XXVI CONTENTS 



Chapter XXI 



REDUCTION PROCESSES — NITRATE REDUCTION 



Reduction processes in the soil. Transformation of nitrates by micro- 

 organisms. Nitrate assimilation. Utilization of nitrates by micro- 

 organisms as sources of oxygen. Reduction of nitrates to gaseous nitro- 

 gen and oxides of nitrogen. Formation of nitrogen gas from organic com- 

 pounds. Denitrification in the soil. Importance of nitrate-reduction in 

 the soil. Reduction of other substances in the soil 542 



Chapter XXII 



FIXATION OF ATMOSPHERIC NITROGEN BY MICROORGANISMS 



Non-symbiotic fixation of nitrogen. Source of energy. Chemistry of de- 

 composition of carbohydrates. Respiration and nitrogen fixation. Pro- 

 tein synthesis by Asiotobacter. Chemistry of process of non-symbiotic 

 nitrogen fixation. Influence of available nitrogen compounds upon nitro- 

 gen fixation. Influence of salts upon nitrogen fixation. Influence of 

 organic matter upon nitrogen fixation. Influence of reaction. Influence 

 of moisture and temperature upon nitrogen fixation. Soil cultivation and 

 nitrogen fixation. Importance of non-symbiotic nitrogen-fixation proc- 

 esses in the soil. Symbiotic Nitrogen Fixation. Relation between the 

 bacteria and the host plant. Chemistry of nitrogen-fixation by symbiotic 

 bacteria. Production of gum by legume bacteria. Influence of reaction 

 on the growth of Bad. radicicola and nodule formation. Nodule formation 

 and nitrogen fixation. Influence of environmental conditions. Importance 

 of symbiotic nitrogen fixation in the soil. Associative action of legumes 

 and non-legumes 558 



Chapter XXIII 



TRANSFORMATION OF SULFUR BY MICROORGANISMS 



Sources of sulfur in the soil and processes of transformation. The nature of 

 oxidation of sulfur and its compounds in the soil. Reduction of sulfur and 

 its compounds. Formation of H2S in the decomposition of organic matter. 

 Sulfur oxidation and transformation of minerals 600 



PART D. SOIL MICROBIOLOGICAL PROCESSES AND SOIL FERTILITY 



Chapter XXIV 



THE SOIL AS A MEDIUM FOR THE GROWTH AND ACTIVITIES OF MICROORGANISMS 



The soil as a culture medium. Soil composition and microbiological activi- 

 ties. The mineral composition of the soil. The physico-chemical role of 

 organic matter in the soil. Colloidal condition of soils and microbiological 

 " activities. Soil solution. Soil reaction and microbiological activities. 

 The soil atmosphere. Soil temperature. Growth of microorganisms in soil 

 in pure and mixed culture. The idea of a soil population 619 



