The Plastic Limit of Soils


Soil comes in a variety of forms that are beneficial or detrimental depending on the circumstances. In addition to the pH range and the organic soil content, soils can achieve different states depending on the soil water content. These states are solid, semi-solid, plastic and liquid. Experimenters have developed methods for testing the plastic limit of soil.


Soil that reaches a plastic limit begins to crumble. The soil plastic limit determines the soil compressibility, permeability, shrink swell, shear strength and compactibility. Soils with high compactibility can quickly decrease the air pockets found in the soil when heavy objects exert pressure on the soil, according to Thomas Tel Ford. Shear strength determines the soil's ability to resist parallel or tangential force. Shrink swell is a measure of how the soil shrinks or swells depending on its water content. Permeable soil allows water to pass through it.


The plasticity index is the range at which the soil exhibits plastic characteristics. High plastic characteristics indicate the soil has more clay, while low plastic characteristics indicate the soil has more silt, according to Texas A&M University. Thus, plastic soils tend not to drain water very well.


Testers measure the plasticity of the soil by taking 20 grams of the soil and placing the soil in a container on top of a wetter portion of soil. The testers then roll the sample with their palm or fingers on a glass plate, reform the soil into a ball and knead the soil a little bit to remove water, according to Texas A&M University. The testers repeat these steps until the soil crumbles. The testers determine the plastic limit by the amount of water released before the soil crumbles.


Testers can purchase tools that will help them test the plastic limit, according to Shambhavi Impex. The tools include a glass plate with ground ends and one frosted side, a brass or stainless steel rod with resistance to corrosion, a flexible spatula, moisture containers, a porcelain basin and a plastic wash bottle.


The plastic limit results give the testers an idea about the clay content of the soil, which impacts construction and agricultural decisions. Clay soils might require more tilling to increase the soil aeration so plants growing in the soil receive adequate water and oxygen. Also, the clay soils might cause construction problems since the soil will tend to shrink when not filled with water, which can damage constructions. Also, clay soil floods more often.

Keywords: plastic limit, plastic index, plastic characteristics, clay content, glass plate

About this Author

Charles Pearson has written as a freelancer for two years. He has a B.S. in Literature from Purdue University Calumet and is currently working on his M.A. He has written three ebooks so far: Karate You Can Teach Your Kids, Macadamia Growing Handout and The Raw Food Diet.