Exam 3
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What is a soil colloid?
Particles with a diameter of 0.1 to 0.001µm (10,000µm=1cm)
What are colloids characterized by?
– Their small size (diameter of 0.1 to 0.001µm) – High surface area with some internal surface area – Surface charge – Adsorption of cations and/or anions – Adsorption of water
What are Inorganic colloids?
– Clay minerals – Hydrous oxides – Are usually crystalline – Usually make up the bulk of soil colloids.
What are Organic colloids?
– Highly decomposed organic matter called humus. – Generally, have a greater influence on soil properties per unit weight than the inorganic colloids.
What are phyllosilicates?
layered silicate structure
What are tetrahedral sheets made from?
Si^4+ and O^2- (Si is bonded to 4 Os)
What are Octahedral Sheets made from?
Al^3+ and O^2- (Al is bonded to 6 Os)
What is a 1:1 type mineral?
kaolinite (one tetrahedral to one octahedral sheet)
What is a 2:1 type mineral?
Illite (two tetrahedral to one octahedral sheet)
What are some characteristics of Iron & Aluminum colloids?
• Common in highly weathered soils (like the Oxisols). • Low cation exchange capacities. • Non-expansive clays.
Highly weathered = more _______? Little weathering = more ____?
more oxides, more 2:1 type clays
A 1:1 phyllosilicate clay has: a) 1 tetrahedral sheet to 1 octahedral sheet b) 2 tetrahedral sheets to 1 octahedral sheets c) 1 oxygen to 1 silicon d) 1 aluminum to 2 oxygen
a) 1 tetrahedral sheet to 1 octahedral sheet
What is soil organic matter made of?
Humus, Living, Fresh Residue, and Active Fraction
What is Anion exchange capacity (AEC)?
sum of total anions that a given soil can absorb
What is Cation exchange capacity (CEC)?
sum of total cations that a given soil can absorb
What is Isomorphic Substitution?
process in which one element substitutes another of comparable size in the crystalline structure. Permeant charge.
What is the isomorphic substitution for 1:1 clay?
Mg substitution for Al in the octahedral lattice Si2O3OHO2Al2(OH)3 →Si2O3OHO2AlMg(OH)3
What is the isomorphic substitution for 2:1 clay?
2:1 Clay (e.g. montmorillite) – Mg substitution for Al in the octahedral lattice – Al substitution for Si in the tetrahedral lattice Si2O3OHO2Al2OH2 Si2O3 → Si2O3OHO2AlMgOH2SiAlO3
What is a Milliequivalent (meq)?
one milligram of hydrogen or the amount of any other ion that will combine with it or displace it. Equivalent weight = molecular wt (g)/ valence or charge
What is CEC expressed in?
milliequivalents
What is Flocculation?
– individual particles stick together to form aggregates of particles or floccules. Such aggregates do not move in the soil solution and form the basis for soil structure.
What is Dispersion?
aggregates do not form. Each particle behaves as an individual. – Without aggregation, water, air, and root movement in the soil is inhibited. – Dispersion is not a desirable characteristic of productive soils.
In a 1:1 type phyllosilicate clay, what cation is replaces the aluminum, and what sheet is it replaced on? a) Mg2+, the tetrahedral sheet b) Mg2+, the octahedral sheet c) Si4+, the tetrahedral sheet d) Si4+, the octahedral sheet
b) Mg2+, the octahedral sheet
What soil pH has the highest cation exchange capacity (CEC)? a) pH 4 b) pH 5 c) pH 6 d) pH 7
d) pH 7