What is Port silt loam, Oklahoma's state soil, and does it flood?

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

In short: Port silt loam is Oklahoma's official state soil, a deep, well-drained floodplain soil covering about one million acres in 33 counties. It floods occasionally or frequently for very brief periods between March and October. For homes, its floodplain location matters more than anything else, and soil scientists also note low strength and some shrink-swell.

What is Port soil?

Port is an official USDA soil series classified as "Fine-silty, mixed, superactive, thermic Cumulic Haplustolls" ¹. Flood plains are among the landscape positions that periodically receive fresh sediment ⁶.

The soil formed in calcareous loamy alluvium of Recent age on narrow flood plains, with slopes of 0 to 3 percent ¹. It occurs in the Central Rolling Red Plains and Central Rolling Red Prairies of Oklahoma, Kansas and Texas and is rated as extensive ¹. Its type location is in Grady County, about 7 miles east of Chickasha, and the series was established in Jackson County in 1942 ¹.

Oklahoma's state soil

The Governor and Oklahoma Legislature named Port silt loam the state soil in 1987 ². It covers about one million acres in 33 of the state's 77 counties, occurring in more counties than any other soil, mainly in western and central Oklahoma ².

In Oklahoma, Port is common along streams in the OKC Metro, North-Central (Red Prairies and Wheat Belt) and Southwest (Lawton and Red Plains) regions, on Quaternary alluvium and terraces.

What does a Port soil profile look like?

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

• Flooding: occasional or frequent, very brief, March to October ¹.
• Drainage: well drained; permeability moderate ¹.
• Climate: mean annual precipitation about 32 inches (range 26-40) ¹.
• Native vegetation: tall grasses with pecan, black walnut, bur oak and eastern cottonwood ¹.
• Depth to bedrock and water table: not stated ¹.

What does Port soil mean for foundations?

Flooding comes first

Port forms on flood plains and floods occasionally or frequently ¹. The Soil Science Society of America summary identifies occasional flooding as Port's main limitation ². The official flood zone for any address is shown on FEMA's Flood Insurance Rate Maps, searchable in the Flood Map Service Center ⁴. See creeks and flood history and severe weather effects.

Strength and shrink-swell

The same summary notes that low strength and shrink-swell limit Port's use for local roads and streets ². The USDA rates shrink-swell as linear extensibility, from low (under 3 percent) through moderate (3 to 6 percent) to high and very high ⁵. The rated value for each mapped unit is in Web Soil Survey ³.

For how soft or wet alluvium can compress under a building, see soft soil consolidation. For how water can strip soil from under footings and slabs, see erosion, undermining and washout voids.

How do you confirm the soil at a property?

• Web Soil Survey shows the mapped soil unit and soil data for an area, and can generate custom reports ³.
• FEMA's map service center provides the FIRM panel and a printable flood map (FIRMette) for an address, and notes that later map changes (LOMCs) should be checked ⁴.

Related

• Pulaski soil
• Kiomatia soil
• Erosion, creeks and slopes
• Soil series in the OKC metro
• Soil series index