308 AGRICULTURE 



because they are not so adhesive. All soils that are lacking 

 in humus tend to become dense and resisting in structure. 



Causes affecting soil structure. The chief adhesive 

 force holding soil particles together in clusters, grains, or 

 lumps is the water films that surround the particles. Each 

 separate particle is covered by a thin film of water, whose 

 effect is much the same as a film of rubber. Let a number 

 of small soil particles, each surrounded by its water film, 

 come into contact, and their individual films all merge into 

 one, and by its tension unites these particles in a single gran- 

 ule, or cluster. These clusters are in a similar way joined 

 into still larger clusters, and so on until, in fine clay soils, 

 one continuous mass is formed. As clay soils dry out the 

 films break, shrinkage occurs, and the surface cracks open. 



The greater adhesive power of clay soils comes from 

 the fineness of their particles. The larger the number of 

 particles in a given mass of soil, the greater the aggregate 

 surface of these particles, and hence the greater the amount 

 of water films needed to bind the particles together. Sandy 

 soils do not form into granules, or lumps, because the aggre- 

 gate surface of the particles is not sufficient to supply the 

 binding force of water films necessary to hold them together. 



It has been carefully estimated that the particles of a 

 cubic foot of soils of different textures have the following 

 amounts of surface: 



Coarse sandy loam 40,000 square feet. 



Sandy loam 65,000 " 



Silt loam 100,000 " 



Clay soil 150,000 " 



From these comparisons it is clear that the water films 

 are several times greater in area in clay soils than in sandy 

 soils, hence the tendency to adhesiveness in clay soils is 

 correspondingly increased. 



