22 A MANUAL OF FORESTRY. 



The remedy is to break through the impermeable 

 stratum, so as to connect the upper and lower permeable 

 layers of the soil. According to the nature of the pan 

 and its depth below the surface one of the following 

 methods of treatment must be adopted : 



(a.) Deep ploughing. 



(b.) Trenching. 



(<?.) Raising the surface-level. 



If the lowest part of the pan is not deeper than 

 2 feet below the surface and not too hard, ploughing may 

 be adopted ; the pan, being brought to the surface and 

 exposed to the atmosphere, soon disintegrates. Pans 

 situated at a greater depth than 2 feet must be broken 

 through by trenching with spade, hoe, pick, or crow- 

 bar. In either case the operation is expensive, hence it 

 is usual to treat only part of the area, in strips, patches, 

 or holes. Strips may be 2 to 3 feet broad, separated by 

 unbroken ground 4 to 6 feet in breadth, so that the 

 actual work is restricted to about one-third of the area. 

 Patches may be of various size, down to holes about 

 12 inches square. 



If the pan is very thick and goes to a greater depth 

 than 3 feet, it may be cheaper to raise a portion of the 

 land by cutting ditches at intervals and placing the 

 excavated earth on the intermediate strips, thus pro- 

 viding the latter with a depth of soil sufficient for the 

 production of trees. 



The treatment of impermeable strata is always an 

 expensive matter, so much so in many cases that the 

 utilization of the area becomes altogether unprofitable. 

 In some cases the expense may be avoided, by planting 



