IMPORTANCE FOR WOOD- PROD QCTION'. 583 



By niixiiiy- soils of various degrees of compacity with humus, 

 a suitable degree of porosity may be secured, as is often seen 

 in lands periodically flooded. Where humus is produced, 

 especially in mountainous districts, it does not become mixed 

 with the soil, but merely covers it, rarely penetrating it more 

 than an inch or two. The admirable condition of the soil, 

 however, which is termed " fresh," is usually accompanied 

 by a suitable degree of porosity ; when fresh, a soil is as it 

 were raised and porous, without being saturated with water ; 

 whilst dry soil, unprotected by a soil-covering of litter and 

 humus, becomes more quickly compact and superficially hard, 

 the more exposed it is to be beaten down by the falling 

 rain-drops. 



Besides, as the particles of humus become more and more 

 oxidised and pass into fresh chemical combinations, such as 

 soluble salts, the process of diffusion causes movements within 

 the soil, which highly assist in increasing its porosity and 

 activity, whenever it is protected by a suitable soil- covering. At 

 the same time, this process is assisted by the decomposing dead 

 roots of felled trees remaining in the soil and by the tunnelling 

 of earthworms, mice, moles, lizards, grubs, insects, &c., 

 whenever the soil is protected by a soil-covering. 



(c) Maintenance of a steady temperature in the soil. — The 

 presence of the soil-covering acts further, in maintaining a 

 fairly steady temperature in the soil, a condition, the importance 

 of which should not be underrated for any trees, and especially 

 for those which are shallow-rooted. Just as the closed leaf- 

 canopy of a forest secures it from wide ranges of temperature, 

 when compared with cleared land, so also, extremes of temperature 

 in the soil are moderated by the soil-covering : the fact that 

 this is very beneficial to roots spreading in the upper layers of 

 the soil has been conclusively proved by the disastrous results 

 of removing litter. 



(d) Fertility of the soil. — Humus has finally most important 

 effects on the fertility of forest soil. It is well-known that 

 undecomposed humus does not afford sufficient nutriment for 

 plants, but nevertheless it influences the fertihty of the soil 

 very considerably ; in the first place, by its physical action on 

 soil, and secondly, by providing from its own decomposition 



