THE SOIL PARTICLE 75 



amount of water they become sticky and impervious. On dry- 

 ing, they shrink with the absorption of considerable heat. On 

 wetting, again swelling occurs. The absorptive capacity of 

 clay material for water, gases, and soluble salts is very high, 

 due to the presence of colloidal material. 1 As material in a 

 colloidal condition is very finely divided, it is found largely 

 in the heavier types of soil. Some clays carry very large 

 amounts of material in a colloidal state. Silt possesses the 

 same properties of plasticity, cohesion, and absorption as does 

 clay, but to a less extent, because the particles of the former 

 are larger than those of the latter. The presence of silt and 

 especially clay in soil imparts to it a heavy texture, with a 

 tendency to slow water and air movement. Such a soil is 

 highly plastic, but becomes sticky when too wet, and hard 

 and cloddy when too dry. The expansion and the contraction 

 on wetting and drying are very great. The water-holding 

 capacity of a clayey or silty soil is high. Such soils are spoken 

 of as heavy because of their working qualities in the field in 

 contrast to the easily tilled sandy soils. 



42. The mineralogical and chemical characteristics of 

 soil separates. — From the mineralogical standpoint there 

 are often considerable differences between the soil separates, 

 especially when the sands and clays are compared. Quartz 

 would naturally be expected to persist and because of its low 

 solubility would very soon be dominant not only in the coarser 

 separates but in the silt and clay as well. Other minerals, 

 such as the feldspars, hornblende, mica, and augite being less 

 resistant would concentrate in the finer separates. This tend- 



1 The colloidal state — when material is in a very fine state of division, 

 approaching but not attaining a molecular condition (true solution), it 

 assumes certain characteristic properties, such as high absorption for 

 water, gases, and salts in .solution. It may also, under certain conditions, 

 cause a marked increase in plasticity and cohesion. The colloidal con- 

 dition is purely physical and depends -on fineness of division, the 

 particles being molecular complexes. Material in a colloid state is 

 heterogeneous and is dispersed through a second material called the 

 dispersive medium. 



