NATURE OF SOIL 231 



one, which by its tension unites these particles in a single 

 granule, or cluster. These clusters are in 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 cracks open. 



The greater adhesive power of clay soils is because of 

 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 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 sandy 

 soils, and hence the tendency to adhesiveness in clay soils 

 is correspondingly increased. 



Soil structure and tilth. You have seen some seed 

 beds that were granular or full of lumps, with very little 

 fine earth for packing about the seeds. Others are finely 

 pulverized, and favorable for plant growth. The condition 

 presented by the soil with reference to plant growth is 

 called its tilth. 



Tilth depends chiefly on soil structure. Dense heavy 

 soils that have a tendency to form into lumps when plowed, 

 or that easily bake after rain, make it difficult to maintain a 



