Physical Properties of Soil
Soil Structure
Texture is used in reference to the size of soil particles, whereas soil structure is the description of how individual soil particles (sand, silt, and clay) are arranged into soil aggregates (also called peds) and reflects both physical and chemical weathering. Peds are held together and in place through the adhesion of organic substances, iron oxides, clays or carbonates. Aggregates play a major role in determining a soil’s friability. A soil is friable when moist soil crumbles into aggregates when crushed with only light pressure.
Soil Structure Dynamics
Soil structure is not static, but continuously modified by various processes. Weather has a large impact, primarily through the action of wetting and drying the soil that can cause structure to collapse or disaggregate under wetting, and crack under drying. Freezing and thawing of soils have a large impact on soil structure development. The amount and nature of colloidal clay influence the formation of aggregates. The greater the amount of clay in a soil, the greater is the tendency to form aggregates. Calcium ions associated with clay generally promote aggregation, whereas sodium ions promote dispersion. Sodium ions are monovalent, relatively large and they are the prominent cation adsorbed to clay particles in some soils in arid and semi-arid regions.
Structural Ped Characteristics
Structure is one of the defining characteristics of a soil horizon. A soil exhibits only one structure per soil horizon, but different horizons within a soil may exhibit different structures. Most soils exhibit some type of aggregation and are composed of peds that can be characterized by their shape (or type), size (fine, medium, coarse), and grade (such as strong, moderate, or weak) of the structural units (peds) are defined.
Soil Structure Shapes
The four principal soil ped shapes are spheroidal, platy, block-like, and prism-like. Table 4.2 will help in visualizing these structural types (and some subtypes).
Soil Structure Size
In addition to structure type, structure is also described by soil scientists based on ped size, and structure strength. Ped size is simply the average size of peds within different size categories. The sizes are grouped into different ranges of diameter, such as fine, medium, coarse, etc. The range “descriptor” may change as well, such as thick, medium, and thin for platy structure.
Soil Structure Grade
The final characteristic used to describe soil structure is Grade. Grades of soil structure are identified on the basis of the visibility in a horizon and stability of aggregates. Stability of aggregates refers to their resistance to destruction by water. Terms for grade of structure are identified as follows:
Aggregate Stability
Aggregate stability is the measured ability of a soil to hold together and maintain structure despite disruptive forces such as raindrops; water and/or wind erosion impacts; trampling/compaction; shrinking and swelling conditions, or tillage. Aggregation affects erosion, movement of water, and plant root growth. Desirable aggregates are stable against rainfall and water movement. Aggregates that break down in water or fall apart when struck by raindrops release individual soil particles that can seal the soil surface and clog pores.
Slake Test
A simple test that demonstrates aggregate stability is the “slake test.” This is done by simply placing a small amount of a similar soil collected from different management settings (e.g., plowed field and the adjacent undisturbed fence row) in separate plastic cups containing distilled water and gently swirling for a few seconds.
Soil Structure Affect on Water Infiltration
Soil structure is a key attribute of soil quality and health that significantly impacts water infiltration. Some soils build a large, solid, structureless mass (referred to as massive); and others consist of small, porous aggregates that tend to have a uniform rounded shape—referred to as granular, a desirable structure for the growth of crops.
Soil Structure Degradation
Soil degradation refers to the loss of land’s physical, chemical, biological, and ecological qualities due to either natural or human-caused disturbances. Some examples of soil degradation processes are the exhaustion of nutrients and organic matter, soil erosion, acidification, desertification, and pollution.
Causes of Structural Degradation
The main causes of soil degradation and, consequently, the main threats to its ecological functions are water erosion, conventional tillage, overgrazing, organic matter decline, compaction, and production and irrigation methods.
Click on the following topics for more information on physical properties of soil.
