WETTING AND DRYING, AND STRUCTURE 107 



required to sink a knife-edge into a puddled clay soil, 

 different samples of which were subject to drying and 

 rewetting a different number of times. 



1. Soil dried once 100.00 



2. Soil dried twenty times 31.44 



3. Soil dried twenty times 30.60 



4. Soil dried twenty times 32.05 



Average 31.40 



* 



From this table it appears that the effect of twenty 

 times drying is to reduce the force necessary to pene- 

 trate the soil a given uniform distance to one-third of that 

 for the untreated sample. This is certainly a large change. 



This fact has many practical applications. It should 

 be observed that the change in structure is not associated 

 with continual wetness, nor is it any more identified 

 with a continued dry state. In neither case is the force 

 necessary to change the structure brought to bear on 

 the particles. This is exerted in the drying process. 

 It is a well-known fact that soils which are continually 

 wet are usually in bad physical condition. In the drain- 

 age of wet land, it is found that the soil is at first very 

 refractory; but, when good drainage is established, 

 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 struc- 

 ture, 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 ways already noted. 



