Initial Publication Date: January 13, 2021

Physical Properties of Soil and Soil Formation

Chapter Summary:

To begin to understand soil, simply dig a hole! Looking at the exposed soil reveals several obvious physical aspects. Initially, you see that soil has many colors - it is not simply brown or red or black. Next, you notice patterns to the color. The upper portion of the soil is darker (browner or blacker) than the materials below, and the colors appear in horizontal bands or layers. A closer examination reveals that plant roots are more common in the upper layers. Furthermore, the soil is composed of clumps, clods, or aggregates that are made up of still smaller particles. There is also a different feel from the top layer to the bottom in terms of how easy it is to dig or break apart. These simple observations identify many of the key physical characteristics, or morphology, of soil: color, texture, structure, consistency, and horizons. Soil scientists use these characteristics to create a soil's profile description (also referred to as the soil morphology).

Educator Background

Soil Color

Color is perhaps the most obvious and easily determined soil property. You can infer from a soil's color extremely important site characteristics, such as drainage, mineral weathering, and water content. In surface layers, organic matter darkens the soil, usually masking all other coloring agents. Soil color is influenced primarily by soil mineralogy – telling us what is in a specific soil. Soils high in iron are deep orange-brown to yellowish-brown. Those soils that are high in organic matter are dark brown or black. Color can also tell us how a soil "behaves" – a soil that drains well is brightly colored and one that is often wet and soggy will have a mottled pattern of grays, reds, and yellows.

Color determination can be quite subjective. In general, people perceive colors differently, so soil scientists use a standard reference known as the Munsell color system. The system describes three components of color: hue, the dominant wavelength of the reflected light; value, the lightness or darkness of a color; and chroma, the relative strength of the hue. Determining the variation in color throughout the soil is important. Many land-use decisions are based on these colors and the fact that they do not change from season to season.

Soil Texture

Pick up a handful of soil and you can feel how fine or coarse it is. That feel comes from the size and relative proportion of mineral particles in the soil and is known as the soil texture.

Soils with high sand content tend to have large pores and therefore allow gases and water to move through them rapidly. Silt particles are relatively inert (as are sand particles) and have smaller pore spaces, resulting in slower movement of gasses and water. Clay particles are so small they are not visible to the naked eye, yet their properties control many other soil properties. Clay particles are characterized not only by size but also by their shape and crystal structure. Unlike sand and silt particles, which are often slightly weathered primary minerals, clays are most often secondary minerals, which are minerals that have formed in soil as opposed to being inherited from the parent material or bedrock.

Clay's negative charge and reactivity give it the plasticity and cohesiveness that allow it to be molded into pots and bowls. Because of their stickiness when wet, soils high in clay are worked only with difficulty.

To the right is the soil texture triangle. If you know the percent of clay, sand and/or silt, you can draw lines into the triangle to figure out what textural category the soil belongs in.


Soil Structure

Another physical characteristic of soil is its structure. How does structure differ from texture? Soil structure is the arrangement of individual soil particles (texture) into a larger grouping, sometimes called peds or aggregates. These aggregates are classified according to their shape: granular, angular blocky, subangular blocky, platy, wedge, prismatic, and columnar. In addition, some soils don't have aggregates and are referred to as structureless.

What makes for good or poor soil structure? A well-structured soil has an abundance of pores for water and air to move readily into the soil (rather than running off) and to root systems. Soils with good structures have aggregates that are difficult to move and so are resistant to the erosive effects of wind and water. Soil structure forms naturally through many processes including freezing/thawing, wetting/drying, plant root penetration, and burrowing by animals/insects. In addition, soil structure can be improved through management/amendments, and degraded by compaction.