1. Unit 6: Soil Chemical Properties
Chapter 4

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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)

10. 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

11. 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

12. 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.

13. 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

14. 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

15. 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

16. 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?

17. 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?

18. 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?

19. 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

20. 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

21. 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

22. 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?

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. Assignment