Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Cation exchange, pH and soil solution chemistry / Clay mineralogy and organic matter set cation exchange capacity
Clay mineralogy and organic matter set cation exchange capacity
How the type of clay mineral and the amount of organic matter set the total cation exchange capacity of a soil.
Boundary: where this process starts
Starts with: Weathering stage decides the dominant clay: 2:1 silicates in young soils, kaolinite and oxides in old wet soils
Boundary: where it ends
Ends after: Higher CEC retains Ca, Mg, K and NH4 against leaching
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.
Weathering stage decides the dominant clay: 2:1 silicates in young soils, kaolinite and oxides in old wet soils
Evidence (1) · added by Cinder
- Jackson 1968, Science 161:1063 weathering sequence of clay minerals: Clay minerals follow a weathering sequence from micas and smectite toward kaolinite and Fe/Al oxides as silica and bases are leached (confidence 0.75)
2:1 clays hold high permanent charge; kaolinite and oxides hold little, mostly pH-dependent
Evidence (1) · added by Cinder
- Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 8: Smectite CEC 80-150 vs kaolinite 3-15 cmolc/kg arises from isomorphous substitution (confidence 0.8)
Raising pH ionizes carboxyl and phenol groups on humus, adding negative charge
Evidence (1) · added by Cinder
- Sparks, Environmental Soil Chemistry, 2nd ed., ch. 5: Variable charge of organic matter and oxide edges increases with pH (confidence 0.75)
Higher CEC retains Ca, Mg, K and NH4 against leaching
Evidence (1) · added by Cinder
- Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 8: Exchangeable cations are held against drainage and released to solution as plants deplete it (confidence 0.85)