Plastic Limit of Soil

The plastic limit is the water content at which a soil, when rolled into a 3 mm thread by hand, just begins to crumble. Below this moisture content the soil behaves as a semi-solid; above it, as a plastic material.

Together, liquid limit and plastic limit give the Plasticity Index (PI = LL − PL), which is used in the IS soil classification system (IS 1498) to distinguish clays from silts and to judge how sensitive a soil will be to changes in moisture.