x.] CLAY SOILS 285 



compound silicates of primitive rocks, and the reduced 

 percolation which they permit, all clays are compara- 

 tively rich in soluble mineral salts. Many of them show 

 crystals of selenite (CaS0 4 2H 2 0) in the subsoil com- 

 paratively near to the surface ; magnesium sulphate is 

 often abundant, and strongly impregnates the water 

 obtained from the wells or the occasional springs to be 

 found in the clays. In the Weald of Kent the shallow 

 wells in the clay yield water that is almost undrinkable, 

 containing, as it does, from 150 to 450 parts of dissolved 

 matters per 100,000, consisting chiefly of the sulphates 

 (with some chlorides) of magnesium and calcium. The 

 sulphates often originate from the oxidation of finely 

 divided iron pyrites. The presence of ferrous salts 

 and other unoxidised iron compounds has already 

 been alluded to as a source of sterility in clay soils 

 particularly where the subsoil has been incautiously 

 brought to the surface. In the cultivation of all land 

 it is important to keep the surface soil on the top, and 

 to attempt to deepen the staple with care ; but this is 

 particularly the case with clays, where the land may 

 easily be injured for years by over-deep ploughing. No 

 soils show more marked change than the clays do in 

 passing from soil to subsoil, both in chemical com- 

 position and physical texture. 



Many clay soils, especially when undrained, possess 

 a great tendency to accumulate hydrated ferric oxide 

 some few inches below the surface, at about the level 

 to which the soil is ordinarily aerated. This deposit 

 sometimes forms a continuous layer or " pan " ; in drier 

 climates it becomes a kind of " crowstone " gravel, made 

 up of little nodules of hydrated oxide of iron, contain- 

 ing also manganese. This material frequently forms 

 a serious obstacle to cultivation, and requires to be 

 broken up with a crowbar or a subsoil plough before 



