CHAPTER X 

 SOIL STRUCTURE 



While texture is the term used in reference to the size 

 of the particles in a soil mass, the_word structure is em- 

 ployed in reference to the arrangement of the grains. 

 The structural condition of the soil is very important to 

 plant growth, since the circulation of air and wate r are 

 so necessary to normal development. The structural 

 condition may be loose or compact, hard or friable, granu- 

 lated or non-granulated, as the case may be. Of these 

 conditions, granulation, especially in heavy soils, is of 

 vital importance, since it is really a summation of all 

 favorable structural conditions. By granulation is meant 

 the drawing to geth er of the small part icles around a 

 suitable nucleus, so that a crumb structure is produced. 

 The grains thus cease to function singly. The impor- 

 tance of such a structural condition on a heavy soil is 

 very obvious. The soil becomes loose because of the 

 larger units, air moves more freely, and water not only 

 drains away readily when in excess, but responds with 

 celerity to the capillary pull of the plant Before the 

 promotion of granulation and the factors that function 

 therein may be clearly discussed, however, two properties 

 of particular importance, especially in soils of fine tex- 

 ture, must be considered. These properties are plasticity 

 and cohesion. 



112. Plasticity. — Any material w T hich allows a change 

 of form without rupture, and which will retain this form 



170 



