CONTENTS 



CHAPTER PAGE 



I. The Soil and the Plant 1 



The nature of the soil. Weathering of rocks. Effect of 

 cUmate upon chemical composition of soil. Soil formation. 

 Movement of water in soil. Organic matter of soil. Soil 

 phases. Role of microbes in plant growth. The nature of 

 plant nutrients. Absorption of nutrients by plants. Sum- 

 mar}'. 



II. The Microbe .\nd Its Activities 21 



The microbe and its importance in the soil. Nature of soil 

 microbes. Nutrition of soil microbes. Bacteria of the soil. 

 Activities of soil bacteria. The fungi of the soil. The 

 actinomyces of the soil. The algae of the soil. The protozoa 

 of the soil. Worms and insects in the soil. Soil organisms 

 causing plant and animal diseases. Symbiotic relationships. 

 Summary-: The complex soil population. 



III. The Soil Population and Its Distribution 44 



The occurrence of microbes in soil. Relation of microbes to 

 plants and animals. QuaUtative and quantitative distribution 

 of microbes in soil. Methods for determining numbers of 

 microorganisms in the soil. Methods for studying activities 

 of soil microbes. Summary of methods. Direct microscopic 

 methods. Plate method for counting microbes. Elective 

 culture method. Abundance of bacteria in soil. Influence of 

 soil conditions and treatment upon the distribution of bacteria. 

 Influence of organic matter. Influence of moisture. Influence 

 of reaction. Influence of season of year. Development of 

 actinomyces in soil. Development of fungi in soil. Develop- 

 ment of protozoa in soil. Development of algae in soil. 

 Lower invertebrates in soil. The activity of the soil popu- 

 lation as a whole. Summary. 



IV. Role of Microbes in the Decomposition of Organic Sub- 



st.\nces in the Soil 75 



Principles of decomposition of organic matter by microbes. 

 Composition of plant and animal substances. Factors affect- 

 ing decomposition. Decomposition of sugars and their deriva- 

 tives. Decomposition of hemicelluloses. Decomposition of 

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