Unit 6: Soil Chemical Properties Chapter 4. Objectives Definition & importance of soil colloids & their effect on the soil Definition & importance of

Unit 6: Soil Unit 6: Soil Chemical Chemical

PropertiesProperties

Chapter 4Chapter 4

ObjectivesObjectives

Definition & importance of soil Definition & importance of soil colloids & their effect on the soil colloids & their effect on the soil

Knowledge of humus and its roleKnowledge of humus and its role Identification of CEC and the role of Identification of CEC and the role of

cation exchangecation exchange Effect of soil pH, soil acidityEffect of soil pH, soil acidity How soil solution, buffering, & cation How soil solution, buffering, & cation

saturation percentagesaturation percentage

IntroductionIntroduction

Soils can’t be managed & production Soils can’t be managed & production can’t be optimized w/out basic can’t be optimized w/out basic knowledge of soil chemistryknowledge of soil chemistry

Colloid – solid substance whose Colloid – solid substance whose particles are very small, but have particles are very small, but have very large surface areavery large surface area– Primarily humus & claysPrimarily humus & clays

Colloids tend to stick togetherColloids tend to stick together

Soil ClaysSoil Clays

Most are crystalline in structureMost are crystalline in structure Term Term clayclay actually carries three actually carries three

meaningsmeanings1.1. Particle size fraction >2 microns in sizeParticle size fraction >2 microns in size

2.2. Name for a group of minerals w/ Name for a group of minerals w/ specific compositionspecific composition

3.3. Soil textural classSoil textural class


The Origin of ClaysThe Origin of Clays– Usually have specific compositionUsually have specific composition– Newly formed crystals, usually different Newly formed crystals, usually different

from primary mineralsfrom primary minerals Reform following dissolution of other Reform following dissolution of other

mineralsminerals

– Kind of clay formed determined by Kind of clay formed determined by proportions of the different ionsproportions of the different ions Silica, aluminaSilica, alumina If some materials leached away, clay types If some materials leached away, clay types

are changed or formation rates changeare changed or formation rates change


Some clays form from slight alteration of Some clays form from slight alteration of primary minerals (micas)primary minerals (micas)

– Vermiculite, hydrous micaVermiculite, hydrous mica

– Soils may have clays from ocean or Soils may have clays from ocean or sediment redepositsediment redeposit Inherited claysInherited clays – clay sediment formed in a – clay sediment formed in a

different climatedifferent climate Modified claysModified clays – changed by further – changed by further

weathering of original claysweathering of original clays Neoformed claysNeoformed clays – new clays formed by – new clays formed by

crystallization of ions from solutioncrystallization of ions from solution


Nature of ClaysNature of Clays– Because of crystalline nature – composed of Because of crystalline nature – composed of

definite, repeating arrangements of atomsdefinite, repeating arrangements of atoms

– Made up of OMade up of O22 atoms, Si, Al held together w/ atoms, Si, Al held together w/ +/- charged ions+/- charged ions

– Clay particles may also be known as Clay particles may also be known as micellemicelle

– Clays have net negative charge, will attract Clays have net negative charge, will attract and hold positively charged ions (and hold positively charged ions (cationscations)) K, Na, NHK, Na, NH44, Ca, Mg, H, Al[OH], Ca, Mg, H, Al[OH]22, Al, Al


Amounts held vary w/ type of clay & vary w/ Amounts held vary w/ type of clay & vary w/ charge of the ioncharge of the ion

Plant roots use some exchangeable cations Plant roots use some exchangeable cations as nutrientsas nutrients

Leaching may remove several cationsLeaching may remove several cations Cations can be replaced by other cationsCations can be replaced by other cations

– As they are exchangedAs they are exchanged– If their charge is more positiveIf their charge is more positive


Charge on ClaysCharge on Clays– Isomorphous substitutionIsomorphous substitution

Clays act like weak acids, release H ions from Clays act like weak acids, release H ions from bonding sites (sites deprotonated)bonding sites (sites deprotonated)

Now has an open site to attract another elementNow has an open site to attract another element– Ions will be attracted that are of similar weight & chargeIons will be attracted that are of similar weight & charge

Amount of deprotonation depends on soil pHAmount of deprotonation depends on soil pH Sites formed known as Sites formed known as cationcation exchangeexchange sites sites Other ions compete to be Other ions compete to be adsorbedadsorbed to these sites to these sites Amount of negative charge = soil’s Amount of negative charge = soil’s CationCation

Exchange Capacity (CEC)Exchange Capacity (CEC)


– Clays w/ layers spread apart allow soil Clays w/ layers spread apart allow soil solution to pass through the layerssolution to pass through the layers Montmorillonite, vermiculiteMontmorillonite, vermiculite Have accessible exchange sites along their Have accessible exchange sites along their

surfacesurface– Nutrients can leach easierNutrients can leach easier– Clays shrink/swell – not suited for building & Clays shrink/swell – not suited for building &

constructionconstruction

– Tightly bonded layers, little room Tightly bonded layers, little room between micellesbetween micelles Kaolinite, chloriteKaolinite, chlorite Don’t swell when wetDon’t swell when wet Can use to make pottery, tile, etc.Can use to make pottery, tile, etc. May have relatively lower CECMay have relatively lower CEC


Silicate ClaysSilicate Clays– Picture a deck of cardsPicture a deck of cards

Each card is layer of a clayEach card is layer of a clay Each layer held together magneticallyEach layer held together magnetically

– Amorphous silicate claysAmorphous silicate clays – lack – lack crystallinitycrystallinity Typically occur where weathered products Typically occur where weathered products

existed, but not sufficient time/condition for existed, but not sufficient time/condition for crystal dev.crystal dev.

Common in soils forming from volcanic ashCommon in soils forming from volcanic ash


– Kaolinite & halloysite – Kaolinite & halloysite – residues from residues from extensive weathering in high-rainfall, extensive weathering in high-rainfall, acidic soilsacidic soils Net negative charge is lowNet negative charge is low

– What does that mean for CEC?What does that mean for CEC?

Strong H bonding layers togetherStrong H bonding layers together– Doesn’t allow HDoesn’t allow H22O to penetrateO to penetrate

– No swellingNo swelling

Common in southeastern U.S.Common in southeastern U.S.


– Montmorillonite & saponiteMontmorillonite & saponite Swelling/sticky claysSwelling/sticky clays Belong to group called Belong to group called smectitessmectites Water easily penetrates clay layersWater easily penetrates clay layers

– Shrink/swell is commonShrink/swell is common Bentonite – Bentonite – impure deposit of impure deposit of

montmorrilonite used to seal earthen montmorrilonite used to seal earthen ponds/lagoons, thickens paints, ties up ponds/lagoons, thickens paints, ties up toxics in feeds, cosmeticstoxics in feeds, cosmetics

Found in soils w/ little/no leachingFound in soils w/ little/no leaching Poorly drained soils, soils developed from Poorly drained soils, soils developed from

limestone, flood plains of riverslimestone, flood plains of rivers


– Hydrous mica, illite – Hydrous mica, illite – fine-grained mica, fine-grained mica, structure similar to montmorillonitestructure similar to montmorillonite Tight bonds don’t letter water penetrateTight bonds don’t letter water penetrate

– Slight to moderate swellingSlight to moderate swelling Named after state of ILNamed after state of IL

– Vermiculite – Vermiculite – occurs in different formsoccurs in different forms Used for insulation, potting soil, packing Used for insulation, potting soil, packing

materialmaterial– What does this tell you about its structure?What does this tell you about its structure?

Swells very littleSwells very little Extremely high CECExtremely high CEC Most often an accessory soil, not dominantMost often an accessory soil, not dominant


– Chlorites – Chlorites – hydrated Mg & Al silicateshydrated Mg & Al silicates Similar to vermiculiteSimilar to vermiculite Restricts swellingRestricts swelling Actually has net positive chargeActually has net positive charge

– What effect does this have?What effect does this have?

Sesquioxide Clays – form under Sesquioxide Clays – form under extensive weathering, leaching in extensive weathering, leaching in warm climateswarm climates– Small amounts exist in many soilsSmall amounts exist in many soils– Can be dominant, or accessoryCan be dominant, or accessory


Usually predominant soil in humid, hot, well-Usually predominant soil in humid, hot, well-drained soilsdrained soils

Usually shades of red & yellow colorsUsually shades of red & yellow colors

– Don’t swell, not stickyDon’t swell, not sticky– Have high P adsorption capacityHave high P adsorption capacity

What does this result in?What does this result in? What problems might it cause?What problems might it cause?

Organic ColloidsOrganic Colloids

Humus – temporary, intermediate Humus – temporary, intermediate product left after decomposition of product left after decomposition of plant & animal remainsplant & animal remains– Continues to decompose slowlyContinues to decompose slowly– Humus particle = organic colloidHumus particle = organic colloid

Consists of various chains of carbon atomsConsists of various chains of carbon atoms

– Has negative chargeHas negative charge What does this mean?What does this mean? What effect does it have on the soil?What effect does it have on the soil?

Organic ColloidsOrganic Colloids

– CEC is many times greater than clay CEC is many times greater than clay colloidscolloids What conclusion does this give you?What conclusion does this give you?

– Humus exerts considerable influence on Humus exerts considerable influence on the soilthe soil In what form?In what form?

Cation ExchangeCation Exchange

Soil colloids will attract & hold positively Soil colloids will attract & hold positively charged ions to their surfacecharged ions to their surface

Replacement of one ion for another Replacement of one ion for another from solution = from solution = cation exchangecation exchange

Adsorbed cations resist removal by Adsorbed cations resist removal by leaching, can be replaced by other leaching, can be replaced by other ions by ions by mass actionmass action

Takes place on clay/humus colloids & on Takes place on clay/humus colloids & on root surfacesroot surfaces


Most commonly held cations – Ca, K, Mg, Most commonly held cations – Ca, K, Mg, H, Na, Al, NHH, Na, Al, NH44

– Proportions of these cations change Proportions of these cations change constantly due to leaching, plant absorptionconstantly due to leaching, plant absorption

Cation Exchange MechanismCation Exchange Mechanism– Secure cations & keep them available to Secure cations & keep them available to

the plants for potential absorptionthe plants for potential absorption– Water moving through the soil may Water moving through the soil may

move/remove some cationsmove/remove some cations


For every cation that is adsorbed, one goes back For every cation that is adsorbed, one goes back into soil solutioninto soil solution

Some may precipitate out & form insoluble saltsSome may precipitate out & form insoluble salts– Affects soil aggregates, and nutrient availabilityAffects soil aggregates, and nutrient availability

– Plants absorb soil N as it is made availablePlants absorb soil N as it is made available Well-vegetated soils lose less N than bare soilsWell-vegetated soils lose less N than bare soils

– Rate of movement decreases as strength of Rate of movement decreases as strength of adsorption increasesadsorption increases Ex. Lead & cadmium from sewageEx. Lead & cadmium from sewage

– Held tightly to soil clays and allowed to filter slowly out Held tightly to soil clays and allowed to filter slowly out rather than pollute waterrather than pollute water


– What effect does liming soil have?What effect does liming soil have? How does it work in the soil…specifically?How does it work in the soil…specifically? What changes might we expect after liming?What changes might we expect after liming? What else might it change in the soil?What else might it change in the soil?

Cation Exchange CapacityCation Exchange Capacity– CEC – quantity of exchangeable cation CEC – quantity of exchangeable cation

sites/unit wt. of dry soilsites/unit wt. of dry soil Measured in centimoles/kg of dry soilMeasured in centimoles/kg of dry soil

– Which soils will have higher CEC? Which soils will have higher CEC? Sand/Clay?Sand/Clay?


– Amounts of exchangeable cations can Amounts of exchangeable cations can be high (even at 24-36”)be high (even at 24-36”)

– CEC level typically constant – as long a CEC level typically constant – as long a soil humus/clay content is the samesoil humus/clay content is the same

– Labs measure CEC w/ soil analysisLabs measure CEC w/ soil analysis– Can estimate, if you know soil clay & Can estimate, if you know soil clay &

organic matter contentorganic matter content


Importance of Cation ExchangeImportance of Cation Exchange– Plant nutrients Ca, Mg, K are supplied to Plant nutrients Ca, Mg, K are supplied to

plants mainly from exchangeable formsplants mainly from exchangeable forms Exchangeable pools of Ca, Mg, K are major Exchangeable pools of Ca, Mg, K are major

sources of these nutrients for plantssources of these nutrients for plants Amount of lime required to raise pH of an Amount of lime required to raise pH of an

acidic soil increases as the CEC increasesacidic soil increases as the CEC increases Cation exchange sites hold Ca, Mg, K, Na, & Cation exchange sites hold Ca, Mg, K, Na, &

NHNH4 4 ions & slow their release by leachingions & slow their release by leaching Adsorb many metals present in wastewater Adsorb many metals present in wastewater

& prevent pollution to ground/surface waters& prevent pollution to ground/surface waters

Anion Exchange & Anion Exchange & AdsorptionAdsorption

Anions – negatively charged ions – Anions – negatively charged ions – sulfate, nitrate, phosphate, chloride, sulfate, nitrate, phosphate, chloride, etc.etc.– NotNot held on CEC sites held on CEC sites– Anion exchange sitesAnion exchange sites – positively charged – positively charged

sites, or ligand exchange sitessites, or ligand exchange sites– Highest Highest Anion Exchange Capacities (AEC)Anion Exchange Capacities (AEC) – –

occur in amorphous silicate claysoccur in amorphous silicate clays AEC’s generally lowAEC’s generally low Low pH relates to high AEC valuesLow pH relates to high AEC values

Soil pHSoil pH

Indication of the acidity/basicity of the soilIndication of the acidity/basicity of the soil– At pH 7.0 – HAt pH 7.0 – H++ ions equal OH ions equal OH-- ions ions– 10x change between each whole pH number10x change between each whole pH number

pH 5.0 is 10x more acidic than pH 6.0pH 5.0 is 10x more acidic than pH 6.0

– Typical soil pH ranges from 4.0 to 10Typical soil pH ranges from 4.0 to 10 Most plants grow well from 5.5 to 8.5Most plants grow well from 5.5 to 8.5

– Strongly acidic soils undesirable – develop toxic levels Strongly acidic soils undesirable – develop toxic levels of Al & Mn, microbe activity greatly reducedof Al & Mn, microbe activity greatly reduced

– Strongly alkaline soils have low micronutrient Strongly alkaline soils have low micronutrient availability, P may be deficientavailability, P may be deficient

Soil pHSoil pH

Soils can become acidic as rainfall Soils can become acidic as rainfall leaches nutrients awayleaches nutrients away

What is more difficult to alter, soil What is more difficult to alter, soil acidity or alkalinity?acidity or alkalinity?– What do you alter each one with?What do you alter each one with?– ChelatesChelates – fertilizer forms that can be – fertilizer forms that can be

added to protect soil nutrientsadded to protect soil nutrients

Soil pHSoil pH

Importance of Soil pHImportance of Soil pH– Affects solubility of mineralsAffects solubility of minerals

More soluble in slightly acidic soilsMore soluble in slightly acidic soils Most crops do best at pH – 6.5Most crops do best at pH – 6.5

– Plants preferring acid soilsPlants preferring acid soils Azaleas, rhododendrons, blueberries, Azaleas, rhododendrons, blueberries,

pineapplepineapple

– Plants preferring basic soilsPlants preferring basic soils Barley, sugar beetsBarley, sugar beets

– High Ca demandHigh Ca demand

– Alfalfa – neutral/slightly basic pHAlfalfa – neutral/slightly basic pH

Soil pHSoil pH

– Also affects soil microbesAlso affects soil microbes Decreased soil microbe activity w/ acidic soilsDecreased soil microbe activity w/ acidic soils Slow/stop decomposition of beneficial Slow/stop decomposition of beneficial

materialsmaterials Decreased N availabilityDecreased N availability

Basic Cation Saturation PercentageBasic Cation Saturation Percentage– Base Saturation PercentageBase Saturation Percentage – proportion – proportion

of basic cations to the total cationsof basic cations to the total cations More acidic the soil, the lower the BSPMore acidic the soil, the lower the BSP At pH 7.0, BSP is essentially 100%At pH 7.0, BSP is essentially 100% Aids in the decision on how much lime to addAids in the decision on how much lime to add

Equilibrium & BufferingEquilibrium & Buffering

SolutionSolution – solvent in which solubles – solvent in which solubles are dissolvedare dissolved– Soil water w/ nutrients dissolved in itSoil water w/ nutrients dissolved in it– Soil nutrients must be in solution to be Soil nutrients must be in solution to be

absorbed by plantsabsorbed by plants Plants & microbes need Ca to thrive, absorb Plants & microbes need Ca to thrive, absorb

it from soil solutionit from soil solution Clay & humus adsorb Ca readilyClay & humus adsorb Ca readily Water can leach Ca awayWater can leach Ca away

Equilibrium & BufferingEquilibrium & Buffering

Most soils resist appreciable pH changesMost soils resist appreciable pH changes– Resistance to change – Resistance to change – buffering capacitybuffering capacity

Increases as CEC increasesIncreases as CEC increases

– Soils high in humus and/or Soils high in humus and/or montmorillonite or vermiculite clay – high montmorillonite or vermiculite clay – high buffering capacitybuffering capacity

– Organic & clay soils – much higher CEC, Organic & clay soils – much higher CEC, more strongly buffered than sandy soilsmore strongly buffered than sandy soils

AssignmentAssignment

Documents

Unit 6: Soil Chemical Properties Chapter 4. Objectives Definition & importance of soil colloids & their effect on the soil Definition & importance of