IO4 



Studies in Forestry 



[CHAP. V. 



not be made soluble for imbibition by the suction-roots, nor 

 could transpiration through the foliage and assimilation possibly - ^ 

 take place; it assists likewise in regulating the ^temperature-^' '' 

 of the soil, by making stiff soils milder and loose soils more 

 binding. But excess of soil-moisture interferes with the 

 normal processes of decomposition, both of the soil and of 

 the humus ; it leads to the development of injurious acids and 

 acid solutions, and tends to the formation of marshes ; seed 

 and roots also suffer from want of oxygen or insufficient 

 aeration of the soil, and hence become easily infected with 

 fungoid diseases; whilst, in consequence of its poor con- 

 ductivity, and of the amount of heat necessary for raising the 

 temperature of water, it both retards the natural activity of the 

 soil, and increases danger from frost. Fineness of the soil- 

 particles favours hygroscopicity, or absorption and retention 

 of moisture ; whilst permeability, or diffusive capacity relative 

 to moisture, is proportional to their coarseness. Soils are in 

 these respects classifiable as follows : 



The addition of humus tends, however, to modify the 

 various characteristic differences exhibited in these respects. 



With regard to the degree of moisture they contain, the 

 following classification of soils has been adopted at all the 

 sylvicultural experimental stations in Germany : 



Wet, when water flows out, without pressure being applied, on a clod 

 being lifted up. 



