i.] SOIL AND SUBSOIL 29 



the clay became mottled, grey and yellow mixed, till 

 at a depth of 4 feet practically the whole was a dark 

 blue unweathered clay, owing its colour to iron pyrites 

 and glauconite or kindred silicates of iron protoxide. 

 One of the greatest distinctions between soil and subsoil 

 lies in their respective texture ; in humid climates like 

 our own the soil is almost invariably composed of coarser 

 grains than the subsoil, though in arid climates soil 

 and subsoil appear to be almost uniform. This is due 

 to the rain, which constantly percolates through even the 

 stiffest soils and washes down the finest particles ; in 

 heavy rains also, water runs off the surface into the 

 ditches, carrying with it the finest particles of the soil 

 and leaving behind the coarser grains on the surface. 

 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 



