144 NATURE AND PROPERTIES OF SOILS 



is a flocculating action. This agglomeration, as already ex- 

 plained, is tlie drawing together of the finer particles of a 

 soil mass into granules. When calcium hydroxide is mixed 

 with water containing fine particles in suspension there is 

 almost immediately a change in the arrangement of the par- 

 ticles. They first draw together in light, fluffy groups, or floc- 

 cules, which then rapidly settle so that the supernatant 

 liquid is left clear or nearly so. This phenomenon is termed 

 flocculation, because of the peculiar appearance of the 

 aggregates. This flocculating tendency when lime is added 

 goes on in the soil as well as with suspensions, although more 

 slowly. In general, the lime tends to satisfy the absorptive 

 capacity of the colloidal material and by throwing down these 

 colloids develops lines of weakness. The cohesive power of 

 the soil is thus localized and agglomeration must necessarily 

 occur. The various forms of lime differ in their flocculating 

 capacities, calcium oxide and hydroxide being very active, 

 while calcium carbonate is relatively inactive in this regard. 



It must not be inferred that lime is generally added for its 

 flocculating influence. It is used primarily for other reasons, 

 the amounts applied being in general too small to have very 

 much influence on the structural condition of the soil. "War- 

 ington/ however, reports a statement of an English farmer 

 to the effect that by the use of large quantities of lime on 

 heavy clay soil, he was enabled to plow with two horses instead 

 of three. It is generally true that soils rich in lime are well 

 granulated, and maintain a much better physical condition 

 than soils of the same texture that are low in lime. 



80. Tillage.^ — Tillage aims to accomplish three primary 



^ Warington, E., Physical Froperties of Soi>ls, p. 33, Oxford, 1900. 



*For a very complete review of the theory and practice of plowing 

 and cultivation, with a complete bibliography: Sewell, M. C, Tillage: 

 A Review of the Literature; Jour, Amer. Soe. Agron., Vol. II, ISTo. 7, 

 pp. 269-290, Oct, 1919. 



The following books upon the mechanics of tillage may prove helpful: 



Davidson, J. B., and Chase, L, W., Farm Machinery and Farm Motors; 

 New York, 1908, 



The Oliver Plow Booh; South Bend, Ind , 1920. 



