Skip to main content

Oklahoma Agriculture in the Classroom

Agricultural Facts

Soil Science

Soil Science
"The nation that destroys its soil destroys itself."
- Franklin D. Roosevelt
  • Without soil we would have no food, no clothing and no shelter.
  • Soils are a combination of minerals, organic matter, air and water that is constantly changing because of the organisms that live there.
  • Soils are home to two or more tons of living things per acre. One 1/4 tsp fertile soil = 62,000 algae, 111,000 fungi, 50 nematodes, 25 million+ bacteria and more.
  • Organic material is beneficial to the soil because it increases water-holding capacity, serves as a reservoir for plant nutrients such as nitrogen and provides food for the living things in the soil.
  • There are three basic categories of particles that exist in soils. Clay is the smallest of particles, feels sticky and often stains the fingers. Silt feels smooth and soft and is somewhat slick. Clay and silt particles cannot be seen with the naked eye. Sand particles are the largest and feel gritty.
  • Soils are distinguished by observing the percentage of each of the three types of particles contained in the soil. These mixtures are called loams. If soil has more sand it is a sandy loam, more silt, a silty loam, or more clay, a clay loam.
  • The combination of particle types dictates how the soil will handle, drain and hold water. Soils that contain a large percentage of clay hold water tightly but drain poorly. Plants growing in clay suffer from a lack of air around the roots because the tiny clay particles absorb so much water.
  • Sandy soils provide ample air supply to roots because the particles are large; however, sand does not hold water well, and it drains quickly. The water becomes unavailable to thirsty plants.
  • The best soils for growing plants combine the airiness and drainage of sand with the water-holding capacity of clay.
  • Soil is made up of four layers.
    1. The first layer is organic matter formed by plants and insects that have died and dead leaves that have fallen.
    2. Just under that is the topsoil. Topsoil is the best place for plants to take root and grow. It is a mixture of air, water, organic material and minerals. In most places the topsoil is between six and 20 inches thick.
    3. Under the topsoil is the subsoil. It is usually lighter in color and less productive than the topsoil. It is made of clay or sand and has very little organic material.
    4. The final layer is bedrock, which is solid rock. Between the subsoil and bedrock is a layer of small rocks that have started to break off from the bedrock. This is called the parent material because most of what makes up the soil was once part of the bedrock.
  • Soil comes in many different colors, from black to yellow to deep red.
  • The red color in much of our soil in Oklahoma is caused by a high iron content in the soil, like the color of a rusty pipe.
  • There are over 550 different kinds of soil in Oklahoma—more different varieties of soil than just about any area this size in any other part of the world.
  • Oklahoma's state soil is port silt loam. Port silt loam is dark brown to dark reddish brown and is derived from upland soil materials weathered from reddish sandstones, siltstones and shales. The natural soil supports a native, undisturbed vetetation of tall prairie grass and native trees, including pecan, walnut, bur oak and cottonwood.
  • Soil erosion is what happens when soil is washed or blown away. In most places, trees and plants hold soil in place. When that vegetation is removed, winds and rains can carry the soil away.