12 



SOIL COMPOSITION, CLASSES AND TYPES 



Silt particles are soil grains medium in size between sand and clay. 

 When moistened, silt has a velvety "feel, '^ but not sticky like clay. 



Sand, silt and clay as used here refer simply to size of individual 

 soil grains and nothing more. 



Organic matter in most soils occurs largely as a well-decomposed 

 residue, black or dark in color and coating the soil grains. Fre- 

 quently plant fiber of recent growth can be distinguished. 



Humus may be defined as a black, waxy, complex substance 



Coarse sondj 



tedium SaU 



Fine sandi 



Silt 



\ne silfi 

 . Coahse cjlay 



"&: 



Very Fine day 



FlQ. 5. — ^Diagram illustrating the sizes of soil gr^^ins The coarsest particles grade 



into the finest 



coating the soil grains, and which is derived from partial decay 

 of organic matter. All humus is organic matter, but all organic 

 matter is not humus. 



The term humus is sometimes used as a general term meaning 

 organic matter. In these chapters we shall use the term ''organic 

 matter" since that is more readily imderstood. 



Soil is a Complex Medium. — ^What we call soil is something 

 more than a mere mixture of sand, silt, clay and organic matter. 

 It is a composite, the framework of which is mineral matter. 

 Aside from organic matter it contains: 



(a) Wal&r (moisture) which in reahty is a dilute solution con- 

 taining weak acids and small amounts of any soluble substance 

 found in the soil. 



(6) Soi7 organisms — bacteria, fungi and worms. These have 



