Clays + Colloids Flashcards
What is cation exchange capacity?
CEC
the soil’s ability to attract, retain and exchange cation elements at specific pHs
Which soil textures have high CEC and why?
CLAY + OM
because they are negatively charged particles and cations are positively charged
Which 3 cations are used in the largest amounts by plants?
Calcium, Magnesium and Potassium
Explain clay sheets?
Each clay particle is made up of individual layers or “sheets”
each layer has CEC on it, which is why clay has very high CEC
each layer has external and internal negatively charged surfaces
How does cation exchange work?
Negatively charged soil particles (clay) attract a swarm of positively charged cations
these ions will form a weak bond with the clay
A soil with high CEC will be fertile or non-fertile? Why?
FERTILE because they have a higher amount of exchange sites for cations (able to exchange more)
A soil with high CEC will resist changes in pH better or worse? why?
RESIST PH CHANGE BETTER
because it has a strong ability to hold onto positively charged ions so there is a buffer to pH change
Will a sandy/coarse soil or a fine/clay/OM soil have more resistance to pH changes? why?
FINE/CLAY/OM MORE RESISTANT
because sandy soils have a low CEC = less sites for the particle to hold onto cations
How does CEC work?
Occurs through mass movement of soil water
ions will exchange and move through the soil
root respiration will release H+ ions into soil in exchange for a cation (Mg, K, or Ca)
In comparison to respiration and photosynthesis, how important is CEC?
Just as important! plants need to be able to take up the useful nutrients in the soil!
T or F: higher CEC means there is more clay or OM present in the soil
TRUE
T or F: high CEC soils have a greater water holding capacity than low CEC soils
TRUE
Clay = high CEC
Sandy = low CEC
T or F: A soil with high CEC is more susceptible to leaching nutrients? why?
FALSE - High CEC makes a soil LESS susceptible to leaching nutrients because ions are more tightly bound in more places
T or F: Low CEC soils are more likely to develop K and Mg deficiencies? why?
TRUE because they have less exchange sites to hold the ions to, bonds are weaker
Which elements are exchanged in CE?
Soil will exchange the H+ that plant roots exude for the cations essential to plant life - K, Mg, and Ca (and others, but mainly those 3)
What would be the best way to manage inputs for sandy soils? why?
frequent additions of small amounts are better (would be best through irrigation) because a one-time large addition will lead to mass leaching
- would need to be limed more often bc of ability to change pH
What is a colloid?
anything that is small - clays and humus
they have negative and positive charges
adsorb swarms of ions and water molecules
What is adsorption vs replacement?
Adsorption is the DISPLACEMENT of ions
replacement refers to the ease of an ion being removed
how does valence and radius size effect an ions adsorption?
the higher the charge and smaller the radius, the more strongly it will ADSORB to the colloid
- less tightly held (often bigger) cations oscillate further from the colloid surface and are more likely to be DISPLACED
What does the replaceability of cations depend on?
- valence - lower valence can be more easily replaced
- ion size - larger ions have more replaceability power
- relative amount of nutrients in the soil
Examples of colloids and their CECs?
kaolinite = low CEC allophone = middle CEC geothite = highest CEC
What are there 4 major types of colloids? where are they found? and give examples
- Crystalline silicate clays
- temperate environments
- ex. smectite - hydrous iron and aluminum oxide clays
- tropical soils
- ex. kaolinite - Allophane and amorphous clays
- volcanic - Humus
- organic residue
What are the two building blocks of clay structures?
- Tetrahedral Sheets (silicon)
2. Octahedral sheets (Al + Mg)
What is a tetrahedral sheet?
Created from the bond between Oxygen and Silicon
1 Si^4 for 4 O^2-s -
But, all of the negative ions cannot be satisfied, so they continue to bond with one another and create chains/sheets
there will always be a remaining negative charge (why it can’t be satisfied) = WHY CLAYS ARE NEGATIVE + HAVE HIGH CEC
What is an Aluminum Octahedral sheet?
A bond between Al and Oxygen
1 Al^3+ for 6 O^2-s
The Als share 2 O ions between the compounds, and like the tetra sheets, the negative charges cannot be fully satisfied, so they continue to bond and make sheets = WHY CLAYS ARE NEGATIVE + HAVE HIGH CEC
Building together, tetrahedral and octahedral sheets make what type of minerals?
clay minerals
What is isomorphous substitution?
Iso = same
Morphous = shape
essentially moving the same shaped molecule into a position within a structure of the lattice
-ve charge in the end
In TETRA: Al3+ will sub for Si4+
in OCTA: coordination of Mg2+, Fe2+, or Fe3+ will sub for Al3+
T or F: Clays are always secondary minerals?why?
TRUE they are created from the chemical weathering of crystals
How are clays classified? what are the 2 groups?
Based on the number and arrangement of tetrahedral and octahedral sheets in the crystal layers
1: 1 clays
2: 1 clays
What is a 1:1 clay? Give an example
these are built from the pattern of 1 tetrahedral sheet and 1 octahedral sheet repeating.
TETRA / OCTA / TETRA / OCTA
these layers are tightly stacked, flat layers with no interlayer spaces = DO NOT EXPAND
ex. KAOLINITE
T or F: Kaolinite will expand when wet or dry. Why or why not?
FALSE
Kaolinite is a 1:1 clay, meaning it has layers so tightly bound together there is no interlayer space for expansion.
*tip to remember, Kaolinite is used in pottery because it doesn’t expand
Does Kaolinite have a high or low CEC? why?
LOW because there is less surface area for exchange sites. The 1:1 layers are so tightly bound and there is no interlayer, the cations can only attach to the edges that have charges so there is not much overall negative charge
T or F: Kaolinite has a low SA and low negative charge? Why?
TRUE
low SA bc of the tight layers and no inner layer
low negative charge because of the low SA and low capacity to adsorb/attract ions
What is a 2:1 clay? Give an example
Each layer is made up of 1 Octahedral sheet sandwiched between 2 Tetrahedral sheets (hence 2:1) with shared O atoms.
TETRA / OCTA / TETRA
There is little attraction between the O atoms in the bottom and top sheets, so this creates a variable SPACE between the layers which can be occupied by WATER or EXCHANGEABLE CATIONS
the distance between the layers is what creates different clays
Ex. smectite
T or F: the distance of the inner layers found in 2:1 clays is what creates different clays
TRUE
T or F: 2:1 clays have a high CEC? Why?
TRUE!
because the inner layers allow a higher SA for more cation exchange sites; isomorphous substitutions allow for a negative charge = attraction for positive
Give an example of a 2:1 clay that doesn’t expand?
ILLITE/FINE GRAINED MICA
does not expand because they are large crystals and their structure resembles a 1:1, they’re layers are very close together and have a low internal SA = lower CEC and -ve charge
their inner space also contains POTASSIUM which binds the sheets tightly
What is the difference between Illite and Mica?
Illite:
- contains K, Ca, Mg
- intermediately weathered
- No-little expansion/shrink
- commonly found in Mica soils
- CEC 30
Mica:
- contains K
- commonly found in poorly weathered soils
- NO SHRINK/SWELL
- a PRIMARY MINERAL
Why do some 2:1 clays expand/shrink?
Their layers are held loosely together as they have a large inner spatial layer.
= high SA = HIGH CEC
Give examples of some 2:1 clays that expand?
- Vermiculites:
- medium-high expansion
10-150 CEC
- puffed up clay that you can actually see the layers in - SMECTITES:
- biggest CEC
- swells the most
- common in temperate soils
- inner layer can hold Ca, Na, Mg, and water - Montmorillite (a smectite)
- prominent in least weathered areas
What kind of clay is Chlorite?
A 2:1, but relatively non-expansive
similar particle size and CEC to fine-grained micas
- Mg dominated sheet between tetras
What is an issue that 2:1 clays can cause that will effect humans?
If roads are built on soils dominated by 2:1 clays, the pavement will crack due to the expanding/shrinking nature of the clays when there are temperature/ppt. changes
Colloids: Iron and Aluminum Oxides: what are their CECs like and where do they occur? Are they expansive?
very common in weathered tropical soils - think Australian outback
very low CEC (less than Kaolinite) + very pH dependent
non-expansive
Give 2 examples of Iron and Aluminum Oxide colloids
- hematite
2. Goethite
Colloids: Allophane: where are they found? are they expansive? what is their CEC like?
they are volcanic
non expansive
HIGH CEC
Humus as a colloid: CEC? Is it pH dependent?
High CEC and changes with pH because as the H+ disappears (pH increases), there are more negative ions for exchange
consists of chains of carbon
Humus can also bind to clay and create a healthy, nutritious soil
T or F: Humus/OM has a greater CEC than clay?
TRUE
4-50x higher than clay
Why does OM/Humus have a negative charge?
the separation of OM into smaller units of organic acids causes an overall negative charge in SOM. This -ve charge is balanced by cations in the soil
Why does soil OM/Humus CEC increase with an increase in pH?
organic acid separation depends on soil pH so the CEC associated is pH-dependent
ex. a neutral pH (7ish) soil will have a higher CEC than a soil with a pH of 5
How is CEC determined? (2 things)
By the amount and nature of soil colloids and by pH
Which pH conditions will allow humus/OM to have a very high CEC?
neutral-alkaline (7+)
Which 2 clays will have a high CEC at any pH level?
Smectite (montmorillite) and Vermiculite
Which 2 clays have intermediate CEC?
Fine-grained mica/illite and chlorites
Which 2 clays have a low CEC?
Kaolinite and Fe, Al Oxides
5 properties of colloids of clays and humus
- negatively charged
- very small particle size
- huge surface area
- surrounded by swarms of loosely held positively charged particles
- sites of dynamic activity in soils
T or F: Humus and clays are colloids are humus and clays…
True, all these are interchangeable terms
T or F: Weathering will effect whether a clay is a 2:1 or 1:1?
TRUE - weathering effects clays CEC and can destroy the 2:1 type clays
note: tropical soils are highly weathered = Iron, Al oxides are highly weathered and found in tropical soils
ex. kaolinite is much more weathered than smectite, but smectite can become more like kaolinite with more weathering
