Port Soil Series

Port soil formed from alluvium — silty sediment deposited by rivers flooding repeatedly over thousands of years. Each flood added a thin layer of fine material carrying organic matter, minerals, and nutrients. Over time these layers compacted into a deep, dark soil with a thick, nearly black surface that distinguishes Mollisols from nearly every other soil order.

The defining feature is the mollic epipedon: a surface horizon more than sixteen inches thick that is dark from organic matter, soft when moist, and hard when dry. In Port soil, the mollic epipedon often extends two feet or deeper — the result of repeated flooding that kept adding organic-rich silt to the surface faster than it could decompose.

Port is well drained to moderately well drained, slightly alkaline, and calcareous at depth. The silty clay loam texture holds moisture well between rains, making it dependable farmland in the semi-arid climate of western Oklahoma.

The Port series is named for Port, Oklahoma, a community in Blaine County in the heart of the western Oklahoma river country where this soil is most extensive. USDA soil scientists follow the convention of naming soil series after nearby towns, streams, or geographic features where the series was first described.

The Soil Science Society of America recognizes Port as Oklahoma's state soil because it represents the deep, fertile alluvial soils that sustained both the Kiowa, Comanche, and Wichita peoples who farmed and hunted the river valleys for centuries, and the waves of settlers who arrived after the Land Runs beginning in 1889.

Port was chosen over the red clay upland soils that cover much of Oklahoma because the floodplain alluvial soils along the Washita, Canadian, and North Fork of the Red River are the most productive farmland in the state — the foundation of Oklahoma's wheat and cotton economy.

Digging into Port soil reveals a profile built by rivers over thousands of years. The surface is nearly black — dark from organic matter that accumulated in silty alluvial layers deposited by floods. The darkness continues for two feet or more before the soil gradually lightens and the stratified river sediments become visible deeper down.

Port soil occupies the floodplains and low stream terraces of rivers draining western and southwestern Oklahoma. It sits on the flat to gently sloping bottomlands between the upland red clay plains, where rivers deposit their finest sediment after rains. The Washita River valley, the Canadian River valley, and the North Fork of the Red River are the core Port soil country.

The soil is most extensive in Blaine, Caddo, Washita, Greer, Kiowa, Jackson, and Tillman counties — the river-valley farming belt of southwestern Oklahoma — and extends north along the Canadian River into Custer, Dewey, and Roger Mills counties.

Winter wheat is the signature crop of Port soil. Oklahoma is one of the top winter wheat-producing states in the country, and the deep, moisture-retaining Port floodplain soils produce higher and more reliable yields than the surrounding red clay uplands. The Washita and Canadian river valleys have grown wheat continuously since the late 1800s.

Cotton was the other historic crop. Before mechanization, Port soil bottomlands along the Red River tributaries were planted heavily in cotton by tenant farmers and sharecroppers through the first half of the twentieth century. Today cotton still grows in the irrigated bottomlands of Jackson and Tillman counties.

Grain sorghum and alfalfa are important modern crops. Alfalfa performs especially well on Port soil — the deep, fine-textured profile holds enough moisture between cuttings to support three or four harvests per season without irrigation in normal rainfall years.

Where Port soil floodplains remain unfarmed, the native vegetation is cottonwood-willow bottomland forest — Plains cottonwood, sandbar willow, pecan, and green ash along the river channels, with tallgrass prairie bluestem and switchgrass on the terrace margins.