260 Studies in Forestry [CHAP. xn. 



The increase which may thus be made in the total quantity 

 of mineral nutrients stored up in an available form within the 

 soil cannot, however, be said to be directly due to any inherent 

 quality in tree-growth, but must be credited as primarily due 

 to indirect physical causes which give the sylviculturist an 

 opportunity of rectifying errors made during the past. 



Whenever any woodland soil deteriorates, it is almost 

 certain that one or other, and usually two, three, or even all 

 four of the physical factors 1 Cohesiveness or Tenacity, Rela- 

 tion to Moisture, Relation to Warmth, and Depth have been 

 affected in some prejudicial manner. These physical factors 

 determine, in a far greater degree than its mineral composition, 

 the capacity of any given soil for yielding to timber crops 

 abundant supplies of the nutrient salts requisite for their 

 normal growth, development, and reproduction. All these 

 physical factors, indeed, act and react so closely and essentially 

 on each other, as in point of fact to determine the general 

 quality of any particular soil for present sylvicultural utilization. 

 Whatever timber-crops, and whatever methods of treatment of 

 these, may be calculated to improve the condition of any one 

 of these factors, will therefore provided always that it be not 

 altered to such an extent as to prejudice the utility of any of 

 the other three factors be a step in the direction of conserv- 

 ing the productive capacity of the soil, and of stimulating this 

 when it is capable of being enhanced. 



As was explained at the close of the chapter on ' The Nutri- 

 tion and Food-Supplies of Forest Trees ' (see page 88), Coppice- 

 woods in general, and more especially those in which the 

 crops consist of Ash, Elm, Lime, Oak or Willow, genera of 

 trees making the highest demands on the soil for nutrients, 

 tend more towards the exhaustion of woodland soils than 

 Copse or Coppice under Standards in which only the under- 

 wood is cleared away periodically at comparatively short 



1 See Chapter V on The Characteristic Influences of the different Classes 

 of Soil, &c., p. 102. 



