MODIFICATION OF SOIL STRUCTURE 103 



absorption, or adsorption, as it is sometimes called, 

 is different from chemical absorption, later to be men- 

 tioned, with which it is closely associated. As a result 

 of this property, gases and materials in solution in the 

 soil moisture are attracted to and, loosely held by the 

 surface of the soil particles. It varies with the extent of 

 surface exposed, and is consequently greatest in fine- 

 textured soil. In clay soil, which has a relatively large 

 surface area, it is very large, and is an important factor 

 in the retention of fertilizers. 



42. Conditions affecting structure. The arrange- 

 ment of the particles in a soil may be modified in many 

 ways. Some conditions tend to produce the compact 

 separate-grain structure, while others favor the granular 

 or crumb structure. 



It has been suggested by Whitney, King and others, 

 that much of the formation of granules in the soil is 

 due- to the contraction of the moisture film around the 

 particles, when, for any reason, the moisture content 

 is reduced. It is known that the soil particles tend to 

 be drawn together by this reduction in the soil moisture. 

 Add to this some influence to determine the size of the 

 granules and a binding material to permanently hold 

 the granules together, and the essential conditions for 

 the granular conditions of soil are realized. Several 

 natural conditions, and the various tillage operations, 

 probably exert their influence on granulation in this 

 way. Warington attributes granulation to unequal 

 expansion and contraction of the soil mass, due to the 

 unequal imbibition and loss of water. In such a soil, 

 the cohesive force being different in different parts, 



