PHYSICAL COMPOSITION AND CLASSES OF SOILS 89 



the same soil may be pervious to water and susceptible of success- 

 ful tillage. 



Boiling in water, freezing, working the wet soil, alcohol, ether, 

 sodium or potassium hydroxides, ammonia, and sodium or 

 potassium carbonates, cause the clay to swell and increase its 

 colloidal properties. These agencies, therefore, tend to make 

 clay soils more impervious, sticky, and difficult to work. Lime, 

 magnesia, bicarbonate of lime, certain acids, such as hydrochloric 

 or sulphuric, and certain salts such as sodium chloride, sodium 

 sulphate, calcium sulphate, coagulate the clay particles. These 

 substances, therefore, tend to make clay soils less sticky and more 

 permeable to water. 



If clay is washed and kneaded when wet, it becomes plastic 

 and sticky, and may be moulded into forms which retain their 

 shape and become hard and stony when dried and baked. 

 Advantage is taken of this property in the manufacture of brick, 

 earthenware, and chinaware, but it is not a desirable property in 

 a soil. 



Hydrated oxide of iron, and some of the other bodies which 

 occur in the clay separation do not have the binding properties of 

 true clay, and if present to any considerable extent, the soil may 

 not have the characteristics which would be expected from the 

 quantity of clay in it. Hilgard, 1 for example, finds that a certain 

 clay soil containing 40 per cent, of clay was scarcely as adhesive 

 as another soil containing 25 per cent, of clay, and not nearly as 

 sticky when wet as a third soil containing 33 per cent. clay. The 

 soil first named, however, was rich in ferric hydrate, a large por- 

 tion of which is probably in the clay particles. This accounts for 

 the behavior of the soil. Ferric oxide appears to diminish the 

 tenacity of a clay. 



Rivers which contain little lime are turbid from presence of 

 clay, but 70 to 80 mg. lime per liter precipitates the clay and leaves 

 the water clear. 



Calcareous clays are very sticky when wet, but many of them 

 disintegrate into a mass of crumbs on drying, thereby producing 

 1 The Soil, p. 100. 

 7 



