Cracked soil

Question: Why do some soils develop huge cracks when they dry and others don’t? What is it about the soil that causes this?

Vertisol showing large cracks during a dry period. Note the pocket knife for scale. Photo: Soil Science Society of America Marbut slide collection.

Answer: Some soils have a high content of clay minerals known as smectites. When smectite clays get wet, water moves into a space between the structural units that make up the clay mineral. The presence of the water molecules pushes the structural units apart, causing the clay mineral to expand, or swell.

When these clays dry out, the water molecules are removed from the inter-structural spaces and the clays shrink. When this shrinking takes place in millions of clay structural units in a volume of soil, the shrinking can be enough to create large cracks in the soil. Soils with a high shrink-swell clay content are known as Vertisols.

Vertisols are found in several places in the United States, including the Gulf Coastal Plain of Texas, the Mississippi River Valley from Illinois to the Gulf Coast, central Montana, western South Dakota, and the Red River Valley of North Dakota and Minnesota. Other major Vertisol regions in the world include central India, eastern Australia, and eastern Sudan and South Sudan.

–Answered by Eric Brevik, Dickinson State University

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