Oklahoma is known for its red soil. But what, exactly, makes our soil red? Professor Shiping Deng, Ph.D., of Oklahoma State University, says it’s the iron oxides – and although the oxidized form of iron is actually orange in the pure state as observed in the laboratory, it shows up bright red in the soil because of the fine crystalized hematite particles scattering light.
Hematite comes in a variety of colors, from black or silver-gray, brown to reddish-brown, or even red tones, and while the color may vary, all hematite contains rust-red streaks, and its name derived from the Greek word for blood.
Deng goes on to say that iron oxide forms in the process of ‘soil weathering’ in tropical and subtropical climates, where iron minerals in rocks and sediment are exposed to oxygen and water, and undergo oxidation, which is observed as rusting. Simply put, soil weathering is the process by which rocks and minerals break down into smaller particles, thus forming soil.
The formed pigment will persist and remain on soil particles if conditions remain aerated, or in the presence of oxygen.
So why, then, is Oklahoma home to so much red dirt as compared to other states and regions? Oklahoma’s climate is conducive for the rocks to oxidize because of our warm and dry climate. The relatively dry conditions of a semi-arid climate help preserve oxygenated air in the soil and reduce leaching, which prevents iron from being removed from the soil profile.
The soil profile consists of layers (or what is called ‘horizons’) beginning with the surface horizon; some soils have an organic horizon that lays on top of the surface horizon, followed by the subsoil, and then the substratum.
Deng shares that the variation of the soil parent material (rocks) and the climate make red soil common in certain parts of Oklahoma. Higher rainfall may cause iron to leach from the soil and reduce the iron content. Because water drives oxygen out of the soil, it creates reduced conditions.
However, red soil is not exclusive to Oklahoma, although we’re known for it. It can be found in other U.S. states such as Georgia, Alabama, Mississippi, Arizona and New Mexico.
Clay soil has shaped the way people farm here in Oklahoma.
“In the past 30 years, the farming community in Oklahoma and nationwide have implemented many management systems to improve soil health and sustainability,” says Steven L. McGowen, a state soil scientist.
There are many ways to manage clay soil that helps it become more usable for agricultural purposes, such as reduced tillage, no-till, cover crops and regenerative agriculture. These management practices “aim to retain the soil moisture, build organic matter, restore natural soil microbial communities, improve nutrient cycling and improve overall soil health,” McGowen says.
McGowen explains that NRCS soil health systems focus on four major factors to protect and feed the soil system: (1) minimize disturbance, (2) maximize cover, (3) maximize biodiversity and (4) maximize living roots.
“By implementing management practices that follow these principles, crop producers and ranchers can care for the soil resource and sustain production potential for the future,” says McGowen.
