2(Ji TRANSACTIOXS OF ROYAL SCOTTISH AKBORICULTURAL SOCIETY. 



upon the nature and the condition of the materials composing 

 it.^ Organic matter amounts to 80 per cent, of the total 

 weight of the covering, dried in open air. Of that mass, the 

 greater part is made up of non-nitrogenous compounds. This 

 humic covering or leaf-mould absorbs a certain amount of 

 water, which is proportionate to its thickness, but becomes 

 more nearly constant as the thickness increases. Another 

 portion of the fallen water percolates into the subsoil. This 

 is greatest with a layer, 20 centimetres deep, of leaf-mould. 

 Evaporation is lessened in this layer, till, after a certain thick- 

 ness, it remains constant. Upon the temperature of the soil, 

 the dead covering also exerts a moderating influence, in that 

 it does not undergo so wide a thermal variation range as does 

 inorganic earth. 



In all these ways the materials of the covering and the humus 

 which arises from it constitute the natural manure of forest soil. 



However, this influence of the covering on the physical and 

 chemical constitution of deeper layers of the soil may become 

 injurious, if the detritus decomposes in unfavourable conditions. 



In a mould soil, there is first a superficial layer, ^ the most 

 decomposed and exhausted by drainage ; then a zone rich in 

 soluble food-stuffs, and yet deeper, the crude earth, very little, 

 if at all, modified. But where there is acid humic decomposition, 

 the dissolving and washing away of the salts is of much greater 

 import. Often there arises, thus, at no great depth, in the 

 layer of active decomposition, a hard stratum of agglutinated 

 humic substances which had first been dissolved and then 

 reprecipitated. This organic grit or pan (Ortstein-Alios) con- 

 stitutes an impermeable layer which ofters a mechanical obstacle 

 to the penetration of roots, water, or air into the soO beneath 

 it, and becomes very injurious to the vegetation and healthiness 

 of the country. Before a good many wells and pits had been 

 dug through the pans of the laudes of Gascony, this region, 

 now one of the healthiest of France, was noted for its barren- 

 ness and unhealthiness. 



From the foregoing, we see that humus deserves to rank 



' E. Ramann, Die Waldstreu unci Hirer Bcdeutung fiir Boden und Wald, 

 Berlin, 1890. 



'•^ E. Ramann, iMr Ortstein Uiid dhnliche Sectcnddrbildu/igen in den 

 Alluvial- und Diluvial-sanden, Berlin, 1886. 



