Unit 6: Soil Chemical Properties Chapter 4

Objectives Definition & importance of soil colloids & their effect on the soil Knowledge of humus and its role Identification of CEC and the role of cation exchange Effect of soil pH, soil acidity How soil solution, buffering, & cation saturation percentage

Introduction Soils can’t be managed & production can’t be optimized w/out basic knowledge of soil chemistry Colloid – solid substance whose particles are very small, but have very large surface area Primarily humus & clays Colloids tend to stick together

Soil Clays Most are crystalline in structure Term clay actually carries three meanings Particle size fraction >2 microns in size Name for a group of minerals w/ specific composition Soil textural class

Soil Clays The Origin of Clays Usually have specific composition Newly formed crystals, usually different from primary minerals Reform following dissolution of other minerals Kind of clay formed determined by proportions of the different ions Silica, alumina If some materials leached away, clay types are changed or formation rates change

Soil Clays Soils may have clays from ocean or sediment redeposit Some clays form from slight alteration of primary minerals (micas) Vermiculite, hydrous mica Soils may have clays from ocean or sediment redeposit Inherited clays – clay sediment formed in a different climate Modified clays – changed by further weathering of original clays Neoformed clays – new clays formed by crystallization of ions from solution

Soil Clays Nature of Clays Because of crystalline nature – composed of definite, repeating arrangements of atoms Made up of O2 atoms, Si, Al held together w/ +/- charged ions Clay particles may also be known as micelle Clays have net negative charge, will attract and hold positively charged ions (cations) K, Na, NH4, Ca, Mg, H, Al[OH]2, Al

Soil Clays Amounts held vary w/ type of clay & vary w/ charge of the ion Plant roots use some exchangeable cations as nutrients Leaching may remove several cations Cations can be replaced by other cations As they are exchanged If their charge is more positive

Soil Clays Charge on Clays Isomorphous substitution Clays act like weak acids, release H ions from bonding sites (sites deprotonated) Now has an open site to attract another element Ions will be attracted that are of similar weight & charge Amount of deprotonation depends on soil pH Sites formed known as cation exchange sites Other ions compete to be adsorbed to these sites Amount of negative charge = soil’s Cation Exchange Capacity (CEC)

Soil Clays Clays w/ layers spread apart allow soil solution to pass through the layers Montmorillonite, vermiculite Have accessible exchange sites along their surface Nutrients can leach easier Clays shrink/swell – not suited for building & construction Tightly bonded layers, little room between micelles Kaolinite, chlorite Don’t swell when wet Can use to make pottery, tile, etc. May have relatively lower CEC

Soil Clays Silicate Clays Picture a deck of cards Each card is layer of a clay Each layer held together magnetically Amorphous silicate clays – lack crystallinity Typically occur where weathered products existed, but not sufficient time/condition for crystal dev. Common in soils forming from volcanic ash

Soil Clays Kaolinite & halloysite – residues from extensive weathering in high-rainfall, acidic soils Net negative charge is low What does that mean for CEC? Strong H bonding layers together Doesn’t allow H2O to penetrate No swelling Common in southeastern U.S.

Soil Clays Montmorillonite & saponite Swelling/sticky clays Belong to group called smectites Water easily penetrates clay layers Shrink/swell is common Bentonite – impure deposit of montmorrilonite used to seal earthen ponds/lagoons, thickens paints, ties up toxics in feeds, cosmetics Found in soils w/ little/no leaching Poorly drained soils, soils developed from limestone, flood plains of rivers

Soil Clays Hydrous mica, illite – fine-grained mica, structure similar to montmorillonite Tight bonds don’t letter water penetrate Slight to moderate swelling Named after state of IL Vermiculite – occurs in different forms Used for insulation, potting soil, packing material What does this tell you about its structure? Swells very little Extremely high CEC Most often an accessory soil, not dominant

Soil Clays Chlorites – hydrated Mg & Al silicates Similar to vermiculite Restricts swelling Actually has net positive charge What effect does this have? Sesquioxide Clays – form under extensive weathering, leaching in warm climates Small amounts exist in many soils Can be dominant, or accessory

Soil Clays Don’t swell, not sticky Have high P adsorption capacity Usually predominant soil in humid, hot, well-drained soils Usually shades of red & yellow colors Don’t swell, not sticky Have high P adsorption capacity What does this result in? What problems might it cause?

Organic Colloids Humus – temporary, intermediate product left after decomposition of plant & animal remains Continues to decompose slowly Humus particle = organic colloid Consists of various chains of carbon atoms Has negative charge What does this mean? What effect does it have on the soil?

Organic Colloids CEC is many times greater than clay colloids What conclusion does this give you? Humus exerts considerable influence on the soil In what form?

Cation Exchange Soil colloids will attract & hold positively charged ions to their surface Replacement of one ion for another from solution = cation exchange Adsorbed cations resist removal by leaching, can be replaced by other ions by mass action Takes place on clay/humus colloids & on root surfaces

Cation Exchange Most commonly held cations – Ca, K, Mg, H, Na, Al, NH4 Proportions of these cations change constantly due to leaching, plant absorption Cation Exchange Mechanism Secure cations & keep them available to the plants for potential absorption Water moving through the soil may move/remove some cations

Cation Exchange Plants absorb soil N as it is made available For every cation that is adsorbed, one goes back into soil solution Some may precipitate out & form insoluble salts Affects soil aggregates, and nutrient availability Plants absorb soil N as it is made available Well-vegetated soils lose less N than bare soils Rate of movement decreases as strength of adsorption increases Ex. Lead & cadmium from sewage Held tightly to soil clays and allowed to filter slowly out rather than pollute water

Cation Exchange Cation Exchange Capacity What effect does liming soil have? How does it work in the soil…specifically? What changes might we expect after liming? What else might it change in the soil? Cation Exchange Capacity CEC – quantity of exchangeable cation sites/unit wt. of dry soil Measured in centimoles/kg of dry soil Which soils will have higher CEC? Sand/Clay?

Cation Exchange Amounts of exchangeable cations can be high (even at 24-36”) CEC level typically constant – as long a soil humus/clay content is the same Labs measure CEC w/ soil analysis Can estimate, if you know soil clay & organic matter content

Cation Exchange Importance of Cation Exchange Plant nutrients Ca, Mg, K are supplied to plants mainly from exchangeable forms Exchangeable pools of Ca, Mg, K are major sources of these nutrients for plants Amount of lime required to raise pH of an acidic soil increases as the CEC increases Cation exchange sites hold Ca, Mg, K, Na, & NH4 ions & slow their release by leaching Adsorb many metals present in wastewater & prevent pollution to ground/surface waters

Anion Exchange & Adsorption Anions – negatively charged ions – sulfate, nitrate, phosphate, chloride, etc. Not held on CEC sites Anion exchange sites – positively charged sites, or ligand exchange sites Highest Anion Exchange Capacities (AEC) – occur in amorphous silicate clays AEC’s generally low Low pH relates to high AEC values

Soil pH Indication of the acidity/basicity of the soil At pH 7.0 – H+ ions equal OH- ions 10x change between each whole pH number pH 5.0 is 10x more acidic than pH 6.0 Typical soil pH ranges from 4.0 to 10 Most plants grow well from 5.5 to 8.5 Strongly acidic soils undesirable – develop toxic levels of Al & Mn, microbe activity greatly reduced Strongly alkaline soils have low micronutrient availability, P may be deficient

Soil pH Soils can become acidic as rainfall leaches nutrients away What is more difficult to alter, soil acidity or alkalinity? What do you alter each one with? Chelates – fertilizer forms that can be added to protect soil nutrients

Soil pH Importance of Soil pH Affects solubility of minerals More soluble in slightly acidic soils Most crops do best at pH – 6.5 Plants preferring acid soils Azaleas, rhododendrons, blueberries, pineapple Plants preferring basic soils Barley, sugar beets High Ca demand Alfalfa – neutral/slightly basic pH

Soil pH Basic Cation Saturation Percentage Also affects soil microbes Decreased soil microbe activity w/ acidic soils Slow/stop decomposition of beneficial materials Decreased N availability Basic Cation Saturation Percentage Base Saturation Percentage – proportion of basic cations to the total cations More acidic the soil, the lower the BSP At pH 7.0, BSP is essentially 100% Aids in the decision on how much lime to add

Equilibrium & Buffering Solution – solvent in which solubles are dissolved Soil water w/ nutrients dissolved in it Soil nutrients must be in solution to be absorbed by plants Plants & microbes need Ca to thrive, absorb it from soil solution Clay & humus adsorb Ca readily Water can leach Ca away

Equilibrium & Buffering Most soils resist appreciable pH changes Resistance to change – buffering capacity Increases as CEC increases Soils high in humus and/or montmorillonite or vermiculite clay – high buffering capacity Organic & clay soils – much higher CEC, more strongly buffered than sandy soils

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