Tarrant County Soils

Many people just dismiss the ground beneath their feet as “dirt,” a nonliving mess that’s a hassle when it gets tracked through the house our pets and covers the floor. However, the ground underneath our feet is created from weathered geologic materials (rocks) that are degraded by biological organisms and climatic actions over long time periods. This soil is mixed well with mineral particles of different sizes, shapes and pores, degraded organic matter, living organisms, and water and air spaces that differ in response to climate changes (moisture and temperature) and our actions (plowing, watering, compacting, etc.). The nutrients found in soils are responsible for life as we know it -- the source of almost all of the chemicals that a plant needs to convert sunlight into food for itself and other organisms.

The soil that is currently in the yards and gardens in your neighborhood has taken hundreds, if not thousands, of years to form. Rocks that are millions of years old are eroded and weathered down into particles that contain minerals. Dead plant and animal matter are decomposed by soil organisms into humus that contain nutrients. All of this material get mixed together to form the soil.

Soil is a lot more complicated than many people think. Soil is defined and classified by two different components. The first component is the minerals, nutrients, organic matter, and parent rock material in the soil. Different soils have different proportions of all of these materials. Some soils have high amounts of nutrients and minerals, while other soils may lack certain nutrients and minerals and make it harder for larger plants to grow. This is easy to see when gardening. Some gardens are on landscapes that have healthy soil that can support many different rose bushes and pansies while other soils that haven’t been taken care of need fertilizer to help the plants grow. By mowing the yard, raking leaves, and keeping your garden pristine may look nicer, it might actually be hurting the soil because the nutrients can’t decompose back into the ground. This is why composing food scraps, newspapers, and grass and tree clippings can help your garden thrive.

The other component that defines soil is the pore space, or the amount of space between grains of sand, that contains the air and water that all plants need to grow Soils have different sizes of pore spaces and this also impacts the plant life growing in the soil. Clay soils have smaller pore spaces, so they can hold onto water easier than sandy soils. But at the same time this means clay soils have a harder time draining once the soils gets too wet. Sandier soils drain well, but may need more watering since they don’t hold onto the water as well. The soil also contains thousands of different species of bacteria, fungi and protozoa, nematodes, and mites that improve the health of the soil. The health of the soil and the types of soil in an area affect what plants grow in the environment, and therefore impacts the ecosystem that develops in that area.

Tarrant County has four different soil types throughout the area because Tarrant Country is on four different parent rock materials. These soil types affect what plants can grow in the area, and helps explain why Tarrant County has four different ecoregions. Tarrant County sits on shale, limestone, and sandstone bedrocks and therefore has many different soil types. The shale and limestone bedrock creates a thick, black clay soil in the Blackland Prairies referred to as “black gumbo” that is great for grasses and agricultural crops to grow on, but is not stable enough to hold larger, woody plants such as trees. The clay also locks in moisture and doesn’t drain quickly because the clay particles are small and are able to compact closer together, reducing the pore space. The black gumbo clay is the primary reason why prairies formed in that area and not forests. On the other hand, the sandy and gravelly bedrock of the Cross Timbers creates a deep sandy soil that is great for trees, causing the development of the forests that the area is named after. This soil’s particles are larger, allowing greater pore space between the particles and therefore water is able to travel more freely and drain faster than clay soils.

Soil isn’t just nutrients, minerals, air, and water. Soil organisms live in the soils and change the makeup of the soil, and make it habitable for plants. In a handful of healthy soil there are billions of organisms represented by thousands of different species. These soil organisms break down organic matter, such as plant and animal remains, into the nutrients that plants need to survive. These soil organisms are extremely important to healthy soil. The soil is a giant recycling system, breaking down dead organisms and matter into nutrients for the growth of living plants. All plants, and the life that depends on plants, are dependent upon a healthy soil to grow and thrive, and healthy soil depends upon that life to form more soil.

When soils are degraded, plants can’t survive and the ecosystems begin to collapse. Soils are degraded in various ways. The primary cause of soil degradation is erosion. Removing native vegetation increases erosion risk in an area, and makes it extremely hard for the native vegetation to come back. For example, the Blackland Prairies has an extremely valuable soil that is great for agriculture. But due to unsustainable agricultural practices, much of the soil is eroding one hundred times faster than when the native vegetation was growing. It also causes the need for fertilizers and other soil treatments to make the soil productive, when before the soil was giving life. In the past 150 years, half the topsoil in the world has been lost to erosion. Soil not only grows our food, shelter, and clothing, it provides valuable services. Soil filters the water we drink, lessens the impacts of droughts and floods, and regulates the climate. Soil is an extremely important part of the ecosystem that we live in, and by understanding soil we can help conserve our natural resources.

© The Botanical Research Institute of Texas. The BRIT Ecoscapes Project is made possible by a grant from the Ann L & Carol Green Rhodes Charitable Trust.