ADII12S1VKNKSS Oi'' TllK SOIL. 185 



difficult to work. The state of dryness Las greut iufluence 

 oa 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 h;is 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 dryhig 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 per meal "le to water, must have the best 

 effects in making such soils easy to work. 



The English piactice 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 docs 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 Avater 

 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, 



