Source of nitrogen in the plant 40 Relation of the free nitrogen to leguminous plants 42-44 Relation of nitrogen in organic forms, as ammonia salts, and nitrates to the plant 46-50 Nitrification and its conditions 51 Ash constituents of the plant 53 Methods of research for ascertaining essentialness of ash constituents of plants 53 (_a_) Artificial soils, (_b_) water-culture 53-55 Method in which plants absorb their food-constituents 55 Endosmosis 55 Retention by soils of plant-food 57 Causes of retention by soils of plant-food 59 Manuring 60 "Field" and "pot" experimentation 60 PART II.--PRINCIPLES OF MANURING. CHAPTER I.--FERTILITY OF THE SOIL. What constitutes fertility in a soil 65 I. Physical properties of a soil 66 Kinds of soils 67 Absorptive power for water of soils 67 Absorptive power for water of sand, clay, and humus 68 Fineness of particles of a soil 69 Limit of fineness of soil-particles 69 Importance of retentive power 70 Power of plants for absorbing water from a soil experiments by Sachs 73 How to increase absorptive power of soils 74 Amount of water in a soil most favourable for plant growth 75 Hygroscopic power of soils 75 Capacity of soils for absorbing and retaining heat 76 Explanation of dew 77 Heat of soils 78 Heat in rotting farmyard manure 78 Causes of heat of fermentation 79 Influence of colour on heat-retaining power 80 Power of soils for absorbing gases 81 Gases found in soils 81 Variation in gas-absorbing power of soils 82 Absorption of nitrogen by soils 82 Requirements of plant-roots in a soil 83