28 THE ORIGIN OF SOILS [chap. 



Naturally, this loss of the finer particles is greater as 

 the soil is more worked and made open to percolation 

 and washing ; to some extent it is counterbalanced by 

 the work of worms bringing the fine mould to the 

 surface from below, so that the difference is least in 

 an old pasture. Per contra, it is greatest in an old 

 garden soil, where the constant working and further 

 opening of the soil by the introduction of bulky manure 

 often results in so complete a washing down of all the 

 finer particles that the soil proper loses its power of 

 cohering, falls into dust when dry, and is popularly said 

 to be " worn out" 



In addition to its humus the soil is nearly always 

 richer than the subsoil in all the essential elements of 

 plant food, despite the fact that crops have been raised 

 on it for generations ; the crops, in fact, have been the 

 cause of the difference, for the deeper roots draw food 

 from the subsoil and leave it behind on the surface as 

 the plants decay. Potash is perhaps an exception in 

 this connection; being essentially a product of the 

 weathering of felspar, and removable from the soil by 

 water containing carbonic acid, it is often more abundant 

 in the comparatively unweathered subsoil. The richness 

 of the humus, its greater warmth and the freer access 

 of air also cause it to be more abundantly supplied with 

 those organisms which play such an important part in 

 preparing the food of the higher plants: as will be 

 seen later, subsoils become almost without living 

 organisms at a very slight depth. 



For all these reasons, — the absence of humus, and of 

 the organisms associated with it, the comparative poverty 

 in inorganic plant food, the presence sometimes of unoxi- 

 dised material, and on stiff soils the great change of 

 texture, — the subsoil is often comparatively unfertile 

 and may be almost barren. Desirable as it is to work 



