3 o THE ORIGIN OF SOILS [chap. 



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 the soil 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 materia], 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 

 the subsoil and open it to the access of air and the free 

 penetration of roots, all methods of cultivation should 

 be avoided that would bury the surface soil and bring 

 the subsoil to the top. A plough which inverts the 

 soil should not go below the former limit of cultivation, 

 and if it is desired to deepen this limit, it should be 

 done by degrees, half an inch or so each year. Immense 

 damage has been done to the fertility of many of the 

 heavier soils by rash ploughing with steam, especially 

 where the old " lands " were thrown down, burying the 

 fertile soil in the furrows and baring the raw clay on 

 the tops of the ridges. 



General Classification of Soils. 



In considering any general classification of soils we 

 have to extend our survey far beyond the British 

 Islands, which contain only one group of soils, the 

 products of the action upon the varied range of under- 

 lying rocks of the characteristic temperate climate with 

 a moderate, evenly distributed rainfall. 



The most satisfactory general classification of soils 



