Contents xix 



PART II 

 FARM DRAINAGE 



CHAPTER XII 

 PRINCIPLES OF DRAINAGE (pages 415-466) 



PAGES 



TJie Necessity for Drainage. Removal of injurious salts 



Better soil ventilation Makes the soil more firm 416, 417 



The Demands for Air in the Soil. Supply of free oxygen 



To lessen denitrification Facilitates chemical changes. . 418, 419 



How Drainage Ventilates the Soil. Permits roots and bur- 

 rowing animals to go deeper Develops shrinkage 

 checks Favors granulation of soil Barometric and 

 temperature changes Suctional effect of rains 419-421 



Too Thorough Aeration of the Soil. Leads to destruction of 



humus Care of open soils 421,422 



Drainage Increases the Supply of Available Moisture for 

 Crops. Deeper root penetration Stronger capillarity 

 Stronger nitrification Deeper ground water more 

 available 422, 423 



Soil Made Warmer by Drainage. By lessening surface 

 evaporation By lowering specific heat Observed 

 differences of temperature 423-425 



Importance of Soil Warmth. Relation to germination 



Hastens development of plant -food 425-428 



Conditions under which Land Drainage Becomes Desirable. 

 Lands subject to frequent overflow Lands with strong 

 underflow near surface Tidal plains Flat lands with 

 heavy subsoils 428 



Origin of Ground Water and its Relation to the Surface. 

 Vertical movement of rains Surface of ground water 



Lines of flow Growth of rivers 429-435 



