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Soil structure

Soil structure refers to the arrangement and combination of primary soil particles—such as sand, silt, and clay—into secondary units known as aggregates or peds.^[1]^[2] These aggregates form through the binding of particles by organic matter, clay, and other cementing agents, creating a network of solids and pore spaces that defines the soil's physical architecture at scales smaller than the soil horizon.^[3]^[1] Soil structure is classified by the size, shape, and grade (distinctness) of aggregates, with common types including granular (rounded, crumb-like for surface soils), blocky (cube- or prism-shaped in subsoils), platy (layered, often indicating compaction), and structureless (massive or single-grained, typical of sandy or compacted soils).^[2] The grade of structure ranges from weak (poorly formed aggregates) to strong (well-defined and stable), influencing the soil's overall stability and behavior.^[1] Factors such as organic matter content, microbial activity, root growth, and management practices like tillage or compaction play key roles in forming and maintaining these structures.^[2]^[3] The importance of soil structure lies in its control over critical soil functions, including water infiltration and retention, aeration, root penetration, and nutrient cycling.^[1] Well-structured soils feature a balance of macropores (larger than 0.08 mm for rapid drainage and root access) and micropores (smaller for water and nutrient retention), promoting biological activity and plant productivity.^[1]^[2] Poor structure, such as from compaction, reduces porosity and hydraulic conductivity, leading to erosion, runoff, and limited crop growth, while good structure enhances soil fertility and sustainability in agricultural and natural ecosystems.^[3]^[2]

Fundamentals

Definition and Components

Soil structure refers to the arrangement of primary soil particles—sand, silt, and clay—into secondary particles known as aggregates or peds.^[4] These peds form through the grouping of individual particles, creating a three-dimensional framework that defines the soil's physical architecture.^[2] In contrast, soil texture describes the relative proportions of these primary particles based on their size distribution, without regard to their spatial organization.^[5]^[6] Peds, the basic units of soil structure, vary in size, shape, and stability, influencing the soil's overall cohesion and porosity. Sizes range from micro-aggregates smaller than 0.5 cm to macro-aggregates exceeding 5 cm in diameter.^[7] Common shapes include granular (resembling crumbs), blocky, prismatic, and platy forms, with stability often determined by resistance to water dispersion or mechanical disruption.^[8] Examples of simple aggregate types include crumbs, which are small, rounded peds typical in surface horizons, and clods, larger irregular masses often resulting from soil disturbance.^[9]^[10] Soil structure also encompasses pore spaces, which are voids between and within peds that facilitate air and water movement. Macropores, larger than 80 µm and located between peds, allow rapid drainage and root penetration, while micropores, smaller than 30 µm and found inside peds, retain water and nutrients through capillary action.^[11]^[12] Peds exhibit a hierarchical organization, where micro-aggregates (less than 250 µm) form the building blocks of larger macro-aggregates (greater than 250 µm), stabilized by organic binding agents at different scales.^[13] This multi-level assembly creates a complex network essential to the soil's physical properties.^[14]

Importance to Soil Functions

Soil structure significantly influences water dynamics in soils by facilitating infiltration, retention, and drainage. Well-structured soils with stable aggregates create macropores that enhance water infiltration rates, reducing surface runoff and allowing deeper percolation, while micropores aid in retention for plant availability.^[15] Poor structure, such as in compacted soils, impedes these processes, leading to reduced infiltration and increased drainage limitations due to fewer large pores.^[16] This balance is crucial for maintaining soil moisture during dry periods and preventing waterlogging.^[17] Aeration and gas exchange are also governed by soil structure, as pore spaces allow oxygen diffusion to roots and soil organisms while permitting carbon dioxide escape. Aggregated structures promote adequate aeration by maintaining interconnected pores, supporting aerobic microbial processes essential for decomposition.^[18] In contrast, dense or poorly structured soils restrict gas movement, fostering anaerobic conditions that can harm plant roots and beneficial biota.^[19] Root penetration and plant growth depend heavily on favorable soil structure, which provides a stable yet penetrable medium for root expansion. Good structure enables roots to explore larger volumes for water and nutrients, enhancing overall plant vigor and yield in agricultural settings.^[19] Conversely, compact or massive structures limit root growth by increasing mechanical impedance.^[16] Soil structure plays a pivotal role in nutrient cycling by creating habitats that support microbial activity and organic matter decomposition. Aggregates protect organic materials from rapid breakdown, regulating the release of nutrients like nitrogen and phosphorus for plant uptake.^[20] Enhanced microbial communities within structured soils accelerate cycling processes, transforming insoluble nutrients into bioavailable forms.^[21] This fosters soil biodiversity, as diverse pore networks sustain varied microbial populations, including bacteria, fungi, and nematodes, which contribute to ecosystem resilience.^[22] By stabilizing soil particles, structure prevents erosion and compaction, preserving topsoil integrity. Well-aggregated soils resist erosive forces from wind and water, maintaining surface cover and reducing sediment loss.^[20] It also mitigates compaction by distributing mechanical stresses, avoiding the formation of dense layers that degrade soil functionality.^[17] Soil structure serves as a key indicator of soil health, directly affecting tilth—the soil's suitability for cultivation—and workability for agricultural operations. Optimal structure ensures friable tilth, easing tillage and seedbed preparation while minimizing energy inputs.^[6] Quantitatively, it influences bulk density and porosity; for arable soils, bulk densities below 1.4 g/cm³ correspond to porosities of 40-60%, promoting balanced water, air, and root functions, whereas higher densities signal degradation.^[23]^[24]

Formation and Influencing Factors

Natural Formation Processes

Soil structure develops through pedogenesis, the natural process of soil formation that transforms parent material into organized aggregates and peds via four primary mechanisms: additions, losses, transformations, and translocations. Additions introduce organic matter, such as leaf litter and root exudates, along with mineral particles from atmospheric dust or flood deposits, providing the building blocks for aggregation. Losses occur through erosion or leaching, removing soluble salts and fine particles, which concentrates remaining materials and influences pore space development. Transformations involve chemical and physical weathering, where rocks break down into clays and secondary minerals, and organic matter decomposes into humus that binds particles. Translocations move these materials vertically within the soil profile, such as the downward migration of clay and iron oxides, fostering horizon differentiation and stable structural units.^[25] These processes directly contribute to aggregate formation by promoting particle adhesion and organization. Weathering physically fragments rocks through expansion from freezing water or root penetration, while chemical weathering dissolves minerals to form sticky clays that flocculate into microaggregates. Deposition of transported sediments, such as alluvial materials in river valleys, lays down initial layers that subsequent processes restructure. Translocation, particularly illuviation where suspended particles deposit in subsoil pores, creates denser, more coherent peds. Biological activity, including root growth, briefly aids in mixing and binding particles during early stages. Overall, these interactions result in the transition from unstructured mineral grains to cohesive soil fabric.^[26]^[27] The development of soil structure progresses through distinct stages, beginning with loose, unconsolidated particles in young soils like entisols, where minimal pedogenesis yields granular or single-grained textures with high porosity but low stability. As processes intensify, intermediate stages form weakly bound aggregates, evolving into mature profiles with stable peds, such as prismatic or blocky units, in soils like alfisols or mollisols. Climate plays a pivotal role in this progression; in temperate regions, repeated wetting-drying cycles cause clay particles to shrink and swell, compressing and aligning them into blocky structures that enhance water retention. Freezing-thawing in colder climates further clumps particles by ice crystal expansion. Topography modulates these effects, with flat landscapes allowing deeper profile development through reduced erosion, while slopes accelerate deposition at bases and limit structuring upslope due to runoff.^[25] Historically, soil structure evolves over extended timelines tied to environmental conditions, often spanning thousands of years from initial parent material exposure. In humid regions with high rainfall, rapid leaching and organic accumulation can establish mature structures within 5,000 to 10,000 years, as seen in forested podzols. Arid environments, with slower weathering due to limited moisture, may require 20,000 years or more for comparable development, resulting in coarser, less aggregated profiles. Post-glacial landscapes, such as those in northern latitudes, demonstrate accelerated initial structuring over 10,000 to 17,000 years through intense frost action. These timelines underscore pedogenesis as a gradual, site-specific continuum shaped by ongoing environmental interactions.^[26]^[27]

Key Influencing Agents

Soil structure is profoundly influenced by a combination of biotic, chemical, and physical agents that interact to determine aggregate stability and overall soil architecture. These agents control the arrangement of primary soil particles into secondary structures, affecting porosity, water retention, and nutrient cycling. Biological agents, in particular, contribute through living organisms that physically and chemically bind particles, while chemical agents mediate cohesion via mineral and organic interactions, and physical agents impose mechanical stresses that reshape the soil matrix. Biological agents, including plant roots, earthworms, fungi, and bacteria, are essential for binding soil particles and enhancing structural stability. Plant roots mechanically anchor soil particles and secrete exudates such as polysaccharides that act as binding agents, promoting the formation of stable aggregates. Earthworms improve soil structure by burrowing, which increases macroporosity, enhances aeration, and incorporates organic matter into deeper layers, thereby fostering aggregate development. Fungi, especially through mycorrhizal networks, form extensive hyphal webs that enmesh soil particles, stabilizing aggregates against erosion and compaction; for instance, arbuscular mycorrhizal fungi extend root systems and contribute to long-term structural integrity in various ecosystems. Bacteria further aid aggregation by producing extracellular polymeric substances that glue particles together, facilitating the decomposition of organic inputs and the cycling of nutrients that support overall soil cohesion. Chemical agents exert control over soil structure primarily through their effects on particle flocculation and dispersion. Organic matter, particularly humus—the stable, decomposed fraction of plant and animal residues—serves as a cementing agent that binds mineral particles into durable aggregates, improving water-stable structure and reducing susceptibility to erosion. Clay minerals, such as montmorillonite (a smectite clay), influence structure by swelling upon water absorption due to their layered silicate composition and high cation exchange capacity, which can lead to volume expansion and potential disruption of aggregates in wet conditions. Exchangeable ions also play a critical role; for example, high sodium concentrations in sodic soils cause clay dispersion by weakening electrostatic bonds between particles, resulting in poor structure, reduced permeability, and increased crusting. Physical agents, including soil moisture regimes, temperature fluctuations, and mechanical forces like freeze-thaw cycles, dynamically alter soil structure through environmental stresses. Alternating wet and dry moisture conditions promote aggregate formation by inducing shrinkage and cracking that reorganize particles, while excessive moisture can lead to slaking and breakdown of unstable structures. Temperature variations affect microbial activity and organic matter decomposition rates, indirectly influencing binding processes, with higher temperatures accelerating breakdown in some contexts. Freeze-thaw cycles mechanically disrupt and reform soil structure by expanding water into ice within pores, which can fracture aggregates but also create new voids and enhance mixing in colder climates, contributing to granular structures over repeated events. The interplay among these agents amplifies their individual effects on soil structure, creating complex dynamics tailored to environmental conditions. For instance, organic matter enhances microbial binding by providing substrates for bacterial and fungal exudates, particularly in temperate soils where moderate moisture and temperature support active decomposition and aggregate stabilization. In such systems, the combination of chemical cohesion from humus and biological enmeshment from roots and microbes results in resilient structures that resist degradation, underscoring the synergistic nature of these influences.

Classification of Soil Structures

Aggregate and Ped Formation

Soil aggregates represent the fundamental building blocks of soil structure, forming through the cohesion of primary particles such as sand, silt, and clay into stable units. Microaggregates, typically smaller than 0.25 mm, primarily arise from interactions between clay minerals and persistent organic matter, where electrostatic forces and transient binding agents like microbial byproducts facilitate their assembly.^[28] In contrast, macroaggregates, ranging from 0.25 to 2 mm, develop at a larger scale through the enmeshment and binding actions of plant roots, fungal hyphae, and soil fauna, which temporarily encapsulate smaller microaggregates and enhance overall structural hierarchy.^[29] This hierarchical organization, first conceptualized in seminal work on scale-dependent mechanisms, underscores how microaggregates serve as stable cores within macroaggregates, promoting soil stability and function.^[30] Peds, the secondary units of soil structure, emerge as larger assemblages of aggregates, exhibiting distinct characteristics that influence soil behavior. The grade of ped structure denotes its development and stability, ranging from weak (indistinct boundaries and low resistance to disruption) to strong (well-defined boundaries and high cohesion). Ped size classes vary, with fine peds generally under 10 mm and coarse peds exceeding 10 mm, though classifications may extend to very fine (<1 mm) or very coarse (>50 mm) depending on soil context. Type refers to the overall shape of peds, which arises from aggregation patterns but is generalized here without specific morphologies. Stability of peds and aggregates relies on multiple mechanisms that bind particles against disruptive forces like water and tillage. Electrostatic forces, particularly in clay-rich soils, promote flocculation by attracting oppositely charged particles, forming initial microaggregate cores. Cementation occurs through the precipitation of carbonates, iron oxides, or silica, which act as durable glues hardening the structure over time.^[31] Biological glues, including fungal hyphae, bacterial polysaccharides, and root exudates, provide transient yet effective binding, especially in macroaggregates, enhancing resistance to slaking. In sandy soils, structure often manifests as single-grained, where loose, unaggregated particles predominate due to low clay and organic content, leading to high permeability but poor cohesion.^[32] Loamy soils, with balanced sand, silt, and clay fractions, favor aggregated structures, where peds form readily through the aforementioned mechanisms, improving water retention and aeration compared to single-grained types.^[33] Organic matter serves as a key influencing agent in these processes, transiently binding particles during decomposition.^[30]

Primary Structure Types

Soil structure is primarily classified based on the morphology of peds, the natural aggregates formed by soil particles, into several distinct types that reflect the arrangement and bonding of primary minerals and organic matter. These types include platy, prismatic, columnar, blocky, granular, massive, and single-grain structures, each characterized by specific shapes, sizes, and orientations that influence soil behavior.^[34]^[6] The formation of these peds depends on soil composition and environmental conditions, with ped size typically ranging from less than 1 mm to over 10 cm and grade describing the distinctness from weak to strong; further details on size and grade appear in the Aggregate and Ped Formation section.^[7] Platy structure consists of thin, horizontal plates or layers that are longer and wider than they are thick, often forming in compacted surface soils or under waterlogged conditions where pressure from overlying material or saturation promotes horizontal orientation. These peds, typically 1-10 mm thick, restrict vertical water infiltration and root penetration due to their stacked arrangement, leading to poor drainage in affected horizons.^[34]^[35]^[6] Prismatic structure features vertically oriented, elongated columns or prisms with flat tops and sides, commonly developing in subsoil horizons of clay-rich soils where shrinking and swelling cycles create vertical cracks. These peds, often exceeding 5 cm in height and 1-5 cm in width, are prevalent in vertisols, which experience significant moisture fluctuations, and they facilitate moderate water movement while supporting root growth in agricultural settings.^[34]^[7]^[36] Columnar structure resembles prismatic but with rounded or dome-like tops on the vertical columns, arising in sodic soils where high sodium content disperses clay particles and promotes distinct vertical aggregation. These peds, usually 5-10 cm tall, inhibit root penetration and water infiltration due to their density, commonly occurring in saline-sodic environments that limit plant growth.^[34]^[7]^[37] Blocky structure includes angular and subangular variants, forming cube-like or polyhedral peds with roughly equal dimensions, typically 1.5-5 cm across, in clay-rich B horizons where moderate clay content and wetting-drying cycles foster stable aggregation. Angular blocky peds have sharp edges from strong vertical and horizontal cracking, while subangular ones exhibit rounded edges, both providing good stability for agricultural soils by balancing water retention and permeability.^[6]^[7]^[34] Granular or crumb structure comprises small, rounded, porous spheres or irregular crumbs, usually less than 1 cm in diameter, that dominate topsoils with high organic matter content from plant residues and microbial activity. This type enhances soil aeration, water infiltration, and root proliferation, making it ideal for productive surface horizons in well-managed ecosystems.^[34]^[35]^[6] Massive structure, also known as structureless, lacks distinct peds and appears as a coherent, blocky mass without visible aggregation, often in dense clays or heavily compacted soils where binding agents dominate without forming units. This type severely limits water flow, aeration, and root development due to minimal pore space.^[34]^[7]^[35] Single-grain structure consists of loose, unaggregated individual particles, such as in coarse sands, where weak interparticle forces prevent ped formation. This results in rapid water percolation but poor nutrient and water retention, commonly limiting soil fertility in sandy profiles.^[34]^[6]^[35]

Assessment Methods

Field Observation Techniques

Field observation techniques for soil structure involve direct, in-situ evaluation to assess aggregate formation, stability, and overall architecture without relying on laboratory equipment. These methods emphasize qualitative and semi-quantitative assessments that inform soil management decisions by revealing how structure influences water movement, root penetration, and biological activity. Practitioners typically excavate soil profiles to expose natural faces for examination, allowing observation of ped shapes, sizes, and grades across horizons.^[38] Visual and manual assessment begins with digging pits or using augers to access soil layers, where examiners inspect exposed ped faces for characteristics such as shape (e.g., granular, blocky, or prismatic), grade (weak, moderate, or strong based on aggregation distinctness), and porosity (e.g., presence of channels or vughs). For instance, a strong grade indicates well-defined peds that separate cleanly along natural planes, while a weak grade shows diffuse boundaries. Manual tests simulate aggregate stability by gently squeezing or dropping moist soil samples; in the Visual Soil Assessment (VSA) protocol, a 200 mm soil cube is dropped from a height of 0.5–1 m onto a firm surface to evaluate clod breakdown, with good structure evidenced by friable, porous aggregates that resist slaking. Hand tests for consistence, such as rupture resistance—compressing a 25–30 mm specimen between thumb and forefinger for one second—classify cohesion as loose (<8 N force) to indurated (>3 J energy), providing insights into structural integrity under field conditions.^[38]^[39]^[40] Tools essential for these assessments include shovels or spades for pit excavation (ideally 1 m wide for clear exposure), soil augers for sampling deeper horizons without full pits, and hand lenses (×10 magnification) for detailed porosity inspection. The ribbon test, which involves extruding moist soil between thumb and forefinger to form a coherent ribbon, highlights texture-structure interplay by estimating clay content; longer ribbons (e.g., >5 cm) indicate higher clay that promotes prismatic or blocky structures prone to compaction. A pocket penetrometer may supplement observations by measuring penetration resistance in dense layers, with values exceeding 2 MPa signaling poor structure. Moisture status must be noted during testing, as assessments are optimal in moist (friable) conditions to avoid misleading results from overly dry or wet soils.^[38]^[40] Key indicators of structure quality include crack patterns, which reveal compaction or desiccation—wide, deep cracks suggest massive or platy structures limiting infiltration, while fine shrinkage cracks indicate balanced aggregation. Root distribution serves as a proxy, with healthy structures showing extensive, branching roots penetrating horizons without deflection, whereas restricted growth around dense peds signals poor porosity. Earthworm channels and casts are positive indicators of biological enhancement to structure, as their abundance (e.g., >10 channels per square meter) correlates with improved aggregation and macroporosity from bioturbation. These proxies are observed directly in profile faces or surface excavations.^[38]^[41] Standardized protocols ensure consistency, such as those in the USDA Soil Survey Manual, which guide horizon-specific evaluation by recording structure grade, size (e.g., fine: 1–2 mm; coarse: 5–10 mm), and type alongside boundary topography. The FAO Guidelines for Soil Description provide tables for classifying structure (e.g., Table 48 for types and sizes) and porosity abundance (e.g., many voids: 15–40% volume), recommending assessments in recently dug pits under natural light. The VSA field guide offers a scorecard system (scores 0–2) for rapid site evaluation across multiple locations, typically four sites per 5 ha, to benchmark structure against photographic references. These protocols prioritize safety, such as stabilizing pit walls, and integrate observations with site factors like slope and vegetation.^[38]^[40]^[39]

Laboratory Analysis Methods

Laboratory analysis methods provide precise, quantitative assessments of soil structure under controlled conditions, enabling the measurement of aggregate stability, pore architecture, and physical properties that influence soil functionality. These techniques typically involve sample preparation from field-collected cores or aggregates, followed by standardized procedures to evaluate structural integrity and void space characteristics. Unlike field observations, laboratory approaches allow for replication and isolation of variables, such as water content or mechanical stress, to derive metrics like stability indices and density values. Aggregate stability tests evaluate the resistance of soil particles to dispersion and slaking in water, key indicators of structural cohesion. The Kemper wet sieving method, a widely adopted technique, involves pre-wetting air-dried aggregates (typically 1-2 mm in size) to minimize slaking effects, followed by oscillatory sieving through a stack of nested sieves with decreasing mesh sizes (e.g., 2 mm to 0.25 mm) in water for a set duration, such as 30 minutes at 30 oscillations per minute. This process quantifies the proportion of stable aggregates by measuring the mass retained on each sieve after dispersion, providing a stability index that reflects resistance to both slaking (initial disruption due to entrapped air displacement) and mechanical breakdown. Developed as a standard protocol, this method has been instrumental in comparing soil management impacts on structure, with higher retained fractions indicating better stability in soils under conservation tillage. Slaking tests complement wet sieving by focusing specifically on water-induced disintegration; in a typical laboratory setup, intact aggregates (e.g., 5-10 mm) are immersed in water without agitation, and stability is assessed visually or by mass loss after 10-60 minutes, often rated on a scale from 0 (complete dispersion) to 4 (intact), or quantified via image analysis for disaggregation kinetics. This test isolates the effects of rapid wetting on aggregate breakdown, revealing vulnerabilities in poorly structured soils prone to crusting.^[42] Microstructural analysis techniques visualize and quantify pore architecture at the microscale, essential for understanding intra-aggregate voids and connectivity. Thin-section microscopy, a foundational method in soil micromorphology, prepares undisturbed soil samples by impregnating cores with resin, cutting 20-30 μm thick slices, and examining them under a polarizing light microscope to identify fabric types, pore shapes (e.g., channels, vughs), and mineral arrangements. This approach, standardized for descriptive analysis, allows quantification of porosity and pore size distribution through point counting or digital image processing, where voids appear as dark areas against the mineral matrix, providing insights into pedogenic processes like illuviation. X-ray computed tomography (CT) offers non-destructive 3D imaging of soil structure by generating cross-sectional X-ray attenuation maps from rotated samples, reconstructing volumetric data at resolutions down to 1-50 μm depending on the scanner. In soil applications, CT delineates pore networks, aggregate boundaries, and tortuosity, enabling segmentation of macropores (>75 μm) and micropores for connectivity analysis, with studies showing its utility in detecting compaction-induced pore reduction in tilled soils.^[43] Physical measurements in the laboratory directly assess bulk properties tied to structure, such as density and water retention. Bulk density is determined via core sampling, where cylindrical metal rings (e.g., 5 cm diameter, 5 cm height) are driven into undisturbed soil to extract known-volume samples, which are then oven-dried at 105°C and weighed; the ratio of dry mass to volume yields bulk density (typically 1.0-1.6 g cm⁻³), inversely related to porosity and indicative of compaction. This core method ensures minimal disturbance, making it reliable for structured soils. Water retention curves, plotted as volumetric water content versus matric potential (e.g., 0 to -15 bar), are generated using pressure plate apparatus, where saturated soil samples in ceramic plates are equilibrated under increasing air pressure in a sealed chamber, with drainage collected to measure retained water at discrete potentials. This technique, foundational for characterizing soil hydraulic properties, reveals structure effects on pore size distribution via the van Genuchten model fit, where well-structured soils exhibit higher retention at low tensions due to macropores. Field porosity, often cross-referenced, aligns with lab-derived values from these measurements. Advanced metrics from these analyses provide numerical descriptors of structural complexity. The mean weight diameter (MWD) of aggregates, calculated from wet sieving data as MWD = \sum_{i=1}^{n} \bar{x}_i w_i, where \bar{x}_i is the mean diameter of the i-th size class and w_i its weight fraction, quantifies overall stability; values >1 mm suggest resilient structure in agricultural soils. Fractal dimension (D), derived from image analysis of thin sections or CT scans using box-counting methods on pore boundaries, measures irregularity and self-similarity (typically 2.0-3.0 for soil aggregates), with higher D indicating more fragmented, less stable structures prone to erosion. These metrics integrate multiple lab data points for predictive modeling of soil behavior.^[44]

Dynamics and Alterations

Temporal Changes in Structure

Soil structure undergoes dynamic changes over various timescales due to natural environmental processes, influencing aggregate stability and overall soil functionality. Seasonal variations, particularly wetting-drying and freeze-thaw cycles, play a key role in altering ped stability. During wetting-drying cycles, rapid water infiltration can cause slaking, where air-dried aggregates disintegrate upon immersion due to trapped air expansion and differential swelling, reducing stability especially in the initial cycles.^[45] Freeze-thaw cycles further disrupt structure by inducing mechanical stress through ice formation and expansion, leading to aggregate breakdown and increased porosity in near-surface soils, with effects intensifying under repeated exposure.^[46] These cyclic processes can enhance interconnected pore networks over time in some soils, promoting better drainage while temporarily weakening stability during active phases, as macroporosity increases with soil drying.^[47] Over longer timescales, soil structure exhibits improvement in vegetated fallow or grassland conversion compared to continuously cultivated fields, reflecting recovery from natural pedogenic influences. In such systems, prolonged periods without disturbance allow organic matter accumulation and root activity to foster aggregate formation, resulting in enhanced macrostructure stability after at least 10 years, with noticeable changes emerging within 2 years under favorable conditions.^[48] Conversely, intensive cultivation accelerates structural decline through repeated mechanical disruption, though natural recovery in fallow phases can partially counteract this by rebuilding pore architecture and aggregate cohesion.^[49] These dynamics highlight how land rest contributes to sustained structural evolution, distinct from initial formation processes.^[50] Feedback loops between soil structure and weathering rates create self-reinforcing mechanisms for stability. Well-structured soils with stable aggregates limit water and oxygen access to mineral surfaces, thereby slowing chemical weathering and preserving structural integrity over time.^[51] In contrast, poorly structured soils expose more surfaces to weathering agents, accelerating mineral breakdown and organic matter incorporation, which can eventually promote aggregation and stabilize the system through increased binding agents.^[52] This interplay fosters resilience, as enhanced structure reduces further weathering vulnerability, maintaining a balanced pedogenic equilibrium. Recent studies indicate that climate change is altering these temporal dynamics, with shifts in freeze-thaw cycles exacerbating soil erosion and modifying pore structures in various ecosystems as of 2025.^[53]

Degradation Mechanisms

Soil structure degradation refers to the breakdown of soil aggregates and pores, primarily driven by anthropogenic activities that disrupt the physical arrangement of soil particles. These processes compromise soil functionality, including water infiltration, root penetration, and nutrient cycling, often leading to reduced agricultural productivity and environmental impacts. Key mechanisms include compaction, erosion and dispersion, loss of organic matter, and pollution effects such as salinization and acidification.^[1] Compaction occurs when external pressures from heavy machinery or livestock trampling compress soil particles, increasing bulk density and diminishing pore space. In agricultural settings, repeated passes of equipment weighing over 20 tons can compact soil to depths of up to 3 feet (0.91 m), with the first few passes achieving 90-95% of maximum compaction. Bulk densities exceeding 1.6 g/cm³—considered a threshold for most soils—restrict root growth and aeration by reducing macropores larger than 60 µm, shifting the soil toward anaerobic conditions and altering aggregate stability. Livestock concentration in grazed areas exacerbates this by destroying aggregates through hoof pressure, particularly in wet soils where forces are amplified.^[54]^[55] Erosion and dispersion further deteriorate structure by physically removing or destabilizing aggregates via water or wind action, often compounded by sodicity. Water erosion detaches aggregates through runoff on disturbed surfaces, leading to surface crusting and loss of fine particles, while wind erosion strips exposed topsoil, accelerating organic matter depletion and weakening remaining structure. In sodic conditions, where sodium dominates exchange sites (e.g., exchangeable sodium percentage >15%), clay particles swell and disperse upon wetting, clogging pores and forming dense, impermeable layers upon drying. This dispersion, influenced by agents like sodium, renders soils highly susceptible to both erosive forces, impairing infiltration and promoting further degradation.^[1]^[56] Loss of organic matter from intensive tillage and cropping practices diminishes the binding agents essential for aggregate formation, resulting in massive or poorly structured soils. Tillage exposes organic residues to microbial decomposition by increasing soil oxygen, while crop removal limits inputs, causing steady declines in organic matter levels. These binding agents, including microbial byproducts and fungal hyphae, stabilize aggregates; their reduction disrupts this network, leading to cloddy or massive structures that resist crumbling and hinder water retention. Even single tillage events accelerate this loss, with repeated operations compounding the effect on soil cohesion.^[57]^[58] Pollution through salinization and acidification, prevalent in intensive farming since the mid-20th century, disrupts chemical bonds within soil colloids, further eroding structure. Salinization from excessive irrigation raises soluble salt concentrations (e.g., electrical conductivity >4 dS/m), promoting clay dispersion and reducing permeability, as seen in regions like Central Asia where over 60% of irrigated lands suffer secondary salinization as of 2020.^[59] Acidification, often from manure decomposition or fertilizer overuse, lowers pH and forms organic acids that weaken aggregate stability by altering cation exchange and microbial activity. In intensively farmed tropical soils, these processes have degraded structures since mid-20th century expansions, increasing erosion vulnerability and compaction.^[60]

Management and Restoration

Improvement Strategies

Organic amendments, such as compost and manure, enhance soil structure by promoting aggregate formation through increased microbial activity and the production of binding agents like polysaccharides and fungal hyphae. These amendments supply organic matter that serves as an energy source for soil microorganisms, which in turn facilitate the binding of soil particles into stable aggregates, improving overall soil tilth and water retention. For instance, long-term application of compost has been shown to increase aggregate stability and microbial biomass, leading to better soil structure in agricultural fields. Similarly, manure amendments boost soil organic carbon content, which correlates with enhanced aggregation and reduced susceptibility to erosion. Tillage practices play a crucial role in maintaining or restoring soil structure, with reduced or conservation tillage methods minimizing soil disturbance to preserve existing aggregates and prevent compaction. Conservation tillage, which leaves crop residues on the surface, helps maintain pore continuity and increases organic matter accumulation in the topsoil, thereby supporting long-term structural stability. In contrast, deep ripping or subsoiling can alleviate compaction in deeper layers by fracturing hardpans, allowing for better root penetration and water infiltration without excessive disruption to surface aggregates when applied judiciously. Cover cropping and crop rotation contribute to soil structure improvement by leveraging plant roots to physically bind soil particles and create biopores that enhance porosity. Cover crops with extensive root systems, such as legumes and grasses, exude organic compounds that promote microbial activity and aggregate formation, while their decay adds to soil organic matter. Crop rotations that include diverse species further stabilize structure by varying root architectures, which collectively improve soil aeration and drainage over time. Integrating these strategies with proper irrigation management amplifies their benefits, as controlled water application prevents excessive wetting that could lead to compaction while allowing amendments and roots to effectively influence structure. Studies indicate that combining organic amendments, conservation tillage, and cover crops can increase macroporosity and total porosity, with reported gains up to 20-30% in some cases depending on soil type and conditions.^[61] Such integrated approaches underscore their efficacy in enhancing soil functionality. Recent advances include biochar amendments, which further stabilize aggregates (as of 2023).^[62]

Handling Problematic Structures

Hardsetting soils are characterized by their tendency to form a firm, compact mass upon drying after wetting, often resulting in surface crusting that impedes seedling emergence and root penetration.^[63] These soils are prevalent in arid and semi-arid regions, where low organic matter content exacerbates aggregate instability and promotes hardening. Key causes include high clay content with swelling minerals like smectite, which facilitate slaking during wetting, followed by cementation through physical compaction or binding agents such as iron oxides in certain profiles.^[63] Where dispersive, gypsum can be applied at 2-3 t/ha to improve structure by suppressing dispersion.^[64] Remediation of hardsetting soils typically involves targeted amendments to enhance dispersion and maintain moisture. In Australian semiarid zones, such as New South Wales Red Sodosols, lime incorporation at 5 t/ha has demonstrated long-term improvements in soil permeability and reduced hardsetting behavior over 12 years.^[65] Mulching with organic residues, such as straw or crop stubble, helps retain surface moisture, delaying drying and hardening processes under high evaporation conditions.^[66] Other problematic soil structures require specific interventions to restore functionality. Sodic soils, marked by high sodium levels leading to dispersed clays and poor permeability, benefit from gypsum application, which supplies calcium ions to replace sodium and promote flocculation. Rates are typically 2-5 t/ha based on exchangeable sodium percentage (ESP), improving structure and infiltration in calcareous profiles.^[67] For compacted massive soils, where bulk density exceeds 1.6 g/cm³ and restricts rooting, bio-drilling with deep-rooted plants such as daikon radish or alfalfa creates biopores that alleviate compaction without mechanical disturbance. These plants penetrate up to 0.6-1 m deep, forming channels that persist for subsequent crops.^[68] Success in handling these structures is monitored through indicators of enhanced friability, such as reduced penetration resistance (below 2 MPa) and increased aggregate stability.^[69] Post-treatment assessments often involve field tests like the drop-cone penetrometer or wet-sieving for slaking resistance, revealing improvements in crumbly texture and root proliferation within 1-3 years.^[70] While organic amendments from general strategies can support these efforts, targeted monitoring ensures corrective measures address the specific structural deficits.^[69]

References

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None
### Definition of Soil Structure
[2]
Soil Structure | Soils - Part 2: Physical Properties of Soil and Soil Water
Soil structure refers to the arrangement of soil separates into units called soil aggregates. An aggregate Soil separates that are grouped into a unit.
[3]
[PDF] Appendix Basic Soil Science and Soil Fertility
The arrangement of the aggregates determines the soil's structure (Figure A-5). Structure is an important soil characteristic because good structure allows ...
[4]
[PDF] Basic Soil Properties - USDA
Soil structure is the arrangement of soil particles into small clumps, called “peds”. • Soil structure and texture affect how the soil will hold water and ...
[5]
Introduction to Soils: Soil Quality - Penn State Extension
Jan 24, 2023 · While soil texture is the proportion of the three soil particle types (sand, silt, and clay), soil structure refers to how those particles are ...Missing: distinction | Show results with:distinction
[6]
Soil Texture and Soil Structure - CTAHR
Soil texture and soil structure are both unique properties of the soil that will have a profound effect on the behavior of soils.
[7]
Soil Structures - Soil Ecology Wiki
Apr 18, 2023 · Soil structures are defined by clump sizes which may be less than 0.5 cm in diameter, 1.5-5 cm in diameter, or greater than 5 cm in diameter.
[8]
SL 277/MG457: Soils and Fertilizers for Master Gardeners
Soil structure is described by the shape of the soil aggregates and how they form in the soil profile (e.g., platy, columnar, granular, blocky, prismatic, or ...
[9]
Soil Terminology and Definitions | Ohioline
May 4, 2012 · Aggregates: Primary soil particles (sand, silt, and clay) held together in a single mass or cluster, such as a crumb, block, prism or clod using ...
[10]
[PDF] Soil Investigations
Aggregates that occur naturally in the soil are referred to as peds, while clumps of soil caused by tillage are called clods. Structure is formed in two ...
[11]
[PDF] Soil Moisture Management
Micropores inside the peds and macropores between the peds carry air, water and facilitate root penetration.
[12]
[PDF] Scale Model of a Soil Aggregate and Associated Organisms
Pore spaces. Soil pores are classified by size as macropores and micropores. Macropores are larger than 80 µm; meso- pores range in size from 30 to 80 µm. Mi-.
[13]
Organic matter and water‐stable aggregates in soils - TISDALL - 1982
The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient, ...
[14]
No. 1. Tisdall, J. M. & Oades, J. M. 1982. Organic matter and water ...
9 Aug 2025 · Tisdall, J. M. & Oades, J. M. 1982. Organic matter and water-stable aggregates in soils. Journal of Soil Science, 33, 141-163 Commentary on the ...
[15]
Improving Soil Structure for Increased Infiltration and Water Holding ...
Aug 16, 2021 · Soils with good structure have adequate pore space making them well drained while still having good water and nutrient holding capacity.
[16]
Soil compaction | UMN Extension
A compacted soil has a reduced rate of both water infiltration and drainage. This happens because large pores more effectively move water downward through the ...
[17]
[PDF] Soil Health Land Management Physical Soil Properties
Soil structure and aggregation promote: • Water infiltration/internal drainage and water retention. • Soil aeration. • Resistance to erosion. • Organic matter.
[18]
Soil Quality Information
Feb 17, 2025 · Important physical characteristics of soil-like structures and aggregation allow water and air to infiltrate, roots to explore, and biota to ...
[19]
1. Soils & Plant Nutrients | NC State Extension Publications
Soil structure refers to the grouping of individual soil particles into larger pieces called peds or aggregates.<|control11|><|separator|>
[20]
Soil Health - Natural Resources Conservation Service - USDA
Healthy soils cycle crop nutrients, support root growth, absorb water and sequester carbon more efficiently. Reducing Soil Erosion – Soil that is covered year- ...
[21]
Soil biology - University of Minnesota Extension
A farmer's guide to soil biology, including bacteria, fungi and nematodes. Also covers strategies for increasing and decreasing microbial populations.
[22]
microbial communities and their transformative role in soil health ...
Mar 25, 2025 · Among these, microbiomes are essential for nutrient cycling, organic matter decomposition, and soil structure, rendering them crucial markers ...
[23]
[PDF] Bulk Density - Natural Resources Conservation Service
As a rule of thumb, most rocks have a bulk density of 2.65 g/cm3 so ideally, a medium textured soil with about 50 percent pore space will have a bulk density ...
[24]
[PDF] understanding the soil health knowledge
In an analysis of indicators of soil physical quality, Kuykendall (2008) found that bulk density is the best physical indicator of soil quality since it ...
[25]
[PDF] Basic Soil Processes - Celebrating the
Four basic processes occur in soils— additions, losses, transformations (changes), and translocation (movement). A PowerPoint presentation provides some ...Missing: natural | Show results with:natural
[26]
[PDF] Soil formation factors and processes
The evolution of true soil from regolith takes place by the combined action of soil forming factors and processes. ➢ The first step is accomplished by ...<|control11|><|separator|>
[27]
(PDF) Soil Forming Processes - ResearchGate
Mar 10, 2017 · Pedogenesis can be classically described as a set of processes of addition, loss, transformation, and translocation experienced by an in situ ...
[28]
Soil Aggregate Microbial Communities: Towards Understanding ...
May 10, 2019 · Soil aggregates, classified as either microaggregates (<250 μm) or macroaggregates (0.25 to 2 mm), self-organize from clays, carbonates, and ...
[29]
Page not found | ScienceDirect
Insufficient relevant content.
[30]
Organic Matter and Water-stable Aggregates in Soils - ResearchGate
5 Aug 2025 · The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient, ...
[31]
Soil Aggregate - an overview | ScienceDirect Topics
A soil aggregate is a group of primary soil particles that cohere to each other more strongly than to other surrounding particles.
[32]
7. Soil Structure
Soil structure is defined by the way individual particles of sand, silt, and clay are assembled. Single particles when assembled appear as larger particles.
[33]
https://www.fao.org/3/x5871e/x5871e04.htm
[34]
Soil and Soil Water Relationships | VCE Publications - Virginia Tech
Mar 1, 2021 · This publication presents and discusses concepts that are fundamental to understanding soil, water, and plant relationships and the soil water balance.
[35]
Soil Properties, Part 1 of 3: Physical Characteristics | Extension
The arrangement of soil particles into aggregates of definite shape is known as soil structure. Soil structures are also known as peds (Natural Resources ...Missing: components | Show results with:components
[36]
[PDF] Illustrated Guide to Soil Taxonomy
Cryoturbation is the mixing of the soil as a result of intense frost churning due to freeze-thaw cycles. (See Gelic Materials). Generalized Characteristics. 1 ...
[37]
Soil Salinity, Sodicity, and Alkalinity in South Dakota Soils
Dec 5, 2024 · Sodium disperses soil clays and causes them to move downward from the A to the B horizon. Classification of sodium and salt affected soils is ...
[38]
[PDF] Soil Survey Manual 2017; Chapter 3
Internal features, such as color, texture, and structure, are observed from the study of the pedon. Observing Pedons. In order to observe a pedon fully, ...
[39]
[PDF] FIELD GUIDE
To this end, Visual Soil Assessment (VSA) provides a quick and simple method to assess soil condition and plant performance. It can also be used to assess the ...
[40]
[PDF] Guidelines for soil description
These guidelines are based on the internationally accepted Guidelines for Soil. Description (FAO, 1990). Some new international developments in soil information.
[41]
[PDF] IN-FIELD INDICATORS OF HEALTHY SOILS
Healthy soils have a surface layer that isn't dense or dried out with cracks. Healthy soil profiles have: • Granular or aggregated structure. • Roots that grow ...Missing: FAO | Show results with:FAO
[42]
[PDF] Soil Aggregate Stability Kit - USDA ARS
At the end of the test, the fragments were oven-dried at 60°C and re-weighed. Aggregate stability was calculated as the soil remaining as a percent by weight ...
[43]
Application of X-ray computed tomography to soil science
X-ray computed tomography (CT) is a non-destructive and non-invasive technique that has been successfully used for three-dimensional (3D) examination of soil.
[44]
Applications of Fractals to Soil Studies - ScienceDirect.com
This chapter discusses the applications of fractals for soil analysis. The different types of fractal dimensions are discussed that are used in soil science.
[45]
Effect of drying–wetting cycles on aggregate breakdown for yellow ...
Sep 15, 2017 · Drying-wetting cycles cause a significant aggregate slaking, especially within the first two cycles. After that, most aggregates show more slacking resistant.Missing: ped | Show results with:ped
[46]
Effects of Freeze–Thaw Cycles and the Prefreezing Water Content ...
Under the influence of atmospheric temperature changes, seasonal freeze–thaw cycles (FTCs) inevitably occur in near-surface soils. This leads to physical, ...Missing: slaking | Show results with:slaking
[47]
Seasonal variation of soil aggregate stability, porosity and infiltration ...
Successive soil wetting and drying cycles lead a massive soil structure to evolve into a structure with a large number of large interconnected pores ( Pires et ...
[48]
Significant structural evolution of a long‐term fallow soil in response ...
Aug 16, 2020 · Significant soil structural changes, especially under grassland, require at least 10 years after conversion, with some changes visible after 2 ...
[49]
(PDF) Green fallow soil vs. intensive soil cultivation – a study of soil ...
Fallow soils have a significantly better structure than soils under a cultivated field. ... soil structure, the present study is composed of long-term field ...
[50]
[PDF] Impact of different fallow durations on soil aggregate structure and ...
Jun 10, 2019 · Fallow soils have a significantly better structure than soils under a cultivated field. Long-term cultivation leads to the deterioration of soil ...
[51]
Organic matter governs weathering rates and microstructure ...
Jan 1, 2023 · We hypothesize that initial soil structure formation starts with weathering and is severely affected by organic matter.
[52]
[PDF] Embracing the dynamic nature of soil structure
Nov 26, 2021 · Soil structure dictates the character and pace of many critical zone (CZ) processes such as chemical weathering, groundwater flow, and carbon ...
[53]
Variation in Soil Aggregate Stability Due to Land Use Changes from ...
Feb 1, 2023 · In comparison with temperate grassland, soil aggregate stability in alpine grassland was higher in this study. In the 0–20 and 20–40 cm soil ...
[54]
Soil C/N ratios cause opposing effects in forests compared to ...
Aug 25, 2023 · Soil C/N ratios cause opposing effects in forests compared to grasslands on decomposition rates and stabilization factors in southern European ...
[55]
[PDF] Soil Compaction Monograph - UGA Open Scholar
Soil handling equipment can be large and heavy allowing compaction many inches deep. ... In another soil, compaction from a bulk density of 1.25g/cc (~50%. Page ...Missing: livestock cm³
[56]
Soil Health Glossary - Natural Resources Conservation Service
bulk density (Db or BD): The density of soil, i.e., the weight of soil divided by its volume. The BD of agricultural soils normally ranges from 1.0 to 1.6 g/cm3 ...
[57]
[PDF] Saline and Sodic Soils - North Dakota State University
Sodic soils are low in total soluble salts but high in exchangeable sodium, which tends to disperse soil particles and destroys soil structure (Management of ...
[58]
Soil organic matter in cropping systems
Even though they might need to wait until aggregates break apart after wet/dry or freeze/thaw cycles, aggregates guarantee that there is always a snack in the ...
[59]
Managing Soil Health: Concepts and Practices
### Summary of Tillage Effects on Organic Matter Decomposition and Aggregate Disruption
[60]
None
### Summary of Acidification and Salinization Effects on Soil Structure in Intensive Farming Contexts Since the 20th Century
[61]
[PDF] Mapping soil slaking index and assessing the impact of ...
Jan 22, 2021 · In severe cases, crusting or hard setting occurs when slaked and dispersed aggregates coalesce and set hard on drying (Mullins et al., 1990).
[62]
[PDF] Organic matter and water-stable aggregates in soils - Regulations.gov
Hydrous oxides of aluminium and iron cement particles together in water-stable aggregates with diameters greater than IOOpm, especially in soils which contain ...
[63]
Influence of lime and gypsum on long-term rehabilitation of a Red ...
This paper determines the influence of lime and gypsum on the rehabilitation of a degraded sodic soil in a semiarid environment 12 years after application.
[64]
Understanding how management can prevent degradation of ... - NIH
In the short-term, mulch is thought to maintain soil moisture and delay hardening even under high evaporation rates. However, though positive short-term ...
[65]
[PDF] Stabilization of Clay Soils by Portland Cement or Lime— A Critical ...
A good soil stabilizer should provide calcium ions (Ca2+) in sufficient amount so that the monovalent cations, especially Na+, adsorbed on the cleavage surfaces ...
[66]
[PDF] Sustainable Landscaping Practice for Enhaning Vegetation ...
Even deep-rooted, biodrilling plants can't penetrate through highly compacted soils. When conditions do not allow for biological remediation, a form of active ...
[67]
[PDF] Comprehensive Assessment of Soil Health
This is the 3rd edition of the "Comprehensive Assessment of Soil Health" manual, based on the "Cornell Framework" and previously titled "Cornell Soil Health ...Missing: workability | Show results with:workability
[68]
Mapping soil slaking index and assessing the impact of ...
Jan 22, 2021 · In this study the SLAKES application was used to obtain slaking index (SI) values of topsoil samples (0 to 10 cm) at 158 sites to assess aggregate stability in ...