xxii OUTLINE AND TABLE OF CONTENTS 



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e. Conductivity (328). 

 /. Circulation of air (329). 

 g. Water content (330). 



IV. Means of modifying the soil temperature 463 



G. External Factors in Soil-Management 465 



I. Means of modifying the soil 465 



a. Summary of practices (331). 

 II. Tillage 466 



a. Objects of tillage (332). 



b. Implements of tillage (333). 



1. Effect on the soil (334). 



2. Mode of action (335). 



(a) Plows (336). 



(1) Pulverization (337). 



(2) Covering rubbish (338). 



(b) Cultivators (339). 



(1) Cultivators proper (340). 



(2) Leveler and harrow type of cultivator(341). 



(3) Seeder cultivators (342). 



(c) Packers and crushers (343). 



(1) Rollers (344). 



(2) Clod crushers (345). 



III. Other phases of tillage operations 489 



a. Weeds in their relation to crop-production (346). 



1. Objectionable qualities of weeds (347). 



2. Control of weeds (348). 



b. Erosion (349). 



1. By water (350). 



2. By wind (351). 



IV. Adaptation of crops to soil 497 



a. Philosophy of crop-adaptation (352). 



b. Factors in crop-adaptation (353). 



1. Physiological requirements of the plant (354). 



2. Requirements for growth supplied by the soil (355). 



