Soils scientists examine various physical properties of soil to identify the different types. One soil property is texture. Texture refers to the size of the soil particles and how they feel. Sand particles feel gritty and drain water quickly. Silt particles feel floury and hold water well. Clay particles feel sticky and keep water out. Iowa woodlands are usually near rivers and streams, where the soil has more clay content. Topsoil formed under woodlands is thinner and often lighter in color.
The texture of soil depends on the:
- Parent material it came from
- Vegetation cover
- Length of time the soil has weathered
- The topography of an area
- Artificial changes caused by human activities
Soils begin with bedrock. Bedrock is broken down by freezing and thawing, wetting and drying, and other natural forces. The rock breaks down into what is called parent material, the geologic material from which soil forms. The parent material breaks into mineral particles of sand, silt or clay.
Eventually, the soil will reach a point where it can support plant life. Different plants help create different types of topsoil. Prairie grasses and flowers grow close to each other and have extensive, fibrous root systems. These roots help break up rocks and hold soil in place. The roots of most prairie plants are fibrous. This means they branch out many times from the base of the plant and keep branching out as they spread in to the soil. This massive network of fine roots serves the plant well, by being able to absorb even the smallest amount of rainfall. The root network also holds the soil particles together so the heavy rain and strong winds do not erode the fine soil particles away. Iowa’s rich soil developed under prairie plants and was held tightly by them. When prairie plants die, their decomposition returns nutrients to the soil, creating a rich, black silty soil.
When Iowa land was first plowed, the settlers found 14 to 16 inches of topsoil. By 2000 the average was six to eight inches. When the prairie plants were plowed under, the soil was to exposed and vulnerable to erosion. Soil erosion is the process of removing soil materials from their original sites by water or wind. Hard rains that wash across bare soil move Iowa’s black gold into gullies and streams. During dry weather winds can carry loose soil across the countryside. If nothing is done to stop the erosion, the rest of Iowa’s topsoil could be gone in the next 100 to 150 years.