SPECIFIC GRAVITY 41 



drought, a precipitation of the colloid matter must take place 

 on the solid particles of the soil, and tend greatly to increase 

 the stability of existing groups. 



The striking effect of winter frosts upon clay land roughly 

 ploughed in autumn is well known to every farmer. The 

 precious tilth thus acquired must be carefully preserved. 

 The following spring cultivation must be done with grubbers 

 and harrows ; to plough the land again would be to bury 

 the fine surface soil obtained by winter exposure. When a 

 farmer ploughs heavy land in summer time he has generally 

 to wait till the clods have thoroughly dried, and been again 

 moistened by rain, before he can obtain the required tilth. 



These striking changes in the texture of a soil are easily 

 observed, the slower changes are not less important. A 

 perfect cure for an unworkable clay soil is to lay it down 

 to grass. The sod protects the soil from the injurious 

 puddling of the clay by heavy rain. The mischief done by 

 tillage operations ceases. The soil is left to itself, and to the 

 natural influences of the changing seasons. When the land 

 is again ploughed up, it is found that an excellent friable 

 condition of soil has been established. 



On old grass land the action of worms in increasing the 

 depth of friable soil is important. The worm-casts consist 

 of soil which has passed through the worm's body ; this soil 

 when ejected readily breaks down to a coarse powder. 

 Worms do little to increase the surface soil on arable land, 

 but on grass land the benefits of their action are very con- 

 siderable. 



Specific Gravity. The comparative weights of equal 

 volumes of various soil constituents are shown in the 

 following table. 



