Ultisol

Ultisol, commonly known as red clay soil, is one of twelve soil orders in the United States Department of Agriculture soil taxonomy. The name "Ultisol" comes from the word "ultimate" because these soils were thought to form over a very long time in moist, mild climates.

Ultisols are found in humid temperate or tropical regions. They are most famous for the red clay soils of the Southern United States, but they also exist in parts of Africa, Asia, Australia, and South America. In the World Reference Base for Soil Resources (WRB), most Ultisols are called Acrisols and Alisols, with some belonging to the Retisols or Nitisols. Aquults are often Stagnosols or Planosols, and Humults may be Umbrisols.

Introduction

Ultisols, also called red clay soil, come in many colors such as purplish-red, bright reddish-orange, or pale yellowish-orange. In cooler places like Pennsylvania, they can look yellowish-brown or grayish-brown. These soils are usually very acidic, often with a pH lower than 5. Their red and yellow colors come from iron oxide, which does not dissolve in water.

Ultisols lack important nutrients like calcium and potassium, so they are not good for farming without help from lime and fertilizers. They can run out of nutrients quickly and need careful care. However, they can grow crops in many different wet and dry conditions. When there is lots of organic matter, like in the Olympic series, the soil can be quite fertile.

Ultisols are most common in the Southern United States, southeastern China, Southeast Asia, and other warm areas. They formed over very long times and are found as far north as the Carboniferous period when forests first appeared.

Gardening in Ultisol

Ultisol soil, also called red clay soil, can be hard for plants to grow in because it doesn’t have enough organic matter. Gardeners often add special materials to the soil each year to help plants grow.

Testing the soil can show its pH level, which is usually low in Ultisol. Adding lime can help raise the pH level, making the soil better for plants.

Gardeners usually want a mix of minerals, organic matter, and space for air and water in their soil. In places like North Carolina, Ultisol has many minerals but not much organic matter or space for air. Using mulch can help keep the soil cool and let more water reach the plants. Adding compost and manure can increase the organic material, which gives nutrients to plants and improves drainage.

Sometimes, small creatures in the soil break down the organic material and use the nutrients before plants can. To avoid this, adding manure in the fall can be helpful. Some gardeners use raised beds to stay away from Ultisol soil.

Plants that naturally grow where there is Ultisol can do well there. These plants are used to damp, poorly drained soils. The Missouri Botanical Garden suggests plants like tickweed, spotted jewelweed, mealycup sage, Camassia, spring starflower, ostrich fern, sideoats grama, Bouteloua curtipendula, and prairie dropseed.

Suborders

Ultisols can be grouped into different types based on where they are found and what they look like.

• Aquults are Ultisols with water near the surface for much of the year.
• Humults are Ultisols that drain well and have lots of organic matter.
• Udults are Ultisols found in humid climates.
• Ustults are Ultisols found in semiarid and subhumid climates.
• Xerults are Ultisols in temperate areas with dry summers and wet winters.