84 Studies in Forestry [CHAP. iv. 



In trees on which the bark long remains thick and fleshy, 

 larger quantities of mineral deposits are stored up in the rind 

 than in those which tend to the early formation of a rough 

 and rugged cortex. 



As the great bulk of the mineral matter is to be found 

 in the foliage of trees, it therefore follows that woodland crops 

 must make the greatest demands on mineral nutrients just at the 

 time when they develop the greatest density of leaf-canopy 

 that is to say, about the twentieth to thirtieth year for Scots 

 Pine, the thirtieth to fortieth year for Spruce, and the fortieth 

 to fiftieth year for Oak and Beech, on the better classes of 

 soil, or perhaps about a decade later on soils of poorer quality. 

 On the inferior classes of land it is therefore just about these 

 ages that crops of those particular kinds of trees will be most 

 likely to show signs of the soil not being able to supply easily 

 the normal demands made for fully adequate supplies of 

 nutrient salts. 



But, at the same time, the absolute demands in this respect 

 vary enormously in the different genera and species of trees. 

 For the production of any given volume of wood, Weymouth 

 Pine, Scots Pine, and Birch make relatively the smallest demands 

 on mineral nutriment ; after these follow Alder, Spruce, Silver 

 Fir, Hornbeam, Beech, and Oak ; whilst Ash and Acacia 

 make the greatest demands. 



The quantities of the different kinds of mineral nutrients, 

 required annually by the various species of woodland trees 

 for their normal production of wood, range between wide 

 limits for the different individual kinds of trees, as the 

 above table shows. But mere analyses of the timber alone 

 will give no useful key to the total annual requirements; 

 for, as has already been stated, the most of the mineral salts 

 absorbed from the soil are finally stored up or deposited in 

 the foliage during the process of assimilation, and are returned 

 again to the soil after the fall of the leaf. It may, however, 

 in passing, be again briefly remarked that, for the production 



