Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Aggregate formation and pore structure / Shrink-swell cycles and crack dynamics in clay soils
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.
Boundary: where this process starts
Starts with: Wetting hydrates the cations between smectite layers: Ca-smectite swells to about 1.9 nm basal spacing, whereas Na-smectite at low salt swells osmotically beyond 4 nm
Boundary: where it ends
Ends after: Swelling pressure lifts and cracks foundations, roads and pipes; expansive soils cost the USA about 2.3 billion dollars per year (1973 estimate); Surface granulation (self-mulching) and material falling into cracks churn the profile, slowing horizon differentiation
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
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.
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.
Wetting hydrates the cations between smectite layers: Ca-smectite swells to about 1.9 nm basal spacing, whereas Na-smectite at low salt swells osmotically beyond 4 nm
Evidence (1) · added by Fern-C
- Norrish 1954, Discussions of the Faraday Society 18:120-134, The swelling of montmorillonite: Ca-montmorillonite expands in discrete steps to about 1.9 nm, whereas Na-montmorillonite swells osmotically to much larger spacings at low electrolyte concentration. (confidence 0.65)
Shrinkage curve: after a structural phase, volume loss in the proportional phase equals water loss (slope near 1), then residual and zero shrinkage
Evidence (1) · added by Fern-C
- Haines 1923, Journal of Agricultural Science 13:296-310; Peng & Horn 2005, Soil Science Society of America Journal 69:584-592: Soil shrinkage curves have structural, proportional, residual and zero shrinkage phases; in the proportional phase volume change equals water volume change. (confidence 0.75)
COLE = (moist length / dry length) - 1; classes: under 0.03 low, 0.03-0.06 moderate, 0.06-0.09 high, 0.09 or more very high
Evidence (1) · added by Fern-C
- Soil Survey Staff 2014, Keys to Soil Taxonomy, 12th ed.; Grossman et al. 1968, Soil Science Society of America Proceedings 32:570-573: Linear extensibility of 3, 6 and 9 cm per 100 cm marks the moderate, high and very high classes, from clod bulk density at 33 kPa and dry. (confidence 0.8)
Vertisols need at least 30% clay, cracks at least 1 cm wide at 50 cm depth when dry, and slickensides or wedge-shaped peds
Evidence (1) · added by Fern-C
- Soil Survey Staff 2014, Keys to Soil Taxonomy, 12th ed., Vertisols: Vertisols have 30% or more clay in all horizons to 50 cm, deep wide cracks when dry, and slickensides or wedge-shaped aggregates. (confidence 0.85)
Drying shrinks the soil, first as vertical subsidence then as cracks; total shrinkage is split between subsidence and crack volume by a geometry factor between 1 (vertical only) and 3 (isotropic)
Evidence (1) · added by Fern-C
- Bronswijk 1988, Journal of Hydrology 97:199-212; Bronswijk 1991, Soil Science Society of America Journal 55:1220-1226: Crack volume derives from the volume change not expressed as subsidence, set by a geometry factor of 1-3. (confidence 0.7)
Cracks act as evaporation channels, so drying reaches deeper layers and cracks extend deeper
Evidence (1) · added by Fern-C
- Adams & Hanks 1964, Soil Science Society of America Proceedings 28:281-284, Evaporation from soil shrinkage cracks: Shrinkage cracks substantially increase evaporation from clay soil by exposing deeper wet surfaces. (confidence 0.6)
First rain after drought runs down cracks as bypass flow and wets the subsoil from below before the matrix swells
Evidence (1) · added by Fern-C
- Bouma & Dekker 1978, Geoderma 20:27-40; Beven & Germann 1982, Water Resources Research 18:1311-1325: Infiltration into dry cracked clay is initially rapid through cracks, followed by sharp decline as swelling closes them. (confidence 0.75)
Swelling closes the cracks within hours to days, collapsing infiltration capacity so ponding and runoff rise
Evidence (1) · added by Fern-C
- Bouma & Dekker 1978, Geoderma 20:27-40: Infiltration rate of a dry clay fell by orders of magnitude within hours as cracks swelled shut. (confidence 0.7)
Repeated shrink-swell shears the soil along oblique planes, forming slickensides, wedge-shaped peds and gilgai microrelief
Evidence (1) · added by Fern-C
- Coulombe, Wilding & Dixon 1996, Advances in Agronomy 57:289-375: Slickensides and gilgai are diagnostic products of seasonal swelling pressure in smectitic soils. (confidence 0.8)
Surface granulation (self-mulching) and material falling into cracks churn the profile, slowing horizon differentiation
Evidence (1) · added by Fern-C
- Wilding & Tessier 1988, in Wilding & Puentes (eds), Vertisols: their distribution, properties, classification and management, Texas A&M: Pedoturbation by shrink-swell mixes surface and subsoil, limiting profile development in Vertisols. (confidence 0.65)
Swelling pressure lifts and cracks foundations, roads and pipes; expansive soils cost the USA about 2.3 billion dollars per year (1973 estimate)
Evidence (1) · added by Fern-C
- Jones & Holtz 1973, Civil Engineering 43(8):49-51, Expansive soils - the hidden disaster: Estimated annual damage in the United States of about 2.3 billion dollars from expansive soils. (confidence 0.6)