134 SOIL PHYSICS AND MANAGEMENT 



loids consist largely of ferric oxide, ferric hydrate, silicic acid and 

 hydra ted aluminum silicate. These are- formed in the decomposition 

 of rocks. In the decomposition of most feldspars the silicic acid 

 and aluminum silicate are formed, but not all in a colloidal state. 

 Zeolites easily give rise to colloidal silica. While many substances 

 exist in a colloidal state in soils, yet the total amount is not large. 

 Warrington estimates it at never over two per cent. 



2. Clays and Clay Loams. Mineralogically clay is com- 

 posed largely of kaolinite, a hydrous aluminum silicate that is 

 formed from decomposition of aluminous minerals. In addition, it 

 may contain very finely divided particles of quartz, feldspar or other 

 minerals. In fact, clay may be composed entirely of other 

 minerals than kaolinite, although th;s is not usually the case. 

 Physically, clay consists of particles less than 0.001 mm. in diameter 

 (Hopkins), 0.005 mm. (Bureau of Soils) or 0.0023 mm. (Hilgard) 

 (see table on page 124). This is divided into two parts, which may 

 be called clay proper, consisting of particles large enough to be dis- 

 tinguished with the microscope, about 0.0001 mm. in diameter, and 

 a small amount of hydrous aluminum silicate whose particles are 

 very small and constitute part of the mineral colloids. 



(a) Tenacity. Tenacity is that quality of cohesiveness by 

 which substances resist disruption, imparting more or less stability 

 to them. In soils this property is due primarily to colloids. Clays 

 and clay loams, however, possess this property to a high degree. 

 Soils have been divided according to their tenacity into "heavy" 

 and " light." A " light " soil is one that works easily, as sand or 

 peat, and incidentally has a high specific gravity, as sand, or a low 

 specific gravity, as peat, but all possessing very little cohesiveness. 

 " Heavy " soils, on the other hand, are those containing a great deal 

 of clay, and hence possessing a high tenacity. Clays, clay loams, 

 and heavy silt loams and some sandy loams are examples of these. In 

 absolute weight they are not as heavy as the sand soils, but the 

 greater tenacity possessed by them makes them more difficult to 

 plow. Hence the term " heavy " is applied to them. 



A high moisture content decreases tenacity. However, a medium 

 amount of moisture imparts a high degree, as does also an extremely 

 small amount of moisture, as where the soil becomes dry and cloddy. 

 This is due to the hardening of colloids and the deposition of soluble 

 salts as a cementing material between the soil particle*. The tenac- 

 ity of " heavy " soils may be diminished by the addition of or- 



