SOIL MANAGEMENT. 21 



" The main cause of plasticity is probably the presence 

 of gelatinous colloidal matter, together with a certain optimum 

 amount of water. These conditions facilitate the ready move- 

 ment of the particles, while at the same time sufficient force 

 is exerted to prevent the body from splitting apart at the 

 time of movement or when the pressure is removed or the 

 material dried. When highly developed, plasticity promotes 

 ease in puddling. The three factors that affect plasticity to 

 the greatest extent are texture, granulation and moisture. 

 The finer the texture the higher the plasticity, and the more 

 granular a soil the lower is the plasticity or the tendency to 

 puddle when ploughed. The amount of water is the third vital 

 factor. A soil will exhibit its maximum plasticity at a definite 

 moisture content. This point will be somewhere between the 

 flowing, or viscous, condition and the point at which a soil 

 refuses to mould, or, in other words starts to become crumbly. 

 With a soil such as clay, in which the plasticity is high, 

 ploughing should be done when the moisture condition is such 

 that there is no likelihood of puddling, and yet the soil will 

 turn over with a maximum granulating effect . . . 



" In general the greater the plasticity of a soil the higher 

 is its cohesion. Cohesion exists under two conditions — when 

 the soil is wet and when the soil is dry. When a soil is moist 

 cohesion is developed by the moisture films and the colloidal 

 materials that may be present (tenacity) ; when dry by the 

 inter-locking of the particles and the deposition of cementing 

 materials. The greatest force, however, is developed by the 

 drying and shrinking of the gelatinous colloidal material. As 

 a general rule the greater the amount of colloidal material, the 

 more firmly the soil is bound together when dry, or, in other 

 words, the greater is its cohesion. 



" Cohesion is important in tillage operations, in that soils 

 having a high co-efficient of cohesion tend to become cloddy 

 when ploughed and may thus be rendered poor in physical 

 condition. This may be avoided by timing the operation so 

 that the moisture content is somewhere above the point at 

 which excessive cohesion is exerted. As cohesion is not greatly 

 developed, except in a heavy soil, it is only when fine texture 

 is found that such a danger exists. The danger is a double 

 one, for, since high plasticity and high cohesion go together, 

 a soil ploughed when too wet may puddle, while one ploughed 

 when too dry may clod. ..." Every soil has a moisture 

 zone at which neither cohesion nor plasticity is excessive. 



