254 Studies in Forestry [CHAP. xn. 



important constituents required for the growth of trees it is 

 seldom the case that any given soil is deficient in any of these 

 minerals ; although it sometimes happens that it has not suffi- 

 cient moisture to enable it to offer them readily to the rootlets 

 in the shape of soluble salts, in which form alone they can be 

 imbibed by the suction-roots. With respect to the demands 

 they make on fertility, woodland crops differ essentially from 

 agricultural crops, and especially from the more exacting kinds 

 like tobacco, hops, and sugar-beet, which all demand the appli- 

 cation of manure in order to maintain the productive capacity 

 of farm-land at par, and to prevent its rapid exhaustion and 

 deterioration. In the majority of agricultural crops (hay, clover, 

 lucerne, barley, wheat) from 4 to 8 % of the dry tissue remains 

 behind as mineral ash on its combustion, and this even rises 

 to 1 7 % in the case of tobacco, hops, and sugar-beet ; whilst 

 for high-forest crops of timber the total requirements per acre 

 and per annum vary, according to Ebermayer's investigations, 

 with the different species of trees from 4 to 20 Ibs. as regards 

 lime, from 2 to 10 Ibs. as regards potash, and from about i to 

 4 Ibs. as regards phosphoric acid \ Say, for example, that any 

 given land contains only -05 % of potash in a soluble form 

 available for absorption throughout a soil of 1 8 to 20 inches in 

 depth ; then the total amount available per acre would be over 

 a ton and a half, or, roughly speaking, about 3,300 to 3,500 Ibs. 

 per acre (taking the specific gravity of the earth as 1-5). But as, 

 for Silver Fir, the tree which takes up most potash from the soil, 

 only about 10 Ibs. per acre are annually required for the forma- 

 tion of timber, even pure forests of this tree, if worked with 

 a rotation of 100 to 120 years, would only have withdrawn from 

 1,000 to 1,200 Ibs. by the time they attain maturity. They would 

 therefore still leave the soil with an abundance of soluble 

 potash for any kind of subsequent timber crop, even if in the 



1 Article in Forstlich-naturwissenschaftliche Zeitschrift, 1893, p. 227. 

 See also Chapter IV on The Nutrition and Food-supplies of Forest Trees, 

 p. 83. 



