Soils - Part 2: Physical Properties of Soil and Soil Water

Organic matter (humus), manganese and iron are the primary coloring agents in soil. The dark color of many productive soils in Nebraska and throughout the Midwest is due to organic matter. The dark soil color from organic matter at the soil surface aids in the absorption of heat from sunlight to warm the soil.

Soil shades of red, yellow and gray are due to the amount and chemical form of iron and manganese present. Red soils contain oxidized iron. Oxidized iron is also observed on metal objects that have been exposed to the atmosphere. We call it rust. Yellow soils contain hydrated iron. Gray soils indicate chemical reduction of iron and/or manganese due to wetness and lack of oxygen. Yellow and gray coloration can be found in the subsoil of some Nebraska soils which remain wet for some portion of the year. These subsoil colors serve as an important indicator of natural drainage conditions. In a soil series description, these colors are designated by the term “mottles” as for the Sharpsburg series in Table 1.4.

Sloping land that has been eroded excessively may expose subsoil horizons that are lighter in color and possess little organic matter.