) . . 136 

 Nitrate formation. Influence of ammonia upon nitrate forma- 

 tion. Influence of soil reaction upon nitrate formation. 

 Influence of soil aeration upon nitrate formation. Organic 

 matter and nitrate formation. Moisture and nitrification. 

 Nitrate reduction. Importance of denitrification in soil. 

 Summary. 



VII. Transformation of Mineral Substances in Soil through 



the Direct or Indirect Action of Microorganisms 154 



Relationships of microorganisms to the elements occurring in 

 nature. Mineral elements and their inorganic compounds as 

 sources of energy. I'se of inorganic salts as sources of 

 oxygen. Interaction of insoluble inorganic salts with in- 

 organic and organic acids produced by microbes. Carbon 

 dioxide in soil. Nitric acid in soil. Organic acids in soil. 

 Change in soil reaction. Mineral assimilation by micro- 

 organisms. Transformation of sulfur in soil by microbes. 

 Transformation of phosphorus by soil microbes. Transforma- 

 tion of iron by soil microbes. Transformation of potassium by 

 soil microbes. Importance of mineral transformation by 

 microbes in soil formation. 



VIII. Interrelationships between Higher Plants and Soil Micro- 



ORG.ANISMS 181 



Interdependence of higher plants and microbes. Evolution 

 of carbon dioxide. Influence of root excretions. Conditions 



