Soil structure refers to the physical arrangement of solid soil particles and the resulting network of pores and spaces between them. It describes how sand, silt, and clay are organized into larger units. This organization is distinct from soil texture, which is the fixed proportion of those mineral particles by size. While texture is fixed, structure is dynamic and can be improved or degraded through physical, chemical, and biological processes. The quality of this structure is a major factor in soil health, governing how water, air, and plant roots interact with the soil.
Defining Soil Structure: From Particles to Peds
Soil structure is built upon the primary mineral components of sand, silt, and clay, which are defined by their particle size. Structure forms when these individual particles bind together into larger, secondary units called aggregates, or peds. These peds are natural, cohesive units separated by planes of weakness, distinguishing them from artificially formed clods.
The formation of these peds involves physical, chemical, and biological forces. Physical processes like cycles of wetting, drying, freezing, and thawing cause particles to shift and coalesce. Chemical binding occurs through agents such as iron and aluminum oxides, while organic matter acts as an important binding agent when microbial activity produces sticky substances (polysaccharides) that glue particles together.
Plant roots and fungal hyphae also act as physical stabilizers, enmeshing the soil particles into stable macroaggregates. The arrangement of these peds creates the pore space—the voids between the solid material. These spaces are categorized into macropores (large pores between peds) and micropores (small pores within peds), and their distribution dictates the functional behavior of the soil. The size of these peds can range widely, from about 1 millimeter up to 10 centimeters.
Classification of Soil Structure Types
Soil structures are visually classified based on the distinct shape, size, and grade (or strength) of the peds. The shape of the aggregate is referred to as the structure type, and the degree of its distinctness and stability is called the grade. These different structural types tend to be associated with specific horizons, or layers, within the soil profile.
Granular structure consists of small, nearly spherical or crumb-like peds, often less than 0.5 centimeters in diameter. This type is typically found in the surface layer (A-horizon) where there is a high concentration of organic matter and root activity. Blocky structure involves peds that are block-like or polyhedral, ranging from angular (sharp edges) to sub-angular (rounded edges), and are commonly found in the B-horizon (subsoil layer).
Prismatic structure is characterized by vertical columns of soil with flat, level tops, often found in lower horizons. A variation, columnar structure, is similar but has rounded tops, and is frequently found in arid or semi-arid regions due to the presence of soluble salts. Platy structure is composed of thin, flat plates that lie horizontally, often overlapping like stacked sheets. This structure is particularly undesirable as it greatly impairs water circulation and is typically associated with compacted surface layers or E-horizons.
Mediating Water and Gas Exchange
The arrangement of peds directly determines the size and connectivity of the pore network, which in turn regulates the movement and storage of water and gases. Well-structured soil possesses a balanced distribution of macropores and micropores, which facilitates optimal conditions for plant growth. Macropores, the larger spaces between the aggregates, are responsible for rapid water infiltration and drainage. They allow excess gravitational water to move quickly through the soil, preventing waterlogging and surface runoff.
Micropores, the smaller spaces primarily located within the peds, are essential for retaining water that plants can access. Water adheres to the surfaces within these small pores against the pull of gravity, providing a reservoir for plants and soil organisms during dry periods. A good structure also ensures adequate aeration, as macropores fill with air after drainage, allowing for gas exchange with the atmosphere. This continuous exchange is necessary to supply oxygen for root respiration and microbial activity, while allowing carbon dioxide to escape.
Poor structure, such as massive or highly platy arrangements, restricts both air and water movement. Platy layers, for instance, slow down infiltration and impede the vertical movement of roots, creating physical barriers. Conversely, a crumbly, granular structure allows roots to easily navigate the soil, accessing nutrients and water, and contributing to plant stability.