40 SOILS AND FERTILIZERS 



ditions being equal. This does not mean that a clay soil is 

 easier to keep in good tilth than is a loam soil, but under 

 favorable conditions the small particles have greater plastic- 

 ity and cohesion and hence form granules more readily. 



41. Wetting and drying. — As a soil becomes dry there 

 is a contraction of volume in which process lines of cleavage 

 or cracks occur and clods are formed. If these clods be 

 again wetted and partly dried without working, they will 

 separate into smaller clods and finally a granular structure 

 will be produced. This is illustrated by the greater ease 

 with which clods may be worked down after a rain and 

 partial drying, than when they remain perfectly dry. Land 

 in need of drainage is usually in poor tilth, while after drain- 

 age this condition gradually improves. 



42. Freezing and thawing. — The " heaving " of roots 

 during winter is an indication that frost has a disrupting 

 action on the solidarity of the soil. Roots are pried out 

 because the surface of the soil rises when freezing occurs 

 and sinks when melting takes place. Water that is held 

 between soil particles freezes when the temperature of the 

 surrounding soil falls below the freezing point. As water 

 freezes it expands, the effect of which is to force the particles 

 farther apart. The pressure applied by the freezing water 

 is very unevenly distributed. Around the larger water- 

 holding spaces the particles are moved farther than are 

 those adjacent to smaller spaces, because the larger the 

 body of water the greater the expansion when it freezes. 

 The uneven crowding of the particles causes a breaking up 

 of the soil into more or less separate masses and as this pro- 

 ceeds with repeated freezing and thawing there is a pro- 

 nounced formation of granules in a clay or loam soil. 



Fall-plowed land, if left unharrowed, or if too cloddy to 

 work down to a good tilth, will generally be mellow by spring, 

 provided there is much freezing weather during the winter. 



