x iv CONTENTS 



PAGE 



Soil Air 94 



Importance of Soil Air, p. 94. 

 Organic Matter of the Soil 95 



The Uses of Humus, p. 95; Humus of Arid and Humid Soils, 



p. 96. 

 Life in the Soil 97 



Importance of Soil Organisms, p. 97; Soil-Bacteria, p. 97. 



Questions 100 



Laboratory Exercises 101 



Collateral Reading 108 



CHAPTER VI 



MAINTAINING THE FERTILITY OF THE LAND 109-153 



How Soils Became Productive, p. 109; How Rich Virgin Soils 

 Become Less Productive, p. 109; Causes of Decreased Produc- 

 tivity, p. Ill; The Limiting Factor in Crop Growth, p. 112; 

 The Amount of Plant Food in the Soil, p. 113; Value of 

 Chemical Analyses of Soils, p. 113; Materials Used as Fer- 

 tilizers, p. 114. 



Nitrogen 116 



Sources of Soil Nitrogen, p. 116; Nitrogen in Rainfall, 

 p. 116; Nitrogen Fixation by Bacteria on Legumes, p. 116; 

 Fixation of Nitrogen without Legumes, p. 119; Impor- 

 tance of Grasses, p. 120; Losses of Nitrogen from the Soil, 

 p. 121; Forms of Nitrogenous Fertilizers, p. 122; Nitrate of 

 Soda, p. 122; Ammonium Sulfate, p. 122; Dried Blood, Tank- 

 age and Bone Meal, p. 123. 



Phosphorus 123 



Forms of Phosphorus Fertilizers, p. 123; Phosphate Rock, 

 p. 123; Bone, p. 124; Thomas Slag, p. 124. 



Potash 125 



Forms of Potash Fertilizers, p. 125; Kainit, Muriate of 

 Potash and Sulfate of Potash, p. 125; Wood-Ashes, p. 125. 



Lime 126 



Functions of Lime, p. 126; Relation of Crops to Lime, 

 p. 126; How to Tell the Need of Lime, p. 127; Forms of Lime, 

 p. 127; Application of Lime, p. 128. 



