CUANGES OF TllK BUI.K OF TIIK SOIL. 183 



very dry in summer lliey are slow to take up water again, 

 so tliat rain-water stands on the surface for a considerable 

 time without penetrating, and when, after some days, it 

 is soaked up, it remains injuriously long. Light rains 

 after drought do little immediate good to such soils, 

 while heavy rains always render them too wet and cold, 

 unless they are suitably ameliorated. The same is true to 

 a less degree of heavy, compact clays. 



CHANGES OF THE BULK OF THE SOIL BY DRYING AND 

 FROST. 



The Shrinkin^^ of Soils on Drying is a matter of no 

 little practical importance. Tliis shrinking is of course 

 offset by an increase of bulk when the soil becomes wet. 

 In variable weather we have therefore constant changes 

 of volume occurring. 



Soils rich in humus experience these changes to the 

 greatest degree. Tiie surfaces of moors often rise and 

 fall with tlie wet or dry season, through a space of sev- 

 eral inches. In ordinary light soils, containing but little 

 humus, no change of bulk is evident. Otherwise, it is in 

 clay soils that shrinking is most perceptible ; since these 

 soils only dry superficially, they do not appear to settle 

 much, but become full of cracks and rifts. Heavy clays 

 may lose one-tenth or more of their volume on drying, 

 and since at the same time they harden about the rootlets 

 which are imbedded in them, it is plain that these indis- 

 pensable organs of the plant must thereby be ruptured 

 during the protracted dry Aveather. Sand, on the other 

 hand, does not change its bulk by wetting or drying, and 

 when present to a considerable extent in the soil, its par- 

 ticles, being interposed between those of the clay, prevent 

 the adhesion of the latter, so that, although a sandy lonni 

 shrinks not inconsiderably on diyiiig, yet the lines of sepa- 



