What is Bethany soil and how does it affect Oklahoma foundations?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Bethany is a very deep, well-drained soil of central Oklahoma whose subsoil is silty clay with about 35 to 45 percent clay. Oklahoma State University testing rated Bethany's subsoil moderate in shrink-swell, with a largely montmorillonitic fine clay, and noted moderate limitations for building sites. Homes on Bethany soils can move seasonally as the clay wets and dries.

Where does Bethany soil occur?

The USDA Natural Resources Conservation Service (NRCS) Official Series Description (OSD) places Bethany in central Oklahoma and Kansas, in Major Land Resource Area 80A, the Central Rolling Red Prairies ¹. The series covers about 525,000 acres, and its type location is in Oklahoma County about 1 mile east of Wheatland ¹. That puts it squarely in the OKC metro; see also soil series of the OKC metro.

Bethany forms on summits and backslopes of old terraces ("paleoterraces") at 1,000 to 1,500 feet elevation, on slopes of 0 to 5 percent ¹. Its parent material is Pleistocene loess (wind-blown silt) or alluvium lying over Permian shale ¹, the setting covered on the Hennessey Shale and Quaternary alluvium and terraces pages.

What are Bethany soil's key properties?

The "solum" is the upper, weathered part of the soil profile (topsoil plus subsoil). Bethany land is used for small grains, grain sorghum, cotton and alfalfa, with some pasture and rangeland ¹.

How much does Bethany soil shrink and swell?

What Oklahoma State University measured

A 1978 Oklahoma Agricultural Experiment Station bulletin studied Bethany silt loam as one of five benchmark soils of western Oklahoma ⁴. The researchers found:

• Clay rising to about 38.5 percent below the surface ⁴.
• A coefficient of linear extensibility (COLE) reaching 0.095 in the most expansive horizon ⁴.
• Fine clay that is largely montmorillonite, a smectite clay mineral that swells when wet; coarse clay was a mix of illite and montmorillonite ⁴.
• Shrink-swell rated low in the surface layer and moderate in the subsoil, with moderate limitations for building sites and severe limitations for septic filter fields ⁴.

For comparison, NRCS shrink-swell classes put linear extensibility under 3 percent (COLE under 0.03) as low, 3 to 6 percent as moderate, 6 to 9 percent as high and 9 percent or more as very high ³. The rated value for each map unit appears in Web Soil Survey ².

How it shows up in houses

Clay soils swell (heave) when they gain moisture and shrink when they dry, moving shallow foundations up and down ⁵. Uneven moisture, such as a dry edge next to a wet one, does more damage than uniform changes ⁵. Typical signs include stair-step cracks in brick, sticking doors or windows and sloping floors ⁵. These patterns are covered in perimeter drying settlement and perimeter wetting heave.

Drought and wet cycles

The Oklahoma Mesonet measures soil moisture at depths of 2, 4, 10 and 24 inches at most of its sites, which shows how deeply rain soaks in and how fast soils dry ⁶. These readings help explain why movement on clay soils such as Bethany often follows drought and wet spells; see drought and wet cycles.

What should you know about a Bethany lot?

• Look up the map unit and its linear extensibility in Web Soil Survey ².
• Keep soil moisture next to and under the foundation as even as practical; the ASCE guide calls this the single most useful step a homeowner can take ⁵.
• Grade soil away from the house and repair water or sewer leaks promptly, since leaks can cause localized movement ⁵.
• Plant trees farther from the foundation than their mature height ⁵.

Related

• Shrink-Swell and Active Zone
• Clay Mineralogy
• Soil Series - OKC Metro
• Heave vs Settlement
• Soil Series Index