134 MECHANICS. 



jured by excessive wetness, or by extreme drought, as 

 shallow ones. In addition to this advantage, they allow 

 a deeper range for the roots in search of nourishment. 



Soils are deepened by trench-plowing and by subsoiling. 

 In trench-plowing, the common plow with a mould-board 

 is made to enter the earth to an unusual depth, and to 

 throw up a portion of the subsoil, covering with it the 

 top-soil which is thrown under. A subsoil plow, on the 

 contrary, only loosens the subsoil, but does not lift it to 

 the surface. 



The Double Michigan Plow, just described, is strictly a 

 trench-plow, and is one of the best imj^lements for this 

 purpose. 



When the subsoil is of such a character that its mixture 

 with the surface tends to render the whole richer, trench- 

 plowing is best; but when of a more sterile character, it 

 should be only loosened with the subsoil plow, and more 

 cautiously intermixed with the richer portion above. 



It often happens that the subsoil plow is very useful in 

 loosening the soil for the purpose of allowing the trench- 

 plow to run more freely through it. 



The operation of the subsoil plow is shown in fig. 140. 



In using the subsoil plow the less the earth is raised, 

 provided it is well broken to pieces, tlie easier will be the 

 draught. The part which moves under the soil and per- 

 forms this loosening is of course in the form of a wedge. 

 If the subsoil is dry, hard, and not adhesive, a long and 

 acute wedge will run most easily ; but if the subsoil is 

 stony, a shorter wedge will succeed better. For general 

 purposes it should therefore be of medium length. 



Different modes of connecting this wedge to the beam 

 above have been adopted, each possessing its peculiar ad- 

 vantages. Fig. 141 represents a subsoil plow with a 

 single, broad, upright shank, cutting like a wedge, with 



