SOIL STRUCTURE 189 



The fact illustrated above has many practical appli- 

 cations. It should be observed that the change in struc- 

 ture is not associated with continual wetness, nor is it 

 identified with a continued dry state. In neither condi- 

 tion is any force brought to bear on the particles. The 

 force is exerted only during the drying process and the 

 wetting process. It is a well-known fact that soils which 

 are continually wet are usually in bad physical condition. 

 In the drainage of wet land, it is found that the soil is at 

 first very refractory; but when good drainage is estab- 

 lished there is a gradual amelioration of the physical 

 condition, which is primarily a change in structure. On 

 the other hand, in a soil continually in a dry state there 

 is no change in granulation. The improvement of soil 

 structure, as a result of changes in the moisture content, 

 is dependent largely on lines of weakness in the soil mass. 

 Some of these are produced in the process of drying, and 

 others in the ways already listed. 



124. Freezing and thawing. — As will be seen in 

 the consideration of soil moisture, the water is distributed 

 in the fine pores of the soil. When it freezes it forms long, 

 needle-like crystals. This crystallizing force is very 

 great, amounting to about 150 tons when a cubic foot of 

 water changes to ice. In freezing, the crystals gradually 

 grow first in the larger spaces. During this process there 

 is a marked withdrawal of moisture from the smallest 

 spaces, so that the ice crystals in the large spaces may be 

 built up. The soil mass is separated by the crystals, and 

 as the result of even a single hard freeze a wet, puddled soil 

 is shattered into pieces. The repetition of this process by 

 subsequent freezing and thawing will further break up the 

 soil by creating new lines of weakness. The granulating 

 power of freezing and thawing is shown in the following 



