I.] SOIL AND SUBSOIL 27 



below the usual limit of cultivation on arable soils, on 

 pastures at the depth to which the mass of the roots 

 penetrate. The most obvious difference between the 

 two lies in the comparative richness of the staple in 

 decaying vegetable matter or humus, which indeed 

 would be entirely confined to the surface layers were 

 it not for the decay of the deeper roots and the work of 

 worms. To the humus is also due the difference in 

 colour ; not only does the colour deepen towards black 

 as the proportion of humus increases, but by it the 

 sands and clay are to a greater or less extent bleached 

 through the removal of the iron oxides which colour 

 them, hence the inorganic material is lighter and duller 

 in colour in the soil than in the subsoil. In stiff clays 

 the subsoil often shows signs of imperfect oxidation at 

 comparatively slight depths. On an old pasture on the 

 Gault Clay a trench was dug, the top 3 inches were black 

 or nearly so and gradually changed to a stiff brown 

 loam which extended to a depth of 9 or 10 inches, 

 becoming lighter and more distinctively yellow as 

 the admixture of humus diminished ; below this depth 

 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 constantly percolating through even the 

 stiffest soils and washing 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 surlace. 



