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 



Very Fine clay 



FIG. 5. Diagram illustrating the sizes of soil grains. The coarsest particles grade 



into the finest. 



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

 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 understood. 



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) Water (moisture) which in reality is a dilute solution con- 

 taining weak acids and small amounts of any soluble substance 

 found in the soil. 



(b) Soil organisms bacteria, fungi and worms. These have 



