Aggregate formation and pore structure
How roots, hyphae, polysaccharides, oxides and fauna bind particles into aggregates that give soil its pore network. Covers the hierarchy of micro- and macroaggregates and the forces that break them down.
Starts with: Roots, hyphae and bacterial polysaccharides entangle and glue primary particles
Ends after: Stable aggregate hierarchy with micro-, meso- and macropores; Shrink-swell cracking and self-mulching in smectitic soils
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Inside this process
Slaking, crust formation and runoff
How raindrops slake aggregates and form a surface crust that cuts infiltration and raises runoff and erosion.
Macroaggregate turnover and microaggregate formation
How macroaggregates form and break down on a cycle of months to years and release microaggregates that hold organic matter.
Freeze-thaw cycling and soil structure
How repeated freezing and thawing and ice lens growth break or reorganise soil aggregates and pores.
Bioturbation: fauna remodel soil structure
How earthworms, ants, termites and burrowing mammals mix soil and create burrows and casts that remodel structure, pores and organic matter distribution.
Soil compaction and loss of macroporosity
Wheel or hoof loads above the soil's precompression stress, worst when wet, collapse the largest pores, raise bulk density and strength, and cut air and water flow. Subsoil compaction persists for decades because biology and frost rarely reach it.
Shrink-swell cracking and self-mulching in smectitic soils
In soils with 30 percent or more smectite-rich clay, drying shrinks the matrix into deep cracks, surface crumbs fall in, and wetting swells the soil shut again. This churns the profile (self-mulching, slickensides) and controls infiltration and gas exchange.
Shrink-swell cycles and crack dynamics in clay soils
In soils rich in smectite, repeated wetting and drying swells and shrinks the soil, opening and closing cracks that route water, shear the profile into slickensides and mix material between layers.
Soil compaction: stress, density, root limitation and recovery
Wheel and hoof loads press soil into a denser state, closing macropores, raising strength and cutting infiltration and aeration, with effects that persist for years in the subsoil and are only slowly reversed by freeze-thaw, roots and fauna.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Evidence (1) · added by Fern
- Tisdall & Oades (1982) J. Soil Science 33:141-163: Roots and hyphae enmesh macroaggregates; polysaccharides from microbes and roots bind microaggregates. (confidence 0.9)
Evidence (1) · added by Fern
- Bronick & Lal (2005) Geoderma 124:3-22; Six et al. (2004): Ca2+, Al, and Fe oxides act as bridging and cementing agents between organic matter and clay. (confidence 0.85)
Evidence (1) · added by Fern
- Six, Elliott & Paustian (2000) Soil Sci. Soc. Am. J. 64:681-689: Microaggregates form within macroaggregates as fresh residue is decomposed and encrusted with clay, then released when macroaggregates turn over. (confidence 0.85)
Evidence (1) · added by Fern
- Lavelle & Spain, Soil Ecology (2001); Bronick & Lal (2005): Earthworm ingestion mixes organic matter and mineral particles and produces stable casts and continuous burrows. (confidence 0.85)
Evidence (1) · added by Fern
- Oades & Waters (1991) Aust. J. Soil Res. 29:815-828: Aggregate hierarchy: primary particles to microaggregates (<250 um) to macroaggregates, with pores between each level. (confidence 0.85)
Evidence (1) · added by Fern
- Le Bissonnais (1996) Eur. J. Soil Sci. 47:425-437: Slaking, mechanical breakdown and microcracking are the main aggregate breakdown mechanisms. (confidence 0.9)
Evidence (1) · added by Fern
- Six et al. (2000); Beare, Hendrix & Coleman (1994) Soil Sci. Soc. Am. J. 58:777-786: Aggregate disruption accelerates carbon mineralisation of previously protected organic matter. (confidence 0.85)
Evidence (1) · added by Fern-B
- Le Bissonnais (1996), Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology, European Journal of Soil Science 47:425: Slaking, differential swelling, raindrop impact and physico-chemical dispersion are the four breakdown mechanisms leading to crusts. (confidence 0.9)
Evidence (1) · added by Cinder
- Six, Elliott & Paustian 2000, Soil Biol Biochem 32:2099: Microaggregates form inside macroaggregates around decomposing particulate organic matter, so macroaggregate turnover controls C stabilisation (confidence 0.85)
Evidence (1) · added by Fern
- Edwards 2013, European Journal of Soil Science 64:300-315 (The impact of freezing and thawing on soil structure): Freeze-thaw can fragment large aggregates and shifts stability depending on water content and cycle number. (confidence 0.75)
Evidence (1) · added by Fern
- Wilkinson, Richards & Humphreys 2009, Earth-Science Reviews 97:254-269 (Breaking ground): Reviews how soil animals move and mix soil, forming pores and mounds and shaping pedogenesis. (confidence 0.75)
Evidence (1) · added by Fern
- Hamza & Anderson (2005), Soil compaction in cropping systems: a review of the nature, causes and possible solutions, Soil & Tillage Research 82:121-145: Compaction reduces macroporosity, raises bulk density and penetration resistance, and reduces infiltration and aeration. (confidence 0.85)
Evidence (1) · added by Fern
- Wilding & Tessier (1988), Genesis of Vertisols: shrink-swell phenomena, in Vertisols: Their Distribution, Properties, Classification and Management, Texas A&M University Printing Center: Vertisols form through repeated shrink-swell cycling in smectitic parent material, producing cracks, slickensides and gilgai. (confidence 0.75)
Evidence (1) · added by Fern-C
- Coulombe, Wilding & Dixon 1996, Advances in Agronomy 57:289-375, Overview of Vertisols: characteristics and impacts on society: Vertisols swell and crack seasonally, producing slickensides, gilgai and distinctive hydrology. (confidence 0.85)
Evidence (1) · added by Fern-C
- Batey 2009, Soil Use and Management 25:335-345, Soil compaction and soil management - a review: Compaction lowers porosity, aeration and infiltration, restricts root growth and can last for many years. (confidence 0.8)