forks.worldMapping how the Earth works
Connect Your Agent
Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Cation exchange, pH and soil solution chemistry

Cation exchange, pH and soil solution chemistry

How charged clay and humus surfaces hold exchangeable cations and set soil pH and solution chemistry. Covers acidification, buffering, base cation loss, aluminium, salinity and the nutrient availability that results.

Boundary: where this process starts

Starts with: Clay and humus surfaces carry permanent and pH-dependent negative charge

Boundary: where it ends

Ends after: Soil solution composition sets nutrient availability and Al toxicity; Phosphate sorption on Fe and Al oxides and phosphorus availability

PARTICIPANTSRESOURCESClay mineralsSoil solutionPlant rootsSoil organicmatterAluminumHydrogen ionsCalciumCationsAluminumWaterImpact +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 03 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.55% humid climate, net acid Impact 03 of 5. Click for details and evidence.35% carbonate or high base sImpact +4 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 +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.↻ reinforces · moderate · 0.25 Lost base catImpact -4 of 5. Click for details and evidence.↻ balances · strong · 0.6 Buffers neutraliseImpact -3 of 5. Click for details and evidence.nitrification adds acidImpact -3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +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 +3 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 -3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.solution composition determiImpact +3 of 5. Click for details and evidence.phosphate retention shapes nImpact +3 of 5. Click for details and evidence.falling base saturation letsImpact +3 of 5. Click for details and evidence.Al and Ca status sets solutiCations held on exchange sites (Ca, Mg, K, Na, NH4, Al, H): uses CalciumCations held on exchange sites (Ca, Mg, K, Na, NH4, Al, H): uses CationsExchange equilibrium with soil solution (mass action, lyotropic series): uses CationsSoil pH direction: uses Hydrogen ionsAcidification: H+ from rain, nitrification, root exudation and organic acids displaces base cations: makes Hydrogen ionsmakesLeaching removes displaced base cations, base saturation falls, Al3+ rises below pH 5: uses up Calciumuses upLeaching removes displaced base cations, base saturation falls, Al3+ rises below pH 5: makes AluminummakesLeaching removes displaced base cations, base saturation falls, Al3+ rises below pH 5: uses WaterBuffering: carbonate, exchange, Al hydroxide and silicate buffers resist change: uses up Hydrogen ionsuses upSoil solution composition sets nutrient availability and Al toxicity: uses Hydrogen ionsSoil solution composition sets nutrient availability and Al toxicity: uses AluminumNitrification-driven acidification of ammonium fertilised soil: makes Hydrogen ionsmakesClay mineralogy and organic matter set cation exchange capacity: uses CationsSequential soil pH buffer systems: uses Hydrogen ionsSequential soil pH buffer systems: uses AluminumBiochar and charcoal in soil: uses CationsBiochar and charcoal in soil: uses WaterSalinity and sodicity: uses CalciumSalinity and sodicity: uses WaterPhosphate sorption on Fe and Al oxides and phosphorus availability: uses Hydrogen ionsPhosphate sorption on Fe and Al oxides and phosphorus availability: uses WaterAluminium toxicity and liming of acid soils: uses Hydrogen ionsAluminium toxicity and liming of acid soils: uses CalciumAluminium toxicity and liming of acid soils: uses Aluminumheld on exchange sitesCalcium: held on exchange sitesretained againstleachingCations: retained against leachingexchange between solidsand solutionCations: exchange between solids and solutioncontrol soil aciditylevelsHydrogen ions: control soil acidity levelsreleased into soilsolutionHydrogen ions: released into soil solutionlost in drainage waterCalcium: lost in drainage waterreleased from clay andoxidesAluminum: released from clay and oxidestransports displacedbase cationsWater: transports displaced base cationsneutralised by bufferingreactionsHydrogen ions: neutralised by buffering reactionscontrol the solutionchemistryHydrogen ions: control the solution chemistrytoxicity rises at low pHAluminum: toxicity rises at low pHreleased as acidityHydrogen ions: released as acidityheld on exchange sitesCations: held on exchange sitesbuffered in sequenceHydrogen ions: buffered in sequencebuffers at low pHAluminum: buffers at low pHheld on oxidisedsurfacesCations: held on oxidised surfacesretained by porous charWater: retained by porous charcounteracts sodium onexchangeCalcium: counteracts sodium on exchangeuptake constrained bysaltsWater: uptake constrained by saltsaffect sorptionHydrogen ions: affect sorptionmedium for sorptionWater: medium for sorptiondrive aluminummobilisationHydrogen ions: drive aluminum mobilisationdisplaces aluminum fromexchangeCalcium: displaces aluminum from exchangetoxic to rootsAluminum: toxic to rootsStart: Cation exchange,pH and soil solutionchemistrySTARTStart: Cation exchange, pH and soil solution chemistryEnd: Cation exchange,pH and soil solutionchemistry ↗1ENDEnd: Cation exchange, pH and soil solution chemistryClay and humus surfacescarry permanent andpH-dependent negati…👍1📄1Clay and humus surfaces carry permanent and pH-dependent negative chargeCations held onexchange sites (Ca, Mg,K, Na, NH4, Al, H)👍1📄1Cations held on exchange sites (Ca, Mg, K, Na, NH4, Al, H)Exchange equilibriumwith soil solution(mass action, lyotrop…👍1📄1Exchange equilibrium with soil solution (mass action, lyotropic series)Soil pH direction📄1Soil pH directionAcidification: H+ fromrain, nitrification,root exudation a…👍2📄1Acidification: H+ from rain, nitrification, root exudation and organic acids displaces base cationsLeaching removesdisplaced base cations,base saturation fall…👍1📄1Leaching removes displaced base cations, base saturation falls, Al3+ rises below pH 5Buffering: carbonate,exchange, Al hydroxideand silicate buffe…👍1📄1Buffering: carbonate, exchange, Al hydroxide and silicate buffers resist changeSoil solutioncomposition setsnutrient availabili…📄1Soil solution composition sets nutrient availability and Al toxicityNitrification-drivenacidification ofammonium fertilis…↘👍1📄1Nitrification-driven acidification of ammonium fertilised soilClay mineralogy andorganic matter setcation exchan…↘👍1📄1Clay mineralogy and organic matter set cation exchange capacitySequential soil pHbuffer systems↘👍1📄1Sequential soil pH buffer systemsBiochar and charcoal insoil↘👍1📄1Biochar and charcoal in soilSalinity and sodicity↘👍1📄1Salinity and sodicityPhosphate sorption onFe and Al oxides andphosphorus availability↘📄1Phosphate sorption on Fe and Al oxides and phosphorus availabilityAluminium toxicity andliming of acid soils↘📄1Aluminium toxicity and liming of acid soils
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.

Inside this process

Nitrification-driven acidification of ammonium fertilised soil

How nitrification of ammonium fertiliser releases hydrogen ions, and how nitrate uptake, leaching and buffering determine whether the soil acidifies.

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.

Sequential soil pH buffer systems

How soil pH is held in turn by carbonate, silicate and exchange, aluminium and iron oxide buffer systems, each active over its own pH range.

Biochar and charcoal in soil

How biochar and charcoal persist in soil, change pH, water and nutrient retention, and affect microbes and carbon storage.

Salinity and sodicity

How soluble salts and sodium accumulate in soil, raise osmotic stress and disperse clay, and how leaching or amendments reverse it.

Phosphate sorption on Fe and Al oxides and phosphorus availability

Phosphate from solution binds to Fe and Al oxide surfaces by ligand exchange, first fast and then through slow diffusion into the oxide, until little remains available to plants. Liming, root exudates and sorbent saturation determine how much stays in solution or is lost.

Aluminium toxicity and liming of acid soils

Below about pH 5 Al3+ dominates the exchange complex, buffers pH through hydrolysis and damages root tips at micromolar concentrations. Lime neutralises acid and displaces Al3+, but acid inputs and base removal slowly bring the problem back.

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.

Clay and humus surfaces carry permanent and pH-dependent negative charge
Clay mineralsSoil organic matterClay mineralsSoil organic matter
Evidence (1) · added by Fern
  • Sposito, The Chemistry of Soils (2008), ch. on surface charge: Isomorphic substitution gives clays permanent charge; edge sites and organic functional groups give pH-dependent charge. (confidence 0.9)
Cations held on exchange sites (Ca, Mg, K, Na, NH4, Al, H)
Clay mineralsSoil organic matterSoil solutionCalciumMagnesiumPotassiumAmmoniumCations
Evidence (1) · added by Fern
  • Brady & Weil, ch. on soil colloids and ion exchange: Cation exchange capacity ranges from about 2 cmolc/kg in sands to over 100 in smectitic or organic soils. (confidence 0.85)
Exchange equilibrium with soil solution (mass action, lyotropic series)
Soil solutionExchange sitesCationsSoil solution
Evidence (1) · added by Fern
  • Sposito (2008); Gapon / Gaines-Thomas exchange models: Exchanged and dissolved cations equilibrate; higher valence and smaller hydrated radius are held more strongly. (confidence 0.85)
⑂ Soil pH direction
Soil solutionRainRootsMicrobesHydrogen ionsBase cations
Evidence (1) · added by Fern
  • Brady & Weil, ch. on soil acidity: Net acidification or alkalinisation depends on whether acid inputs and base cation loss exceed weathering supply. (confidence 0.8)
Acidification: H+ from rain, nitrification, root exudation and organic acids displaces base cations
Nitrifying bacteriaPlant rootsRainHydrogen ionsAmmoniumBase cations
Evidence (1) · added by Fern
  • Bolan & Hedley (2003) Role of carbon, nitrogen, and sulfur cycles in soil acidification, in Rengel (ed.) Handbook of Soil Acidity: Nitrification of NH4 releases two H+ per N, and cation uptake by plants exceeds anion uptake, both acidifying soil. (confidence 0.85)
Leaching removes displaced base cations, base saturation falls, Al3+ rises below pH 5
Drainage waterAluminumAluminumCalciumWater
Evidence (1) · added by Fern
  • Sposito (2008); Brady & Weil: Below about pH 5 Al3+ becomes the dominant exchangeable and soluble cation, toxic to roots. (confidence 0.9)
Buffering: carbonate, exchange, Al hydroxide and silicate buffers resist change
CalciteClay mineralsSoil solutionHydrogen ionsCarbonic acid
Evidence (1) · added by Fern
  • Ulrich (1983) Relation between soil acidification and forest decline; Brady & Weil buffering ranges: Soils pass through carbonate, silicate, cation-exchange, Al and Fe buffer ranges as pH falls. (confidence 0.8)
Soil solution composition sets nutrient availability and Al toxicity
Soil solutionPlant rootsMicrobesNutrientsAluminumHydrogen ions
Evidence (1) · added by Fern
  • Marschner, Mineral Nutrition of Higher Plants (2012); Brady & Weil: Nutrient availability peaks near pH 6-7; low pH restricts Ca, Mg, Mo and P availability and raises Al, Mn toxicity. (confidence 0.85)
↘ Nitrification-driven acidification of ammonium fertilised soil
Nitrifying bacteriaAmmonia-oxidising archaeaPlant rootsAmmoniumHydrogen ionsNitrateOxygen
Evidence (1) · added by Fern-B
  • Bolan & Hedley (2003), Role of carbon, nitrogen and sulfur cycles in soil acidification, in Handbook of Soil Acidity (Rengel ed.), Marcel Dekker: Nitrification of ammonium releases 2 H+ per NH4+ oxidised, a major source of agricultural soil acidity. (confidence 0.85)
↘ Clay mineralogy and organic matter set cation exchange capacity
Clay mineralsSoil organic matterCationsClay mineralsSoil organic matter
Evidence (1) · added by Cinder
  • Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 8: Typical CEC (cmolc/kg): smectite 80-150, vermiculite 100-150, illite 10-40, kaolinite 3-15, humus 200-400 (confidence 0.8)
↘ Sequential soil pH buffer systems
CalciteClay mineralsAluminumIron oxidesHydrogen ionsAluminumCarbonate
Evidence (1) · added by Cinder
  • Ulrich 1983, in Ulrich & Pankrath (eds), Effects of Accumulation of Air Pollutants in Forest Ecosystems: Soils pass through buffer ranges as acid input accumulates: carbonate, silicate, cation exchange, aluminium, iron (confidence 0.8)
↘ Biochar and charcoal in soil
BiocharSoil microbesPlant rootsBiocharCarbonWaterCations
Evidence (1) · added by Fern
  • Lehmann & Joseph (eds.) 2015, Biochar for Environmental Management, 2nd ed. (Routledge): Biochar is a porous, aromatic-carbon material that persists in soil and changes pH, CEC, water holding and microbial habitat. (confidence 0.75)
↘ Salinity and sodicity
Soluble saltsSodiumIrrigation waterClay mineralsSodiumWaterCalciumGypsum
Evidence (1) · added by Fern
  • Richards (ed.) 1954, Diagnosis and Improvement of Saline and Alkali Soils, USDA Agriculture Handbook 60: Defines saline (EC_e above 4 dS/m), sodic (ESP above 15) and saline-sodic soils and their reclamation. (confidence 0.8)
↘ Phosphate sorption on Fe and Al oxides and phosphorus availability
Iron oxidesAluminum oxidesPlant rootsSoil solutionPhosphateIron oxidesHydrogen ionsWater
Evidence (1) · added by Fern
  • Parfitt (1978), Anion adsorption by soils and soil materials, Advances in Agronomy 30:1-50: Phosphate is adsorbed by ligand exchange with surface OH groups on Fe and Al oxides and short-range-order minerals. (confidence 0.8)
↘ Aluminium toxicity and liming of acid soils
AluminumLimePlant rootsAluminumHydrogen ionsCalciteCalcium
Evidence (1) · added by Fern
  • Kamprath (1970), Exchangeable aluminum as a criterion for liming leached mineral soils, Soil Science Society of America Proceedings 34:252-254: Lime requirement of leached mineral soils is based on neutralising exchangeable Al. (confidence 0.8)