12 SOILS AND FERTILIZERS 



of one grade. The clay particles are exceedingly 

 small ; it would take 5000 of the larger ones, if laid 

 in a line with the edges touching, to measure an inch, 

 while it would take but 50 of the larger medium sand 

 particles to measure an inch. 



5. Clay. The term clay used physically denotes 

 those soil particles less than 0.005 mm - (0.0002 inch) 

 in diameter, without regard to chemical composition. 

 As used in a physical sense clay may be silica, feld- 

 spar, limestone, mica, kaolin, or any other rock or 

 mineral which has been pulverized until the particles 

 are less than 0.005 mm - ^ n diameter. Chemically, 

 however, the term clay is restricted to one material, 

 as will be explained in another part of the work. The 

 physical properties of clay are well known. It has the 

 power of absorbing a large amount of water, and will 

 remain suspended in water for a long time. The 

 roiled appearance of many streams and lakes is 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 accumu- 

 lated surface water must be removed by the slow 

 process of evaporation. 



6. Silt. 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 per- 

 meable, but is not as open and porous as a sandy soil. 



