CH. ii] . Silts and Silty Soils 23 



smaller pore-spaces which retard the movements both 

 of air and water. Further, they show more tendency to 

 stick together. Whether or not they have distinct 

 chemical properties is not clear, nor is it always known 

 precisely from what minerals they arose. But they con- 

 stitute a large part of the soil, and have so characteristic 

 an agricultural effect that they are called by the special 

 name of silt. It is usual in this country to distinguish 

 two grades: silt, the particles of which vary in diameter 

 between 0-04 and 0-01 mm., and fine silt, the particles of 

 which range between 0-01 and 0-002 mm. in diameter, 

 but, as already pointed out, the distinction is rather one 

 of convenience than of reality. 



The fine silt differs in one important respect from 

 elay: it is not flocculated and rendered less sticky by 

 the addition of hme, or by frost or cultivation. Thus 

 if a soil contains sufficient fine silt its stickiness and 

 heaviness cannot usually be remedied by liming, or in- 

 deed by any method known at present. Such soils occur 

 on the Boulder Clays, the Lower Wealden Beds in 

 Sussex, and elsewhere, and they are always a source of 

 trouble : a good instance is seen at the Leeds University 

 Farm at Garforth. The simplest plan is to leave them 

 in grass, but even this device is not entirely satis- 

 factory. 



There is another type of rock which in places has 

 playecT a great part in the formation of soil. Chalk 

 covers a large area of the eastern half of England, in- 

 cluding portions of the counties eastwards of the line 

 joining Lincolnshire and Wiltshire. Chalk is a substance 

 of perfectly definite character entirely distinct either 

 from silica or silicates. It dissolves somewhat in water, 

 and still more readily in water containing carbon 



