(#) Soil colloids Flashcards
True or false, soil colloids are the most chemically active portion of the soil
True
Average soil colloid diameter
<1μ or 0.001mm
What are the types of soil colloids?
Organic
Inorganic
What are the types of Organic soil colloids?
Humus
What are the types of Inorganic soil colloids?
Silicate clays
Allophanes
Fe and Al oxides
what is Micelle?
a silicate clay crystal
Shape of silicate clays
- flaky, laminated, usually hexagonal
Surface area of silicate clays
internal and external
name a silicate clay crystal
Micelle
Name silicate clay’s electro negative charges
net negatively charged
Types of Silicate clays
1:1 (One silica tetrahedon & one aluminum actahedron)
2:1 Expanding type
2:1 Non-expanding type
2:1:1 or 2:2
The type of silicate clay that’s one tetrahedral sheet attached to one side of an octahedral sheet
1:1
Name a 1:1 silicate clay
Kaolinite
CEC of kaolinite
low
True or false, kaolinite has high surface area
False
True or false, kaolinite has low water holding capacity
True
True or false, kaolinite has low shrink-swell capacity. Why?
True, due to strong hydrogen bonding
Which silicate clay has a symmetrical arrangement of two tetrahedral sheets and a central octahedral sheet?
2:1 Expanding silicate clay
Name a 2:1 expanding silicate clay
Montmorillonite
True or false, montmorillonite has low CEC
False
Montmorillonite is a major component in what kind of soil?
Vertisols
What is the D spacing of montmorillonite?
10-17A
True or false, 2:1 Non-expanding silicate clay has the different composition/structure than 2:1 expanding silicate clay
false, they have the same composition/structure, it’s just non-expanding doesn’t expand
Why do 2:1 non-expanding silicate clays not expand?
presence of K+ ions
Name a 2:1 Non-expanding silicate clay
muscovite
Which silicate clay has two silica tetrahedral sheet and two Mg-dominated octahedral sheet
2:1:1 / 2:2 silicate clay
Name a 2:2 silicate clay
chlorites
What are the substitutions in a 2:1 silicate clay’s tetrahedral sheet
Al^3+ & Si^4+ (negative charge)
What does the octahedral interlayer of a chlorite contain?
Mg^2+ & Fe^2+ (positive charge)
Hydrous oxide ore are hydrous oxides of Fe and Al commonly dominant in?
Highly weathered soils of tropics and subtropics
Name hydrous oxides of Fe and Al
Gibbsite and Goethite
True or false, some hydrous oxides of Fe and Al have definite crystaline structure whilst others are amorphus
True
True or false, hydrous oxides of Fe and Al are sticky and plastic
false, non-sticky and plastic
“short-range order minerals”
allophanes
What soil are allophanes prevalent in?
Andisols
True or false, Allophanes have low capacity to adsorb cations
false
Name two allophanes
allophanes & imogolite
What is humus composed of?
C, H, O (non crystaline
What are the 3 sources of negative charges?
1) Isomorphous substitution
2) Dissociation of exposed OH groups at the broken edges of the mineral
3) Dissociation of functional groups in organic colloids
The source of negative charge wherein centrally coordinated cations are replaced by another cation without alteration of crystal structure of the mineral
Isomorphous substitution
In which silicate clay is isomorphous substitution most prevalent?
2:1
In which source of negative charge does OH dissociate at a high pH?
Dissociation of exposed OH groups at the broken edges of the mineral
In which source of negative charge, when at high pH, creates pH dependent negative charge?
Dissociation of functional groups in organic colloids
The interchange between a cation in solution and another cation on the surface of any surface-active material such as clay or organic matter
cation exchange
True or false, cation exchange is the result of positive charges in soil colloids, which attract positive ions (cations) from soil solution
False, it’s the other way around
What is cation exchange capacity (CEC)?
sum of exchangeable cations that a soil can adsorb
What is base saturation?
Proportion of CEC that is made up of exchangeable bases
The quantity of a substance that exactly reacts with, or is equal to the combining value of, an arbitrarily fixed quantity of another substance in a particular reaction.
Equivalent weight
