CONTENTS xi 



PAGES 



CHAPTER VIII 



Soil Formation 



Physical Agencies 72 



Temperature Changes 72 



Wind 73 



Running Water 74 



Action of Waves 75 



Ice 75 



Volcanoes 75 



Chemical Agencies 76 



Atmosphere 76 



Water 77 



Biological Agencies 78 



Bacteria as Soil Formers 78 



Calcium Carbonate 80 



Phosphorus 80 



Sulfur 81 



Iron 82 



Potassium • 82 



CHAPTER IX 



Soil TB'Xtur&y Structure^ and Comfosition 



Soil Texture and Structure 83 



Soil Classes 83 



Soil Texture 84 



Soil Structure 85 



Plant Requirements and Soil Composition 86 



Plant Requirements , 87 



Nitrogen 87 



Phosphorus 88 



Potassium 88 



Calcium 88 



Magnesium 89 



Iron 89 



Sulfur 89 



The Biological Agencies in the Soil 91 



Number of Bacteria in Soils 92 



Factors Influencing Numbers 92 



Variation with Depth 94 



Kinds of Microorganisms 95 



