Physical Properties of Soil
Soil Color
Soil color as other physical properties including texture, structure, and consistence are used to distinguish and identify soil horizons (layers) and to group soils according to the soil classification system called Soil Taxonomy. Soil color reflects environmental conditions, soil forming processes, and other influences on the soil. It generally indicates the status of organic matter and moisture, drainage, and the state of oxidation and reduction. It may also indicate mineralogical composition of the soil such as presence of iron, manganese, and dark minerals.
Munsell Color Chart
The Munsell Color Chart is routinely used for reading soil color in the fields. It consists of colored chips showing hue, value and chroma, the three simple variables that form a color (Figure 4.14). Hue refers to the degree of redness or yellowness of soil. Value refers to the lightness or darkness of soil. Chroma refers to the brightness or dullness of soil. A freshly exposed face or ped is used to determine color. Using moist soil is most common and dry soil may be misted with water. In direct sunlight, the moist soil ped is compared to the Munsell color system to determine the hue, value, and chroma.
Factors Affecting Soil Color
Three major factors influence soil colors: (1) organic matter content, (2) water content, and (3) the presence and oxidation states of iron and manganese oxides in various minerals. Organic matter tends to coat mineral particles, darkening and masking the brighter colors of mineral compounds such as iron oxides. Raw peat is usually brown; well-decomposed organic matter, such as humus, is black or nearly so. Many organic soils have a black color. In most mineral soils, the organic matter content is usually greatest in the surface soil horizons and the color becomes darker as the organic matter content increases. The effect of usually dark brown or black organic matter on soil color is important not only for soil classification purposes, but also for ensuring good thermal properties, which in turn contribute to soil warming and promote biological processes.
Click on the following topics for more information on physical properties of soil.
