xii CONTENTS 



CHAPTER II 



PAGE 



Absorption of Nitrogen from the Soil and the Air 50 



15. Sources of Nitrogen for Plants 50 



16. Absorption of Nitrogen from Soil 51 



17. Nitrates and Ammonium Salts as Sources of Nitrogen. Physio- 

 logical Acidity of Salts 54 



18. The Products of Nitrogen Assimilation and Its Connection with 

 Photosynthesis 55 



19. Assimilation of Atmospheric Nitrogen by Soil Bacteria 56 



20. Assimilation of Molecular Nitrogen by Leguminous Plants 59 



21. The Nitrogen Cycle in Nature. Artificial Nitrogenous Fertilizers. 63 



CHAPTER III 



The Absorption of Mineral Elements 66 



22. The Ash Constituents of the Plant. Their Composition and 

 Origin. The Indispensable and Minor Mineral Elements 66 



23. Importance of the Different Groups of Ash Constituents. Nutrient 

 Elements as Regulators of Vital Processes 68 



24. Sulphur and Phosphorus. Their Function in the Plant and their 

 Cycle in Nature 69 



25. The General Role of Cations and Their Specific Activity 71 



26. Toxic Properties of Single Salts. Balanced Solutions 74 



27. Mineral Nutrition of Plants 76 



28. Means of Determining Soil Fertility. Methods of Pot and Field 

 Experiments 80 



29. Yields as Determined by the Amount of Nutritive Substances. 

 Law of the Minimum. The Action of Growth Factors 84 



30. The Use of Mineral Substances by Various Plants. Absorption 



of Soil Nutrients at Different Stages of Growth 88 



31. Plants with Special Nutrient Requirements 91 



32. Effects of Hydrogen-ion Concentration of the Soil 94 



PART II 

 WATER RELATIONS OF THE PLANT 



CHAPTER IV 



Importance of Water in the Life of the Cell. The Cell as a 



Colloidal Osmotic System 101 



33. Saturation of the Cell with Water as an Essential Condition of 

 Life. The Water Content in Different Parts of the Plant 101 



