Ruston Soil Series

The Ruston soil series is Louisiana's official state soil. It sits on the gently rolling uplands and ridgetops of the West Gulf Coastal Plain in north and central Louisiana — old, stable land surfaces that have been above sea level and weathering for a very long time.

Ruston is an Ultisol, the most heavily leached of the major soil orders in humid climates. Ultisols form where warm temperatures and high rainfall have dissolved and washed away most of the soluble minerals over millions of years, leaving behind iron oxides, aluminum oxides, and stubborn clay minerals.

The result is a soil that looks poor — pale and sandy at the surface, with a bleached layer just below — but has a deep, clay-enriched subsoil that holds water and nutrients where roots can reach them. It is the dominant agricultural soil of north Louisiana and one of the most common upland soils in the Gulf South.

The Ruston series was first described and established in Lincoln Parish, Louisiana, and named for Ruston — the parish seat and home of Louisiana Tech University. USDA soil scientists follow the standard practice of naming series after local geographic features, and Ruston was the center of the region where this soil was first systematically studied.

Louisiana soil scientists and the USDA Natural Resources Conservation Service identified the Ruston series as the most representative upland soil of north Louisiana, the region that defines the state's timber and sweet potato industries. The series is widespread, well documented, and directly tied to the land uses that shaped north Louisiana's economy.

The Ruston series was chosen as the state soil because it captures something distinct about Louisiana's landscape — not the famous delta and marshes of the south, which attract most attention, but the old, red-soiled uplands of the north that have supported farming and forestry for generations.

The Ruston profile is a study in what happens when warm rain falls on the same land for millions of years. At the surface, the soil is pale — fine sandy loam, not much color. Just below is a bleached layer where water has washed minerals downward. Then, abruptly, the soil shifts to reddish yellow and then to red as you enter the argillic horizon where iron oxide has concentrated.

That red clay layer can be several feet thick. Soil scientists call it a Bt horizon with kandic properties — a deeply weathered argillic zone where kaolinite clay dominates and iron is so concentrated it looks like brick. Below it, the soil stays red well past the depth most crop roots reach.

Ruston soil covers the rolling upland terrain of north and central Louisiana, where the West Gulf Coastal Plain rises above the river floodplains and deltas of the south. This is a landscape of pine forests, red clay roads, and small farms on gently sloping ridges.

Lincoln Parish, where the series is named, is near the center of the Ruston soil belt. The series extends through the surrounding parishes — Claiborne, Union, Bienville, Jackson, Winn, Natchitoches, and others — covering much of the land that lies between the Red River to the west and the Ouachita River valley to the east.

Unlike the famous soils of south Louisiana — the rich alluvial silts of the Mississippi delta or the organic soils of the coastal marshes — Ruston soil sits on old, stable uplands that have been forming and weathering since before the Mississippi River built its current delta.

Longleaf pine and loblolly pine are the dominant plants on Ruston soil today, covering millions of acres of north Louisiana uplands in managed timber production. The deep, well-drained profile suits pine well — roots reach deep into the red clay subsoil, and the sandy surface layer dries quickly enough to prevent root rot. North Louisiana's timber industry is one of the most productive in the South, and Ruston soil underlies most of it.

Sweet potatoes are the crop most associated with Ruston soil in Louisiana. The state is among the leading sweet potato producers in the United States, and north Louisiana — particularly Beauregard, Evangeline, and neighboring parishes where similar soils occur — grows most of that crop. Sweet potatoes need deep, loose, well-drained soil to develop without cracking or rotting. Ruston's sandy surface and deep profile provide exactly those conditions.

Cotton was the defining crop of north Louisiana for most of the nineteenth and early twentieth centuries, and Ruston soil supported most of it. As cotton declined, farmers shifted to timber, soybeans, corn, and hay. The red clay subsoil holds enough water to carry row crops through short dry spells, which is important in a climate where summer rainfall is inconsistent.