256 Studies in Forestry [CHAP. xn. 



render them available for the rootlets ; whilst the latter is itself 

 an indispensable nitrogenous food. But this direct action of 

 humus is in reality of secondary importance to its enormous 

 influence in improving the physical conditions of soil, more 

 especially with respect to the regulation of the soil-temperature, 

 and with regard to its power of absorbing and retaining moisture, 

 in preventing the rain from rushing off the surface and in allow- 

 ing it to permeate gradually into the subsoil. 



Though of itself affording no index to the mineral fertility 

 of the land, a favourable admixture of humus throughout the 

 upper layers of soils that are not of themselves fertile goes far 

 towards making them suitable for the more exacting kinds of 

 timber-crops. It is only when the normal progress of the 

 decomposition of the leaves and fallen debris is interfered 

 with by injudicious treatment of the crop such as the mainten- 

 ance of an excessive density of leaf-canopy in crowded planta- 

 tions, or by allowing the leaves to be removed for litter, or to 

 be blown away by the wind that there ever is any practical 

 danger of the humus and the upper layer of dead leaves having 

 an injurious effect upon the productive capacity of the woodland 

 soil which they cover. 



But as a matter of fact, when the formation of humus goes 

 on under the most favourable combinations of moderate warmth, 

 moisture, and atmospheric oxygen whose action is stimulated 

 by fungi like Cladosporium humifadens, Rostr., the decompo- 

 sition takes place so continuously that but a slight covering 

 of leaves shed during the past, and partially also during the 

 previous, autumn litters the ground. Where the soil is exposed 

 so freely that the benefits, which would otherwise be derivable 

 from the humus, are diverted from the tree-crops and unprofit- 

 ably consume*d by a soil-covering of grass or weeds, care should, 

 in the interests of the productive capacity of the soil, be taken 

 to maintain a thicker canopy of foliage overhead, and thus 

 keep down the rank growth of weeds by cutting off the supplies 

 of light and warmth requisite for their growth. But where, on 



