PHYSICAL PROPERTIES OF SOILS 1 7 



tides rather than to their chemical composition. Any 

 mineral when finely pulverized has physical properties 

 similar to clay. 7 



7. Silt. Silt is composed of a great variety of rock 

 fragments. The particles are, in size, between sand 

 and clay. Chemical analysis shows them to be more 

 hydrated than the clay particles. Many of the western 

 prairie subsoils, clay-like in nature, are composed 

 mainly of silt, which imparts characteristics intermedi- 

 ate to sand and clay. While a clay soil is nearly im- 

 pervious to water, and when wet works with difficulty, a 

 silt soil is more permeable, but is not so open and 

 porous as a sandy soil. When a soil containing large 

 amounts of clay and silt is treated with water, the silt 

 settles slowly, while the clay remains in suspension. 

 The fine deposit in ditches and drains, where the 

 water moves slowly, is mainly silt. Soils composed 

 largely of silt deposited by water and mixed with vege- 

 table matter are among the most fertile. 



8. Form of Soil Particles. Soil particles are ex- 

 tremely varied in form. When examined with the 

 microscope they show the same diversity as is observed 

 among larger stones. In some soils the particles are 

 spherical, while in others they are angular. The shape 

 is determined by the way in which the soil has been 

 formed, and also by the nature of the rock from which 

 it was produced. 



