50 An Account of the JSoil [pt. i 



sorted out to some extent and suffered changes on the 

 journey. The amount of sorting and the extent of the 

 change depend largely on climatic factors. 



Effect 071 the organic matter. The mass of mineral 

 particles formed by weathering of the rocks and sorting 

 out by subsequent agencies is not yet soil, although it 

 may be looked upon as the framework of the soil. But 

 it soon covers itself with vegetation which gradually 

 produces the remarkable results dealt with in Chapter 

 III and converts the mineral mass into a true soil. 



The character of the soil is very much affected by the 

 nature of the organic matter present, and tliis is largely 

 determined by the type of vegetation that grows there 

 and the extent to which the decomposition has pro- 

 ceeded. Now both these are cHmatic effects. Under 

 dry conditions the plants tend to be narrow-leaved and 

 tough e.g., pine needles, broom, etc. whilst under 

 moister conditions a larger more leafy type of vegeta- 

 tion arises. These two types of vegetation break down 

 in very different manner in the soil: the large leafy 

 plants yield a large supply of useful humus material, 

 while the shrubbier and more leathery plants of the dry 

 situation do not. There may be plenty of organic matter 

 in these dry soils; the light dry sands of the Sussex 

 heaths sometimes contain as much as 10 per cent, but 

 it exists in the form of undecomposed bracken fronds 

 and similar residues, and is of no agricultural value be- 

 cause it is not properly decomposed. 



Soil losses 



So far we have been considering only the building up 

 of the soil ; we have now to turn to the other side of the 

 account and study the losses that are going on. The 



