48 SOILS AND MANUEES 



or less closely with those of the strata from which they 

 are derived, and may be judged, to some extent, from 

 the nature of the materials of which the latter are com- 

 posed. Thus, in general, soils derived from the Crag, 

 Eocene, Cretaceous, Permian and Devonian sands are light 

 or medium in texture, easily cultivated and usually fertile ; 

 in some cases they are very rich. Those derived from the 

 carboniferous, silurian and Cambrian formations are also 

 light in character, but not usually so fertile as those pre- 

 viously mentioned. The London clay, gault, weald and 

 Jurassic clays as a rule produce stiff cold land, but when 

 well drained and limed these soils are sometimes very pro- 

 ductive. Chalk and limestones generally yield soils of very 

 inferior quality. 



Soils of mixed character are usually richest in plant 

 foods and possess the most suitable physical properties. 

 For this reason loams are to be preferred to either sands 

 or clays, calcareous sands to either calcareous or sandy 

 soils, humous clays to either clay or vegetable soil. 



Soils derived from strata of mixed character are there- 

 fore generally more fertile than those formed from more 

 homogeneous materials. For example, the soils derived 

 from the lower beds of the London clay, in which a quan- 

 tity of sand is present, are generally more productive than 

 those derived from the upper portion, which consists almost 

 entirely of clay. The lower chalk, again, which is of a 

 marly character, produces some good fertile land_, whereas 

 the upper chalk, which is nearly pure, yields very poor 

 ^oils. 



For the same reasons, in districts where two formations 

 meet, the soils are always more fertile than those derived 

 from either formation alone. For example, where the chalk 

 mixes with the London clay, which lies above, or with the 

 green sand below it, richer and more productive soils 

 result than from either the London clay or the green sand 



