150 NATURE AND PROPERTIES OF SOILS 



and to clod when dry are augmented. Therefore in heavy 

 soils a modification in these factors is advisable, through a 

 careful control of moisture and a bettering of the granular 

 structure of the soil. Granulation, while due to some extent 

 to the influence of the water film, is traceable largely to col- 

 loidal matter both mineral and organic. It is really a con- 

 centration of the forces of cohesion and plasticity around num- 

 berless localized foci. Granulation takes place under the in- 

 fluence of wetting and drying, freezing, plants and animals, 

 addition of lime and organic matter, and tillage operations, 

 especially plowing. The farmer exerts a modifying influence 

 on structure most efficiently by increasing the organic content 

 of the soil and by plowing. He is, of course, aided and abetted 

 by natural forces. 



Efficient tillage requires good judgment in the selection of 

 proper implements and mechanical skill in their operation. 

 It demands besides an understanding of the properties of soils 

 and a knowledge of their plant relationships. Sandy soils are 

 easily handled provided sufficient organic matter is main- 

 tained. Such cannot be said of clayey soils. Due to the high 

 cohesion and plasticity of heavy soils the moisture zone for 

 successful tillage is particularly narrow. The ability to detect 

 when this zone has been reached in a clay soil is one of the 

 essentials of its successful management. Another essential 

 is the effective widening of such a zone by granulation oper- 

 ations. 



The optimum moisture condition for tillage is generally near 

 the optimum condition for plant growth — a happy coinci- 

 dence, since by regulating the moisture content for plant devel- 

 opment conditions are rendered most favorable for all soil ac- 

 tivities. It is thus possible to produce in one operation that 

 desideratum in all soil physical operations, an optimum tilth. 



