Ultisols

What Are Ultisols

Ultisols are strongly leached, acidic soils with a clay-enriched subsoil — similar to alfisols in structure, but far more weathered. The name comes from "ultimus" (Latin for last), reflecting their position as one of the most weathered soil orders. Over tens of thousands of years, heavy rainfall has stripped away calcium, magnesium, and other base nutrients, leaving behind iron and aluminum oxides that give these soils their characteristic red, orange, or yellow colors.

The key diagnostic feature is low base saturation — less than 35% in the subsoil. This means the soil holds relatively little calcium and magnesium compared to alfisols or mollisols. In practical terms: ultisols are naturally acidic and nutrient-poor, requiring lime and fertilizer for productive agriculture.

Where Ultisols Are Found

Ultisols dominate the southeastern United States, forming a broad band from Virginia and the Carolinas south through Georgia, Alabama, and Mississippi, and west into Arkansas and east Texas. They are the soil of the Piedmont Plateau, the southern Appalachian foothills, and the Gulf Coastal Plain.

In our dataset, 105 counties show ultisols as the dominant order, heavily concentrated in Alabama (8 counties), Arkansas, North Carolina, South Carolina, Tennessee, and Virginia. Major population centers like Montgomery County, AL and Lee County, AL (Auburn) sit squarely on ultisol terrain.

Why Ultisols Are Red

The vivid reds and oranges of southeastern soils come from iron oxide minerals — primarily hematite (red) and goethite (yellow-orange). In ultisols, the intense weathering has broken down most of the original rock minerals, leaving these iron oxides as the dominant coloring agents.

The color actually tells you something useful: bright red soils tend to be well-drained with good aeration (iron is fully oxidized), while yellow-orange soils indicate slightly more moisture and less complete oxidation. Gray or mottled subsoils signal seasonal waterlogging — a drainage red flag for gardeners and builders alike.

Farming and Gardening in Ultisols

Ultisols present real challenges, but generations of southern gardeners have developed effective strategies. The number one intervention is lime — regular applications to raise pH from the natural 4.5-5.5 range to 6.0-6.5 for most vegetables. A soil test should be your first step.

Organic matter is critical and disappears fast. The warm, humid climate decomposes compost and mulch rapidly, so southeastern gardens need constant organic matter inputs — cover crops, mulch, compost — to maintain soil structure and fertility. Think of it as a renewable subscription, not a one-time fix.

The sandy surface layer dries quickly, but the clay subsoil can stay waterlogged. Raised beds are enormously popular in ultisol country for this reason — they improve drainage while letting you build up the organic matter content of the root zone.

Despite the challenges, ultisols support a long growing season (200+ frost-free days in most of the Southeast), making succession planting and year-round gardening practical. Sweet potatoes, okra, southern peas, and collards are traditional ultisol-adapted crops. With amendment, tomatoes, peppers, and most standard garden crops thrive.

All Counties with Ultisols as Dominant Order(105)

Showing 50 of 105 counties. Search all counties

Other Soil Orders