viii CONTENTS. 



CHAPTER VIII. 



SOIL AND SUBSOIL; CAUSES AND PROCESSES OF DIFFERENTATIATION 

 HUMUS, 120. Soil and Subsoil ill-defined, 120. The Organic and Or- 

 ganized Constituents of Soils, 120. Humus in the Surface Soil, 120. 

 Soil and Subsoil ; Causes of their Differentiation, 121. Ulmin Sub- 

 stances or Sour Humus, 122. Sour Soils, 122. Cultivation Induces 

 Acidity, 123. Humin Substances, 123. Porosity of Humus, 124. 

 Physical and Chemical Nature of the Humus Substances. Table, 124. 

 Chemical Nature, 125. Progressive Changes and Effect on Soils, 126. 

 The Phases of Humification, Wood to Anthracite; Table, 127. 

 Amounts of Humus and Coal formed from Vegetable Matter, 128. 

 Figure, From Port Hudson Bluff, 128. Conditions of Normal Humifi- 

 cation, 129. Eremacausis in the Arid Regions, 129. Black Earth of 

 Russia ; Kosticheff's Table, 130. Losses of Humus from Cultivation 

 and Fallowing, 131. Estimation of Humus in Soils; Unreliability of 

 Combustion Methods, 132. Grandeau Method, " Matiere Noire," 132. 

 Amounts of Humus in Soils, 133. Humates and Ulmates, 134. Mineral 

 Ingredients in the Humus, 134. Functions of the Unhumified Organic 

 Matter, 135. The Nitrogen Content of Humus, 135. Table for Arid 

 and Humid Soils, 136. Decrease of Nitrogen Content in Humus with 

 Depth, 138. Table, Russian River Soils, 139. Influence of the Original 

 Material upon the Composition of Humus, 139. Table of Snyder, 139. 

 Effect of Humus in rendering Mineral Plant Food Available, 140. 



CHAPTER IX. 



SOIL AND SUBSOIL (continued) , 142. ORGANISMS INFLUENCING SOIL-CONDI- 

 TIONS. BACTERIA, 142. Micro-organisms of the Soil. Bacteria, Moulds, 

 Ferments, 142. Numbers at Various Depths, given by Early Observers, 

 142. Investigations of Hohl ; Mayo and Kinsley. Tables, 143. Multi- 

 plication of the Bacteria, 144. Aerobic and Anaerobic Bacteria, 144. 

 Food Materials required, 145. Functions of the Bacteria, 145. Nitrify- 

 ing Bacteria. Figures, 146. Conditions of their Activity. Table, 146. 

 Effects of Aeration and Reduction, 147. Unhumified Organic Matter 

 does not Nitrify, 148 Unhumified Vegetable Matter, Functions in Soils, 

 148. Denitrifying Bacteria. Figures, 148. Ammonia-forming Bacte- 

 ria. Figures, 149. Alinit, 149. Effects of Bacterial Life on Physical 

 Soil Conditions, 149. Root-bacteria, or Rhizobia of Legumes, 150. 

 Figures of Root Excrescences and Corresponding Bacteroids, 152. 

 Varieties of Forms, 154. Mode of Infection, 154. Cultural Results, 

 155. Table Showing Increased Production by Soil Inoculation, 155. 

 Other Nitrogen-absorbing Bacteria, 156. Distribution of Humus in the 

 Surface Soil, 157. Fungi, Moulds and Algae, 157. Animal Agencies 

 Earthworms, Insects, Burrowing Quadrupeds, 158. 



CHAPTER X. 



SOIL AND SUBSOIL IN THEIR RELATIONS TO VEGETATION, 161. Physical 

 Effects of the Percolation of Surface Waters, 161. Chemical Effects ; 

 Calcareous Subsoils and Hardpans, 161. " Rawness " of Subsoils in 



