What is Pratt soil and is it found in Oklahoma?

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

In short: Pratt is a deep, well-drained sandy soil formed in wind-blown sand on dunes, with thin clay-enriched bands called lamellae. It is mapped mainly in south-central Kansas, and its official description is unclear about Oklahoma, so the mapped soil at any Oklahoma site should be confirmed. Its very low clay content means foundation concerns center on loose sand, water and collapse testing rather than clay swelling.

What is Pratt soil?

Pratt is an official USDA soil series classified as "Sandy, mixed, mesic Lamellic Haplustalfs" ¹. "Lamellic" refers to its lamellae, and "sandy" describes its texture ¹. It formed in sandy eolian (wind-deposited) material on dunes on paleo-terraces, and also occurs on terraces and in some places on floodplains ¹. Slopes are 0 to 15 percent ¹.

Where it occurs

The official description places Pratt in south-central Kansas, in the Great Bend Sand Plains (MLRA 79), and calls it of large extent ¹. The series was established in Pratt County, Kansas, in 1910, and the type location is in Reno County, Kansas ¹.

Oklahoma's status is unclear in the record. The description's header lists "KS+OK" and a remark records a 1995 update of the series for Oklahoma, but the distribution section names only Kansas ¹. Web Soil Survey shows which soil is actually mapped at a given location ². Other sandy soils on this site include Tivoli and Dougherty.

What is its typical soil profile?

Depth	Layer	Texture	Clay
0-8 in	Ap	Fine sand	5%
8-24 in	Bt	Loamy fine sand with 11 lamellae of fine sandy loam	9%
24-64 in	E&Bt	Fine sand with 3 lamellae of loamy fine sand	3% (E)
64-80 in	C	Fine sand	5%

Source for the table: ¹. Other properties from the official description ¹:

Clay in control section: 4 to 11 percent; sand 80 to 90 percent ¹.
Drainage: well drained; saturated hydraulic conductivity rapid; runoff low ¹.
Climate: mean annual precipitation 19 to 32 inches ¹.
Use: gentler slopes farmed to grain sorghum, wheat and alfalfa; steeper slopes in native range ¹.

What does it mean for foundations?

Little clay, little shrink-swell

The USDA measures shrink-swell as linear extensibility, with under 3 percent rated low ⁵. With 4 to 11 percent clay and 80 to 90 percent sand in its control section ¹, Pratt lacks the clay content behind seasonal shrink-swell. The rated value for a mapped unit is in Web Soil Survey ².

Lamellae and water

Lamellae are thin layers, each under about 3 inches (7.5 cm) thick, where clay has accumulated in otherwise sandy soil, and they slow water movement ³. In Pratt, these bands sit within rapidly permeable sand ¹. Rapid conductivity means water from roofs and leaks soaks in quickly ¹. For how flowing water can carry sand out from beneath slabs and footings, see erosion, undermining and washout voids.

Windblown sand and collapse

Collapsible soils are unsaturated soils that lose volume when wetted, and engineering guidance lists windblown sand dunes and loess among the main collapsible deposit types ⁴. Pratt formed in eolian sand on dunes ¹, though its official description does not report collapse. Collapse is evaluated by index tests and by oedometer testing under ASTM D4546 ⁴. See collapsible and loess soils and hydrocollapse.

Fill

Pratt slopes reach 15 percent ¹. Where a lot was leveled with fill, see fill consolidation settlement.

Common questions

Is Pratt soil found in Oklahoma?

The official description's header lists Kansas and Oklahoma and notes a 1995 update for Oklahoma, but its distribution section names only south-central Kansas. Check Web Soil Survey to see whether Pratt is mapped at a specific Oklahoma address.

Is Pratt soil expansive?

No. Pratt is sandy, with only about 4 to 11 percent clay in its control section. Clay-driven shrink-swell is not a feature of this soil.

What are the thin bands in Pratt soil?

They are lamellae, thin layers less than 3 inches thick where clay has collected within otherwise sandy soil. They slow water movement slightly.

Can sandy windblown soil collapse?

Windblown sand dunes and loess are among the deposit types engineers check for collapse, a loss of volume when wetted. Lab testing determines whether a specific site is collapsible.

Sources

Official Series Description - PRATT Series USDA Natural Resources Conservation Service
Web Soil Survey - Home USDA Natural Resources Conservation Service
A Glossary of Terms Used in Soil Survey and Soil Classification USDA Natural Resources Conservation Service
Collapsible Soil California Department of Transportation (Caltrans), Geotechnical Services
Linear Extensibility Percent UC Davis California Soil Resource Lab (SoilWeb), citing the USDA-NRCS National Soil Survey Handbook

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