The plasticity number or index of plasticity is an indirect measure of force required to mold the soil. The tensions of the water at the lower plastic limit and upper plastic limit are equivalent to pF values 2.8-3.3 and about 0.5 respectively. In the upper plastic limit the film of oriented water molecules becomes so thick that cohesion is decreased and the entire soil water mass flows freely under an applied force.
The moisture content of Atterberg limits depends upon the amount and nature of the soil colloids present. Orientation of particles and their subsequent sliding over each other takes place at this point, since sufficient water has been added to provide a film around each particle. It represents the minimum moisture percentage at which the soil can be puddled. The different Atterberg limits as stated above represents the moisture content of the change from the friable to the plastic consistency. Different soils are characterized by a specific plastic number or index of plasticity. If refers to the difference between moisture contents of a soil at its upper plastic limit and lower plastic limit.
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