190 SEEDING AND PLANTING 



under irrigation in the semiarid regions of California for the first 

 one or two years after planting. 1 After the trees are once well 

 established on suitable sites they thrive without further irriga- 

 tion. 



6. IMPERVIOUS SUBSOIL 



A hard impermeable substratum, 4 feet or less below the surface, 

 is injurious to forest growth, and the closer it is to the surface the 

 more harmful it becomes. An impermeable substratum may be 

 a compact layer of clay or a layer of sand or gravel intermixed 

 with clay, organic matter, or iron oxide. It is sometimes formed 

 by a deposition of lime and mineral salts at a variable distance be- 

 low the surface. This impermeable layer is called " pan " or "hard- 

 pan." Sometimes it occurs in loose, sandy soil under the action 

 of humic acid. 2 The pan, however formed, seriously interferes 

 with forest growth by obstructing the free movement of soil 

 water. It prevents seepage to lower levels and checks the ascent 

 of water by capillary action during dry weather. The soil above 

 the pan is alternately too wet and too dry for forest growth. 

 This hard, impervious layer also interferes with normal root de- 

 velopment and increases the danger from windfall. Artificial 

 regeneration over a pan is seldom successful. 



All methods of soil treatment to overcome the bad effects of 

 the pan are expensive and can rarely be undertaken in the recla- 

 mation of land for forest growth in the United States. For the 

 most part, when it interferes with forest growth, shallow-rooted 

 species must be used in the regeneration and low increment 

 should be expected. The only effective method for overcoming 

 its injury is by breaking through the impermeable layer, thus 

 effecting a more uniform distribution of soil moisture and deeper 

 root penetration. 



In European practice, when the pan is not too hard or too far 

 below the surface, it is usually broken with heavy subsoil plows 

 specially constructed for this purpose, or by trenching- 3 The 

 pan is brought to the surface where it disintegrates under the 

 action of sun and air. If it is 18 inches or more below the surface, it 



1 McClatchie, A. J.: Eucalypts cultivated in the United States. (U. S. 

 Forest Service, Bui. 35. 1902.) 



2 Schlich, Wm.: Manual of forestry, vol. II, p. 136. London, 1910. 



3 Ibid. 



