Soil structure, aggregation, air and water
Why soil structure matters
Soil structure is a physical system property describing how mineral particles are organised into aggregates and how those aggregates form connected pore networks. This physical organisation governs how water and air move through soil, how long water is retained within the rooting zone, and how nutrients are buffered or lost. Soils with the same texture and climate can therefore function very differently depending on their structural condition.
Soil texture and soil structure
Soil texture refers to the proportion of sand, silt, and clay particles and is effectively fixed at field scale. Soil structure refers to the spatial arrangement of those particles into aggregates and pores. Texture defines the physical limits within which soil operates, while structure determines how effectively those limits are expressed. Structure can degrade or recover over time without any change in texture.
Aggregation and physical stability
Aggregation describes the binding of mineral particles into stable units of different sizes. Stable aggregation creates a hierarchy of pores that underpin soil function. Structural stability depends on resistance to slaking, dispersion, compression, and collapse during wetting, drying, and mechanical loading. When aggregates fail, pore continuity is disrupted and soil behaviour can change rapidly.
Pore networks and water behaviour
Soil structure regulates how rainfall enters soil, how water is stored within pore space, and how excess water drains. Connected macro‑pores allow infiltration and drainage, meso‑pores regulate water movement and root access, and micro‑pores retain plant‑available water. When pore networks are intact, soils buffer rainfall and release water gradually. When pores collapse or disconnect, water movement becomes restricted or short‑circuited, increasing runoff, waterlogging, or deep percolation beyond rooting depth.
Air exchange and gaseous regulation
Air movement in soil depends on the continuity of air‑filled macro‑pores. Effective soil structure enables oxygen to diffuse into soil and carbon dioxide and other gases to diffuse out. Structural degradation reduces air‑filled porosity, increasing the risk of oxygen limitation, anaerobic zones, and disruption of normal root and microbial processes.
Air and water dynamics are physically coupled. When water occupies macro‑pores for extended periods, gas exchange is restricted. Well‑structured soils re‑establish air‑filled pore space after rainfall, maintaining a balance between water retention and aeration.
Nutrient buffering and loss pathways
Nutrients move primarily with water. Soil structure therefore plays a central role in determining whether nutrients are retained within the rooting zone or lost through leaching, runoff, or erosion. Stable aggregation slows water movement and increases contact time between nutrients and mineral or organic retention surfaces. Structural breakdown short‑circuits these pathways, increasing nutrient losses even when nutrient supply is unchanged.
Role of humus and mineral‑associated organic matter (MAOM)
Humus and mineral‑associated organic matter (MAOM) are stabilised organic fractions bound to mineral surfaces. Their role in soil structure is physical rather than nutritional. These fractions contribute to aggregate stability by acting as binding agents at particle interfaces, protecting pore geometry and increasing resistance to compaction and wetting–drying stress. The continuity of humus and MAOM binding supports longer‑term structural persistence.
Roots and biological interactions (bounded)
Roots and soil organisms interact with soil structure by occupying pores, exerting physical pressure, and contributing to aggregate reinforcement. However, these interactions are constrained by the physical structure already present. Continuous pore networks and stable aggregates enable root growth and biological activity; they do not reliably arise from short‑lived biological inputs alone. Soil structure therefore conditions biological expression rather than being guaranteed by it.
Structural degradation processes
Structural degradation occurs through processes such as mechanical compaction, aggregate dispersion during rainfall, and collapse following disruption of humus and MAOM binding at particle interfaces. These processes reduce pore continuity, increase bulk density, and impair water and air movement. The resulting changes increase water stress, oxygen limitation, and nutrient losses.
Boundaries and limits of soil structure
Soil structure influences how effectively soil stores and transmits air, water, and nutrients, and how resilient it is to physical stress. It cannot change soil texture, override climatic constraints, or guarantee nutrient availability in the absence of supply. Structural change is time‑dependent and contingent on stability and protection from repeated disturbance rather than instant or permanent transformation.
How HealthySoil interprets this within the Soil Transition Model (STM)
HealthySoil uses the Soil Transition Model as a diagnostic lens to interpret how differences in soil structure relate to water availability and nutrient behaviour across soils with similar textures and climates. The STM does not redefine soil structure or its mechanisms.to understand performance differences between soils with similar textures and climates.