CHANGES OF THE BULK OF THE SOIL. 183 
very dry in summer they are slow to take up water again, 
so that 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. 
§ 7, 
CHANGES OF THE BULK OF THE SOIL BY DRYING AND 
FROST. 
The Shrinking of Soils on Drying is a matter of no 
little practical importance. This 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. The surfaces of moors often rise and 
fall with the 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 weather. 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 loam 
shrinks not inconsiderably on drying, yet the lines of sepa- 
