CHAP, xii.] Protection of the Soil 267 



of which are species that require to transpire a considerable 

 amount of water through their foliage before they elaborate 

 the requisite amount of mineral food necessary for carrying on 

 active vegetation and building up their ligneous tissue. The 

 latter condition, however, implying an insufficiency of water for 

 the transpiratory requirements of many of the above species, 

 and even of the other less exacting kinds like Beech, sessile 

 Oak, Silver Fir, and Douglas Fir, and generally involving along 

 with this but scanty supplies of mineral food, necessarily limits 

 the choice of trees from among which it is for the time being 

 possible to form woodland crops. As has previously been 

 shown (see page 79), deciduous trees make far higher demands 

 on soil-moisture than conifers ; and among the latter the most 

 suitable species for forming crops on dry soils are the Pines, 

 more especially the Scots and Black Pines on sandy tracts, and 

 particularly the Austrian Pine on land of a limy character. 



In order that the most advantageous degree of soil-moisture 

 may be retained continuously throughout extensive woodland 

 areas, it is necessary that the soil should be protected as well as 

 possible from becoming exposed to the drying and evaporating 

 effects of sun and wind. This object may be best attained by 

 a judicious choice of the trees to form the crop, by avoiding 

 the formation of coppice-woods on soils tending to dryness, by 

 the maintenance of good leaf-canopy without overcrowding 

 (which would act most injuriously on the soil-moisture), by 

 regenerating the woods naturally in place of making large 

 clearances or clear fellings of all the trees on the annual fall, 

 to be followed by sowing a planting artificially, and by taking 

 the ordinary sylvicultural precautions for preventing the dead 

 foliage from being blown away and the normal process of 

 humification being interfered with for, owing to its extremely 

 favourable hygroscopic qualities, humus is of invaluable in- 

 fluence in absorbing, in retaining, and in regulating the dis- 

 posal of the atmospheric precipitations throughout the upper 

 layers of the soil. 



