270 Studies in Forestry [CHAP. xn. 



being blown about so as to form dunes of shifting sand, and in 

 order to hinder the formation of a thin impermeable subsoil layer 

 of xnoorpan by dry heather mould, quartz-sand, and a slight 

 admixture of oxide of iron, the retention of a fairly good canopy 

 and the preservation of all the foliage for humification are 

 essential. When stretches of moorpan soil have been reclaimed 

 by subsoil ploughing and planting up with Scots Pine, the 

 undecomposed layer of heather-humus is often turned into 

 mould of a more ordinary nature. And on reclaimed dunes, 

 the layer of dead needles under the Pine trees not only acts 

 as a mechanical hindrance to the lifting of the sand, but 

 yields to it the beneficial humus that helps to bind it and 

 to keerj it moist. Such light sandy soils are unfortunately 

 too poor to bear densely foliaged crops, or to permit of any 

 underwood being formed with a view to the improvement of 

 the soil. 



V. The Manner in which the Relation of the Soil 

 towards Warmth may be affected by the Nature 

 of the Crop or the Method adopted for its 

 Treatment. 



The manner in which woodland soils respond to variations 

 in the atmospheric temperature is determined more especially 

 by the quantity of moisture contained in them, than by the 

 specific warmth of the different kinds of soil. Colour, Conduc- 

 tivity, Porosity, Composition, and other physical factors all 

 exert direct influence in this direction ; whilst the nature of the 

 soil-covering is also of no little importance. That the quantity 

 of moisture contained is of so much influence may be easily 

 understood from the great latent power that has to be overcome 

 in effecting changes in the temperature of water. And as 

 regards colour, soils prove no exception to the law of radiation 

 and absorption, viz. that dark bodies respond more quickly to 

 variations than those of a similar texture but lighter in colour. 



