284 SOIL TYPES [chap. 



clay soils naturally varies very much; the heaviest 

 clays occur in dry climates, where the percolation 

 has not been sufficiently great to wash away many 

 of the finer particles ; in the east and south-east of 

 England the Oxford and the London Clay, with the 

 Boulder clays derived therefrom, give rise to the 

 most stubborn and intractable clays. On these soils 

 the old practice of an occasional bare fallow is still 

 carried out, and is almost necessary to maintain the 

 soil in good cultivation. As a rule, the strong clay 

 soils have of late years been laid down to permanent 

 pasture; the cost and the difficulty of arable culti- 

 vation (for much wet weather in autumn or spring 

 may render it impossible to put horses on the land 

 for long periods), and the great fall in prices of both 

 wheat and beans, the staple crops of such land, have 

 rendered it necessary to resort to a cheaper method 

 of farming. 



Most clays carry good permanent pasture, because 

 the soil retains enough water to keep the grass growing 

 through any but the longer periods of drought ; in very 

 dry years, however, clay suffers severely from the 

 drought; the surface cracks and the subsoil dries 

 through the cracks ; the resistance also offered by the 

 close texture of the soil to the capillary rise of soil 

 water renders the winter rainfall le$s available to the 

 crop than on soils of lighter texture. The benefits 

 accruing from drainage, in making the soil dry more 

 quickly after rain and resist drought better, have already 

 been discussed. Certain clay soils may be found too 

 close textured to carry good pasture; the soil sets so 

 firmly that aeration becomes very defective, and the 

 vegetation degenerates into surface rooting, stoloniferous 

 grasses like Agrostis alba. 



Owing to their fine division, their origin from the 



