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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Aggregate formation and pore structure / Shrink-swell cracking and self-mulching in smectitic soils

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.

Boundary: where this process starts

Starts with: Soil with 30 percent or more clay dominated by smectite holds water between the layers of expandable clay

Boundary: where it ends

Ends after: Shear between swelling blocks makes slickensides, and uneven mixing builds gilgai microrelief; Wetting swells the clay, closes cracks and cuts infiltration to the slow matrix rate

PARTICIPANTSRESOURCESSmectiteSoil waterClay mineralsSoil cracksSoil aggregatesClay mineralsWaterPore spaceSoil aggregatesImpact +3 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Water entering the cracks weImpact -4 of 5. Click for details and evidence.↻ balances · strong · 0.7 Swelling closes crImpact +2 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact 02 of 5. Click for details and evidence.Soil with 30 percent or more clay dominated by smectite holds water between the layers of expandable clay: uses Clay mineralsSoil with 30 percent or more clay dominated by smectite holds water between the layers of expandable clay: uses WaterDrying shrinks the matrix: Vertisols have linear extensibility of 6 cm or more in the upper metre: uses Clay mineralsDrying shrinks the matrix: Vertisols have linear extensibility of 6 cm or more in the upper metre: uses up Wateruses upCracks open to 0.5 m or more; first rain bypasses the matrix through them into the subsoil: uses WaterCracks open to 0.5 m or more; first rain bypasses the matrix through them into the subsoil: uses Pore spaceSurface crumbs fall into cracks (self-mulching) and mix material downward while swelling pushes material up: uses Soil aggregatesWetting swells the clay, closes cracks and cuts infiltration to the slow matrix rate: uses Clay mineralsWetting swells the clay, closes cracks and cuts infiltration to the slow matrix rate: uses WaterWetting swells the clay, closes cracks and cuts infiltration to the slow matrix rate: uses Pore spaceShear between swelling blocks makes slickensides, and uneven mixing builds gilgai microrelief: uses Clay mineralshold water betweenlayersClay minerals: hold water between layersheld between clay layersWater: held between clay layersshrinks as matrix driesClay minerals: shrinks as matrix driesremoved by dryingWater: removed by dryingbypasses matrix throughcracksWater: bypasses matrix through crackscracks open as channelsPore space: cracks open as channelsmixed downward throughcracksSoil aggregates: mixed downward through cracksswell on wettingClay minerals: swell on wettingswells clay, closingcracksWater: swells clay, closing cracksclosed as clay swellsPore space: closed as clay swellsshear to formslickensidesClay minerals: shear to form slickensidesStart: Shrink-swellcracking andself-mulching …STARTStart: Shrink-swell cracking and self-mulching in smectitic soilsEnd: Shrink-swellcracking andself-mulching …ENDEnd: Shrink-swell cracking and self-mulching in smectitic soilsSoil with 30 percent ormore clay dominated bysmectite holds wat…📄1Soil with 30 percent or more clay dominated by smectite holds water between the layers of expandable clayDrying shrinks thematrix: Vertisols havelinear extensibility …📄1Drying shrinks the matrix: Vertisols have linear extensibility of 6 cm or more in the upper metreCracks open to 0.5 m ormore; first rainbypasses the matr…📄1Cracks open to 0.5 m or more; first rain bypasses the matrix through them into the subsoilSurface crumbs fallinto cracks(self-mulching) and m…📄1Surface crumbs fall into cracks (self-mulching) and mix material downward while swelling pushes material upWetting swells theclay, closes cracks andcuts infiltration …📄1Wetting swells the clay, closes cracks and cuts infiltration to the slow matrix rateShear between swellingblocks makesslickensides, a…📄1Shear between swelling blocks makes slickensides, and uneven mixing builds gilgai microrelief
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)
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Soil with 30 percent or more clay dominated by smectite holds water between the layers of expandable clay
SmectiteSoil waterClay mineralsWater
Evidence (1) · added by Fern
  • IUSS Working Group WRB (2022), World Reference Base for Soil Resources, 4th ed., Vertisols: Vertisols have a vertic horizon with at least 30 percent clay and shrink-swell features. (confidence 0.8)
Drying shrinks the matrix: Vertisols have linear extensibility of 6 cm or more in the upper metre
SmectiteSunWaterClay minerals
Evidence (1) · added by Fern
  • Soil Survey Staff (2014), Keys to Soil Taxonomy, 12th ed., USDA-NRCS, Vertisols: Vertisols are defined by a linear extensibility of 6.0 cm or more between the surface and 100 cm depth. (confidence 0.8)
Cracks open to 0.5 m or more; first rain bypasses the matrix through them into the subsoil
Soil cracksRain and irrigationWaterPore space
Evidence (1) · added by Fern
  • Bouma & Dekker (1978), A case study on infiltration into dry clay soil. I. Morphological observations, Geoderma 20:27-40: Initial infiltration into dry cracked clay is very fast through cracks and ends as cracks close. (confidence 0.8)
Surface crumbs fall into cracks (self-mulching) and mix material downward while swelling pushes material up
Soil aggregatesSoil cracksSoil aggregates
Evidence (1) · added by Fern
  • Wilding & Tessier (1988), in Vertisols: Their Distribution, Properties, Classification and Management, Texas A&M: Self-mulching crumbs falling into cracks drive pedoturbation in Vertisols. (confidence 0.7)
Wetting swells the clay, closes cracks and cuts infiltration to the slow matrix rate
SmectiteSoil waterWaterPore spaceClay minerals
Evidence (1) · added by Fern
  • Bronswijk (1988), Modeling of water balance, cracking and subsidence of clay soils, Journal of Hydrology 97:199-212: Crack volume changes with soil moisture, and closure on swelling restricts water entry to the matrix. (confidence 0.8)
Shear between swelling blocks makes slickensides, and uneven mixing builds gilgai microrelief
SmectiteSoil aggregatesClay minerals
Evidence (1) · added by Fern
  • Yaalon & Kalmar (1978), Dynamics of cracking and swelling clay soils: displacement of skeletal grains, optimum depth of slickensides, and rate of intra-pedonic turbation, Earth Surface Processes 3:31-42: Slickensides develop at an optimum depth where swelling pressure causes shear, and cause continuous turbation. (confidence 0.7)