196 SOILS: PROPERTIES A.Xh MANAGEMENT 



force, the share is sloping and is set to deliver a slanting 

 cut, and the mold board is long and gently inclined. This 

 allows the furrow slice to be turned with little granulation 

 and a minimum expenditure of energy. When maximum 

 granulation and pulverization are desired, the moldboard 

 is short and sharply turned, and the share is less sloping 

 and the cutting edge is less slanting. Such conditions 

 make for the development of more friction and the genera- 

 tion of those internal twisting and shearing stresses neces- 

 sary for good granulation. The sharper the bending of 

 the furrow slice, the greater are the internal stresses set 

 up. While the plow is the very best pulverizing agent 

 when optimum soil moisture conditions prevail, it is also 

 a most effective puddling agent when the soil is wet. 

 Therefore care in the judging of optimum conditions for 

 plowing is a most important feature in the maintenance 

 and encouragement of soil granulation and tilth. 

 / 130. Resume. — The factors controlling the struc- 

 tural onnrlitinn of any soil are found to be p lasticity and 

 .cohesion. As these increase, the tendencies of a soil to 

 puddle when wet and to clod when dry are augmented. 

 Therefore, in heavy soils a decrease in these factors is 

 advisable, through a careful control of moisture and a 

 bettering of the granular structure of the soil. Granu- 

 lation, while due to some extent to the localized influence 

 of the water film, is traceable largely to the colloidal 

 matter which acts as a binding agent. It is really a 

 concentration of the forces of cohesion and plasticity 

 around numberless localized foci. Granulation takes 

 place under the influence of wetting and drying, freezing, 

 plants and animals, addition of humus and lime, and 

 tillage operations, especially plowing. Due to the high 

 cohesion and plasticity of heavy soils, the moisture zone 



