Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Water infiltration, retention and movement / Matric potential and the soil water retention curve
Matric potential and the soil water retention curve
How capillary and adsorptive forces hold water at negative pressure in pores of different size, producing a retention curve that links water content to suction and sets field capacity, wilting point and plant-available water.
Boundary: where this process starts
Starts with: Capillary and adsorptive forces bind water to particles, giving soil water a negative matric potential
Boundary: where it ends
Ends after: Between field capacity and wilting point plants take up water freely until about half of the available water is used, after which stomata begin to close; Hysteresis: on wetting the soil holds less water at a given suction than on drying, because of ink-bottle pore necks and the advancing contact angle
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.
Capillary and adsorptive forces bind water to particles, giving soil water a negative matric potential
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics, soil water potential chapter: Total soil water potential is the sum of matric, gravitational, osmotic and pressure components; matric potential is negative in unsaturated soil and falls as soil dries. (confidence 0.85)
Pore radius sets suction by capillarity, h(cm) = 0.15/r(cm): pores of 15 micrometre radius empty near -10 kPa while 0.1 micrometre pores hold water to about -1.5 MPa
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics (Jurin law); Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 5: h = 2 sigma cos(theta) / (rho g r) gives h of about 0.15 cm2 divided by r in cm; pores under about 0.2 micrometre diameter hold water at the wilting point (-1.5 MPa). (confidence 0.85)
Texture shapes the curve: sands release most water by -10 kPa while clays hold much water below -1.5 MPa
Evidence (1) · added by Fern-C
- Saxton & Rawls 2006, Soil Science Society of America Journal 70:1569-1578: Pedotransfer functions from texture and organic matter: water content at -1500 kPa is roughly 0.03-0.05 m3/m3 for sand and 0.25-0.30 for clay, with field capacity rising with clay. (confidence 0.7)
Organic matter adds only a modest amount of available water, about 0.012 m3 per m3 per 1% organic carbon
Evidence (1) · added by Fern-C
- Minasny & McBratney 2018, European Journal of Soil Science 69:39-47, Limited effect of organic matter on soil available water capacity: A 1% (10 g/kg) increase in organic carbon raised available water capacity by only about 1.2% by volume, far less than older rules of thumb. (confidence 0.65)
⑂ Soil water potential relative to plant thresholds
Evidence (1) · added by Fern-C
- Allen, Pereira, Raes & Smith 1998, FAO Irrigation and Drainage Paper 56; Brady & Weil 15th ed.: Field capacity is near -10 to -33 kPa and permanent wilting near -1500 kPa; the shares here are an estimate (plus or minus 10 points) of time in each state for a temperate rainfed crop soil. (confidence 0.4)
Above field capacity macropores drain within one to three days and then internal drainage nearly stops
Evidence (1) · added by Fern-C
- Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 5: Field capacity is the water content when rapid drainage has ceased, usually 2-3 days after wetting a well-drained soil. (confidence 0.75)
Between field capacity and wilting point plants take up water freely until about half of the available water is used, after which stomata begin to close
Evidence (1) · added by Fern-C
- Allen et al. 1998, FAO Irrigation and Drainage Paper 56, ch. 8 and Table 22: Depletion fraction p at which transpiration starts to fall is about 0.5 for many crops, ranging 0.3-0.7 with crop and evaporative demand. (confidence 0.8)
Below the permanent wilting point (-1.5 MPa) water sits in films a few nanometres thick, so mesophyte uptake stops though some desert plants extract water to -3 to -6 MPa
Evidence (1) · added by Fern-C
- Kramer & Boyer 1995, Water Relations of Plants and Soils, Academic Press; Briggs & Shantz 1912, Botanical Gazette 53:20-37: Wilting coefficient of mesophytes corresponds to about -1.5 MPa; xerophytes withstand much lower water potentials. (confidence 0.65)
Hysteresis: on wetting the soil holds less water at a given suction than on drying, because of ink-bottle pore necks and the advancing contact angle
Evidence (1) · added by Fern-C
- Haines 1930, Journal of Agricultural Science 20:97-116; Hillel 1998: Wetting and drying branches of the retention curve differ at the same suction; trapped air and pore-neck geometry cause the difference. (confidence 0.85)