1 6 SOILS AND FERTILIZERS 



three grades, fine, medium, and very fine. The chief 

 characteristic of sand is non-cohesion of particles. A 

 soil composed entirely of sand has little, if any, agri- 

 cultural value. Sandy soils usually contain from 5 

 to 15 per cent of clay and silt. The relative size of 

 sand, silt, and clay is shown in the illustration. In the 

 coarser grained sand, quartz predominates, while the 

 finer grained is composed mainly of other minerals. 



6. 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. 

 It may be silica, feldspar, limestone, mica, kaolin, or 

 any other rock or mineral which has been pulverized 

 until the particles are less than 0.005 mm - m diameter. 

 Chemically, however, the term ' clay ' is restricted to one 

 material, as explained in Section 74. The physical 

 properties of clay are well known. It has the power to 

 absorb large amounts of water, and will remain sus- 

 pended in water for a long time. The roiled appear- 

 ance of many streams and lakes is due to the presence 

 of suspended clay particles. The amount in agricul- 

 tural soils may range from 3 to 40 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. As clays dry, they shrink, become tena- 

 cious, and are worked with difficulty. Clay soils owe 

 their properties to the fineness of division of the par- 



