14 SOILS AND FERTILIZERS 



due to the presence of suspended clay particles. The 

 amount in agricultural soils may range from 3 to 50 

 per cent. Clay soils, if worked when too wet, become 

 puddled ; then percolation cannot take place, and the 

 accumulated surface water must be removed by the 

 slow process of evaporation. 



6. Silt. — The silt particles are, in size, between sand 

 and clay. Many of the western prairie subsoils, clay- 

 like in nature, are composed mainly of silt. The silt 

 imparts characteristics intermediate to sand and clay. 

 While a clay soil is nearly impervious to water, and 

 when wet works with difficulty, a silt soil is more 

 permeable, but is not as 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 de- 

 posit in ditches and drains, where the water moves 

 slowly, is mainly silt. 



7. Sand. — There are three grades of sand. The 

 characteristics, as permeability and non-cohesion of 

 particles, are so well known that they do not require 

 discussion. A soil composed entirely of sand would 

 have little, if any, agricultural value. Sandy soils 

 usually contain from 5 to 15 per cent, of clay and 

 silt. The relative sizes of sand, silt, and clay are given 

 in the illustration. 



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



