CHAP, xii.] Protection of the Soil 257 



the other hand, a deep layer of undecomposing leaves litters 

 the ground thickly, it prevents air from entering and circulating 

 within the soil, and induces an unfavourable condition with 

 regard to the moisture ; whilst it also tends to the development 

 of certain kinds of free acids of an injurious nature. Such 

 conditions indicate either wetness or want of aeration in the 

 soil, or an excessive density of canopy overhead. In our damp 

 insular climate, however, there is much less danger of the 

 productive capacity of the soil becoming endangered from this 

 cause, than from the opposite extreme, of having the timber- 

 crops more open and of a lighter canopy than is advisable for 

 the prudent husbanding of the means of stimulating the incre- 

 ment in the present crops, and for ensuring that those which 

 may succeed them will enter upon a heritage able to supply 

 the crops with sufficient supplies of nutriment. 



Various tree-crops show different results with regard to the 

 capital in nutrient salts that they leave behind them in the soil 

 when they have reached maturity and fall to the axe. As has 

 been pointed out in a previous chapter (see p. 86), conifers in 

 general, but more especially the Spruce, and in even a more 

 marked degree the Scots Pine, make relatively smaller demands 

 on mineral nutrients than broad-leaved deciduous species, 

 both with regard to their foliage and their timber. Hence 

 tracts of woodland can be greatly improved by the rotation 

 of a coniferous crop, when once the soil has become exhausted 

 and deteriorated either by the free play of sun and wind, or by 

 growing light-demanding, thinly-foliaged kinds of trees (like Oak, 

 Ash, Maple, Larch, and Pine, when once they begin to get 

 broken in canopy) without an admixture of shade-bearing, soil- 

 improving kinds like Beech, Spruce, and Silver or Douglas 

 Firs. Under the good canopy maintained by all the evergreen 

 conifers (for even the light-demanding Scots Pine has a good 

 canopy till about the twentieth to thirtieth year, when its 

 special demands for increased growing-space make themselves 

 apparent) the rich fall of easily decomposed needles forms 



