108 The Control of the Soil [pt. ii 



be grown on certain types of soil. In Fig. 28 are shown 

 mechanical analyses of the soils on which in the south- 

 east of England wheat and potatoes are found to do 

 well. 



Clay soils 

 There are two kinds of clay soils : 



1. Those that arise through the presence of 20 per 

 cent, or more of clay^. 



2. Those that owe their properties to the presence of 

 considerable amounts of fine silt. 



They are indistinguishable to the eye and have many 

 properties in common, but they have this important 

 difference: the "clay" can be flocculated by lime or by 

 exposure to frost while the "fine silt" cannot. Hence 

 the first group can be improved agriculturally by liming 

 but not the second: indeed so far the "silty" clays have 

 proved unmanageable. 



The first group are the typical clays and are widely 

 distributed in this country. The fine particles have cer- 

 tain properties which they impress on the whole soil: 

 they are sticky when wet but set very hard when dry: 

 they swell up on moistening and give out a little heat : 

 they absorb heat and shrink on drying, and thus cause 

 the large gaping cracks seen in dr}^ weather on clay land. 

 The fine particles also impede the movement of water so 

 that the soil is very wet in wet weather but may suffer 

 from drought in very dry weather. 



If the soil is not limed and the drains and ditches 

 are not well looked after, the clay tends to go into the 

 deflocculated form (p. 21) and then all the properties 

 just described are intensified. The soil becomes difficult 

 to cultivate owing to its persistent wetness: autumn 

 ^ I.e., particles less than 0002 mm. in diameter, see p. 16. 



