CONTENTS xi 



PART V. THE EARTH'S CRUST 



PAQK 



CHAPTER XXII. HOW ROCK BECOMES SOIL 279 



Questions for Discussion. The surface of the earth. The surface 

 and water. The work of freezing water. Disintegration of rock 

 by water. Glaciers and the earth's crust. Effect of plants and 

 animals. The soil and its significance. 



CHAPTER XXIII. PHYSICAL STRUCTURE AND FERTILITY 

 OF THE SOIL 288 



Questions for Discussion. Kinds of soil. Structure of the soil. 

 Size of soil particles. Plants and animals as soil-formers. Raw 

 materials for plant food. Loss of nitrogen from soil. The supply 

 of nitrogen. Value of the soil. 



CHAPTER XXIV. SOIL WATER, DRAINAGE, AND IRRIGATION 294 



Questions for Discussion. Amount of water in the soil. Soil water 

 and soil temperature. Amount of water used by crops. Soil water 

 as affected by tillage. Natural drainage of soil water. Artificial 

 drainage of soil water. Irrigation. Source of water for irrigation. 

 Dry farming. Reclamation of swamp lauds. 



CHAPTER XXV. EROSION AND SEDIMENTATION 304 



Questions for Discussion. Removal of the soil erosion. Loss 

 from water erosion. Vegetation and erosion. Deposit of soil from 

 erosion. Erosion by wind. 



CHAPTER XXVI. LIFE IN THE SOIL 312 



Questions for Discussion. Living things in the soil. Interrelation- 

 ship of living things in the soil. Bacteria of the soil. Soils and man. 



PART VI. LIFE UPON THE EARTH 



CHAPTER XXVII. THE PLANT COVERING OF THE EARTH 315 



Questions for Discussion. Abundance of plants. Variety among 

 plant formations. Value of wild plants. Cultivated plants. Plants 

 and soil. How plants secure water. Root structure. Root surface. 

 Water-carrying tissues. Root surface and transplanting. 



CHAPTER XXVIII. HOW FOOD IS USED BY PLANTS ... 328 



Questions for Discussion. Do plants need food ? The kinds of 

 foods. Where food is used. Method of transfer. Digestion of 

 starch. Digestion of other substances. Transference in the plant. 

 Food storage. Use of food by plants. Assimilation and respiration. 



