146 NATURE AND PROPERTIES OF SOILS 



be grouped as follows: plows, cultivators, packers and 

 crushers. 



81. The action of the plow. — The moldboard plow 

 brings about its effects because of the differential stresses set 

 up in the furrow slice as it passes over the share and the 

 moldboard. The soil in immediate contact with the plow sur- 

 face is retarded 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 proper 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 differ- 

 ential, due to the slope of the share and especially to the curve 

 of the moldboard. When the soil is to be turned over with 

 the least expenditure of energy, the share is sloping and is 

 set to deliver a slanting cut, and the moldboard is long and 

 gently inclined. This allows the furrow slice to be turned with 

 little granulation and with a minimum effort. When maxi- 

 mum granulation and pulverization are desired, the mold- 

 board is short and sharply turned, and the share is less slop- 

 ing and the cutting edge less slanting. Such conditions make 

 for the development of more friction and the generation of 

 those internal twisting and shearing stresses necessary for 

 good granulation. The sharper the bending of the furrow 

 slice, the greater are the internal stresses set up. Various 

 types of moldboards and shares designated for special soils 

 and particular operations are on the market. (See Fig. 25.) 



The disc plow is a sharp rolling disc set at such an angle 

 that it slices off and turns over the soil, pulverizing it fairly 

 effectively somewhat after the manner of the moldboard plow. 

 One advantage of the disc plow is its lighter draft, due to 

 a rolling rather than a sliding friction in the soil. In prac- 

 tice it is especially effective on very dry, hard soil. 



While the plow is the very best pulverizing agent when 

 optimum soil-moisture conditions prevail, it is also a most 



