250 



Physics of the Soil. 



313. Subsoil Plow One of the most widely used forms 

 of sub-soil plow is represented in Fig. 87. It is intended 

 to be used in the bottom of an ordinary furrow, one plow 

 following the other in doing the work. 



Extremely good judgment is required in the use of the 

 subsoil plow to avoid puddling, which is sure to result from 

 using the tool when the subsoil is too wet. In humid 

 climates the dangers are greatest in the spring and least 

 in the fall, and it must be kept in mind that the surface 

 soil may be in good condition to plow when the subsoil is 

 much too wet. 



FIG. 87. Sub-soil plow. 



In semi-arid climates the dangers of injuring the soil 

 texture are much less and it is under such conditions that 

 subsoiling is likely to prove most profitable, tending as it 

 does to increase the available moisture for crop production. 



OBJECTS, METHODS AND TIMES OF PLOWING. 



314. Depth of Plowing. The best depth to plow at a 

 given time, on a given soil, for a given crop must be de- 

 cided on the spot after exercising good judgment with a 



