SUBIRRIGATION AND SUBSOIUNG. 435 



immediately below that reached by the customary fre- 

 quent cultivation. It is more frequently observed in 

 orchards which have been subjedled for a few years to 

 irrigation and cultivation as ordinarily pradliced, and 

 seems to be a gradual process. A most urgent appeal 

 was made a few years ago to the United States Agri- 

 cultural Department for a solution of the mystery, but 



FIG. IIO — SUBSOIL PLOW. 



the matter was turned down completely with the usual 

 promptitude so charadleristic of that organization at 

 the time. 



As a matter of fa(5l the trouble is of most simple 

 solution, inasmuch as it is caused by a peculiar adlion 

 of certain chemicals in the soil superinduced by solar 

 influence, the result of which is not unlike the harden- 

 ing of mortar in a plaster bed when negledled by 

 workmen. Usually it exists four to five inches in 

 thickness just below the line of draft of plows or culti- 

 vating implements. That it is a very serious obstacle 

 to the irrigation of the soil to a proper depth can 

 readily be understood. In orchards underlaid by this 

 pan the trees in the hot summer months will wilt in 



