CHAP, vi.] Advantages of Mixed Woods 121 



beneficial influence in enhancing the productive capacity of 

 most classes of woodland soil. 



Pure forests make constant demands of one unvarying sort 

 on the land, so that on soils of merely average quality there is 

 far greater risk of one particular class of nutrients being rapidly 

 diminished and being less readily available than when a variety 

 of species of trees induces a more general demand for the 

 various different kinds of mineral food-substances. Thus, 

 whilst in general the chemical composition of dry woody- 

 substance of the different kinds of timber is, in round figures, 

 approximately constant as follows 1 : 



50 % Carbon, 42 % Oxygen, 6 % Hydrogen, i % Nitrogen, and i % 

 Ash (or mineral residuum), 



yet the nature of the substances found in the ash varies 

 greatly in the different species of trees. Thus, with regard to 

 lime, the principal constituent in the ash, there are 3-5 times 

 as much found in Oak, and 2-3 times as much in Beech, as 

 in the Scots Pine, which, however, exhibits more than Birch. 

 Taking the quantity contained in Scots Pine as unity, then 

 the amount of potash requisite is about 5 times greater in 

 Beech, 3^ times greater in Oak, z\ times greater in Silver Fir, 

 twice as much in Larch and Birch, and about 1} times as 

 much in Spruce. And again, with regard to Phosphoric Acid, 

 the wood of Beech contains in its ash about 2 \ times as much 

 as Scots Pine, Oak 3 times as much, Birch twice as much, 

 Larch and Silver Fir i \ times as much, whilst Spruce contains 

 considerably less. 



There can be little doubt that the theory enunciated by 

 Georg Ludwig Hartig, the father of Dr. Theodor Hartig, and 

 grandfather of Prof. Robert Hartig (of Munich), that the soil 

 can be most thoroughly utilized by an admixture of shallow- 

 rooting species with deep-rooting, in order that each may draw 



1 \Yeber, Die Aufgaben der Forstivirthschaft (Lorey's Handbuch, &c.), 

 1886, vol. i. pp. 72 and 62. More accurate details have already been 

 given in Chapter IV. pp. 82-85. 



