ADHESIVENESS OF THE SOIL. 185 
difficult to work. The state of dryness has great influence 
on this quality. Sand, lime, and humus have very little 
adhesion when dry, but considerable when wet. Soils in 
which they predominate are usually easy to work. But 
clay or impalpable matter has entirely different characters, 
upon which the tenacity of a soil almost exclusively de- 
pends. Dry “clay,” when powdered, has hardly more 
consistence than sand, but when thoroughly moistened its 
particles adhere together to a soft and plastic, but tena- 
cious mass; and in drying away, at a certain point it be- 
comes very hard, and requires a good deal of force to 
penetrate it. In this condition it offers great resistance to 
the instruments used in tillage, and when thrown up by 
the plow it forms lumps which require repeated harrow- 
ings to break them down. Since the adhesiveness of the 
soil depends so greatly upon the quantity of water con- 
tained in it, it follows that thorough draining, combined 
with deep tillage, whereby sooner or later the stiffest clays 
become readily permeable to water, must have the best 
effects in making such soils easy to work. 
The English practice of burning clays speedily accom- 
plishes the same purpose. When clay is burned and then 
crushed, the particles no longer adhere tenaciously to- 
gether on moistening, and the mass does not acquire again 
the unctuous plasticity peculiar to unburned clay. 
Mixing sand with clay, or incorporating vegetable mat- 
ter with it, or liming, serves to separate the particles 
from each other, and thus remedies too great adhesiveness. 
The considerable expansion of water in the act of solid- 
ifying (one-fifteenth of its volume) has already been no-. 
ticed as an agency in reducing rocks to powder. In the 
same way the alternate freezing and thawing of the water 
which impregnates the soil during the colder part of the 
year plays an important part in overcoming its adhesion. 
The effect is apparent in the spring, immediately after 
“the frost leaves the ground,” and is very considerable, 
