SOIL STRUCTURE 195 



are bent. This disturbance of the existing arrangement 

 of particles puts in motion the two forces that have al- 

 ready been discussed, the pull of the water film and the 

 binding power of the colloidal matter. The strength 

 of cohesion between small particles, such as clay, can be 

 realized when one considers the tenacity with which these 

 particles are held together in dried puddled soil. This 

 cohesive attraction is inversely proportional to the square 

 of the distance between the centers of the attracting bodies. 

 Particles that can be brought as closely together as can 

 clay particles may be thus held with great firmness. 

 The effect of tillage when an excess of water is present 

 is to force the particles into large masses and bring about 

 a generalized exertion of the forces of plasticity. The 

 soil then becomes puddled. Tillage when the soil is 

 too dry results either in clodding or in the soiFs becom- 

 ing so pulverized that it becomes puddled on whetting. 

 As already emphasized, proper pulverization by tillage, 

 especially by plowing, may occur only when the soil is 

 in optimum moisture condition. 



129. The action of the plow. — The plow brings about its 

 effects because of the differential stresses set up in the fur- 

 row slice as it passes over the share and the moldboard. 

 The soil in immediate contact with the plow surface is re- 

 tarded by friction, and the layers above tend to slide over 

 one another much as the leaves of a book when they are 

 bent. If the soil is in just the right condition, maximum 

 granulation results ; but if the moisture is too high or too 

 low, puddling or clodding may follow, especially on a 

 heavy soil. Not only does a shearing occur, but this 

 shearing is differential, due to the slope of the share and 

 especially to the curve of the moldboard. Where the 

 soil is to be turned over with the least expenditure of 



