230 AGRICULTURE 



cent, silt and clay combined. Middle western silt loam, 

 such as grows most of our best corn, is three-fourths silt, 

 and fifteen per cent, clay and twelve per cent. sand. River- 

 bottom, clay soils are slightly more than one-third clay, and 

 almost one-half silt. 



4. Structure of Soils 



Take a piece of clay in your hand. Try to crumble it 

 into small particles. Do the same with a piece of loam; 

 with a lump of sandy soil. Note that some soils plow up 

 in great clods, while others break up into small pieces, pro- 

 ducing what is called a mellow condition. Have you no- 

 ticed that in some places the ground cakes and cracks open 

 when it becomes very dry, while in other places it remains 

 soft and unbroken no matter how dry it becomes? These 

 differences are a matter of soil structure. 



Soil structure. By soil structure is meant the mode 

 in which particles adhere to one another, causing them to 

 cling together in solid masses hard to break up, or forming 

 but loosely joined lumps easily pulverized. 



Clay soils are of a heavy, dense, clinging structure, dif- 

 ficult to break apart, and hence hard to plow. Silt loams 

 and sand loams, on the other hand, are friable; that is, 

 they are easily broken up. They plow or pulverize easily 

 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 pf rubber. Let a number 

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

 come into contact, and their individual films all merge into 



