TILLAGE AND STRUCTURE 



111 



This process has several interesting illustrations. 

 In concrete work, the freezing of the material in the 

 wet state, before it has an opportunity to harden, is 

 recognized as decidedly injurious to the strength of 

 the wall. The development and action of the ice crystals 

 may be readily observed in the freezing of any thoroughly 

 wet soil in winter. Such examples are shown in Figs. 31, 

 32 and 33. 



To a much less extent, expansion and contraction 

 of the soil mass caused by variations in temperature 

 may contribute to the formation of granules. Any 

 movement of the particles will tend to produce changes 

 in the cohesive forces, and when the particles can move 

 easily they are drawn together by this attraction. 



46. Tillage.- The effect of tillage upon soil structure 

 is to produce lines of cleavage, and these, when produced 

 by plowing, are multitudinous, and quite uniformly 

 distributed. As pointed out by King, plowing when 

 the moisture content is suitable tends to break the 

 soil into thin layers, which move one over the other, 



Fia. 34. Plow with interchangeable moldboard and share, which adapts 

 it to different kinds of plowing The plow tends to shear the soil into thin 

 layers which are very thoroughly broken up. 



