184 SOILS: PEOPEETJES AND MANAGEMENT 



120, Soil tilth. — The previous data and discussion 

 have clearly shown the very great importance of a crumb 

 structure in the working of the soil in the field. Since 

 good physical condition will reflect itself on crop yield, 

 it is evident that structure must ultimately be considered 

 in relation to all plant growth. This relationship is 

 usually expressed by the term tilth. While , structure 

 refers to the arrangement of the particles in general, and 

 granulation to a particular aggregate condition, tilth 

 goes one step further and includes the plant. Tilth, then, 

 refers to the physical condition of the soil as related to 

 crop growth. It may be poor, medium, good, or excel- 

 lent, according to circumstances. Good tilth may de- 

 mand in some soils maximum granulation, in others 

 only a medium development. Maximum tilth always 

 implies the presence of water, since the best physical 

 relationships cannot be developed without optimum 

 moisture conditions. 



From the curves already presented, it is evident that 

 an optimum moisture condition exists for the proper 

 tillage of a soil, especially one of a heavy character. Also, 

 an optimum moisture condition must exist for proper 

 tilth, and therefore for proper plant development, since 

 adequate tilth is the best physical condition for crop 

 growth. Practical experience and theoretical evidence^ 

 have shown that these two optimum conditions are 

 identical in nearly all cases, a happy coincidence in the 

 practical management of a soil. The optimum moisture 

 condition for plant growth, then, is the proper moisture 

 condition for effective plowing. The optimum condition 



^ Cameron, F. K., and Gallagher, F. E. Moisture Content 

 and Physical Condition of Soils. U. S. D. A., Bur. Soils, BuL 

 50, p. 8. 1908. 



