NATURAL FORCES. 23 



The extent to which they exercise their effect determines the 

 " yield capacity " or " quality " of the locaUty, 



(a) The Action of the Soil. — While the carbon required by trees 

 is principally taken from the atmosphere, the soil provides the 

 mineral substances and the water required for the formation of 

 wood. These are of the same kind as those required by field 

 crops, the most important being potash, lime, magnesia, and 

 phosphoric acid. As regards quantity, it has been proved that 

 an average forest crop requires only about one-half (54 per cent.) 

 of that necessary for an average field crop. The greater part 

 (46 per cent.) of the mineral substances is deposited in the 

 leaves, and, if these are allowed to decay in the forest, their 

 mineral substances are returned to the soil, so that only about 

 8 per cent, are removed with the wood. Consequently, exhaus- 

 tion of the soil is much slower if used for forestry than for the 

 production of field crops ; on the contrary, an improvement of 

 the yield capacity may take place under careful forest manage- 

 ment without artificial manuring. 



Whether, and to what extent, the nourishing substances in the 

 soil become available for the production of wood, depends on the 

 physical properties of the soil, such as its degree of density, the 

 extent to which heat and moisture are retained, and its depth. 

 The latter is essential for the development of a sufficient root 

 system, and it improves the physical properties generally. On 

 the degree of density depends a proper aeration of the soil and 

 the retention of moisture and heat. In this respect, the presence 

 of a suitable layer of humus is of great importance ; it loosens a 

 too firm soil and binds a too loose soil, in either case improving 

 the fertility. 



(b) The Action of the Climate. — ^The productive action of the 

 chemical and physical properties of the soil is possible only if the 

 climatic conditions correspond with the requirements of the 

 species of trees which it is proposed to grow. In addition to a 

 sufficient rainfall, a certain minimum of heat is necessary, both 

 as regards the average temperature of the year and that of the 

 growing season. The degree of heat also influences atmospheric 

 precipitations. 



The temperature of a locality is determined by the geogra- 

 phical position, the elevation above sea level and other local 



