Chapter 3 Flashcards
The most reactive component of the soil.
soil colloid
What are the two types of the soil colloids?
organic colloids
inorganic colloids
The highly decomposed organic matter?
Humus
The type of colloid that is composed of highly decomposed organic matter?
Organic colloid
More reactive type of colloid?
organic colloid
_____ are more reactive chemically and generally have a greater influence on soil properties per unit weight than the inorganic colloids.
Organic colloids
Inorganic colloids include what?
silicate clays
oxides
hydrous oxides
amorphous clays
The size of soil colloid
0.001 mm
The charge of colloids in very acid soils
net positive charge
Exhibit positive as well as negative charge.
amphoteric
The pH at which the surface charge is neutral.
Zero point of Charge
The dissociation of H from hydroxyl group also decreases CEC.
increases CEC
Fe/Al oxide minerals are stable at what pH?
pH 5
Variable charge soils can be limed up to what pH.
pH 5.5
Is it viable to increase the pH of variable charge soils up to pH 6?
no, it is costly because variable charge soils are buffered below or above pH0
What are the effects of lime?
increase pH
precipitate Al to Al(OH)3
replace precipitated Al with Ca
The reaction involved in reacting H+ with OH- with the release of H2O.
neutralization
What is the main value of lime?
provide hydroxyl ions
What is the surface charge if pH0-pH is (-)?
net negative charge
What is the net surface charge if pH0-pH is (+)?
net positive charge
Which has lower pH0:
surface soil vs subsoil
surface soil due to higher OM
soils that have low pH0?
silica
OM
This is the point at which net surface charge is independent of salt concentration so the titration curves should intersect at this point.
Zero Net Charge
The pH0 value of Fe/Al oxides
7-9 ( high AEC)
Parameters that affect the sign and magnitude of charge on variable charge colloids
pH0
soil pH
electrolyte concentration
valence counterion
Dielectric constant
Enumerate the theories of Electrical Double Layer
Helmholtz Double Layer Theory
Gouy-Chapman Diffuse DL Theory
Stern DL Theory
consist of an inner layer of strongly adsorbed counterions and an outer layer of ion swarm.
Stern DL
2 fundamental equations to describe ionic concentration near the surface and potential distribution with distance from the surface:
Poisson equation
Boltzman distribution
Describes the coulombic interaction between charges.
Poisson equation
Describes the change in concentration of counterions near a charged surface.
Boltzman distribution
What happens to DDL at increasing concentration?
decrease thickness
What happens to DDL in monovalent cations?
thicker
What happens to DDL in divalent cations?
thinner
It accounts for the thermal motion of counterions at the charged surface.
Gouy-Chapman Diffuse Double Layer Theory
Gouy-Chapman Diffuse DL theory was developed by Gouy in what year?
1910
What are the sources of Negative Charge in Colloidal Surfaces
Isomorphous substitution
Ionization of exposed hydroxyl group
broken bonds at edges of silica-alumina units
What are the sources of Positive Charge in Colloidal surface
Isomorphous substitution
Protonation of exposed hydroxyl group
if Al3+ is being replaced with Mg2+, what will happen to the charge?
leads to increase in negative
Source of the positive charge of organic colloid at low pH?
protonation of hydroxyl group and NH group
What will happen to the charge if there is IS of Al3+ to Mg2+
gain positive charge
Examples of minerals with permanent charge
2:1 clay minerals
2:2
intergrades
Enumerate minerals with pH-dependent or variable charge
1:1 clay minerals
Fe and Al oxides or sesquioxides
Amorphous aluminosilicate
Amorphous Fe/Al hydrous oxides
Organic colloids
They balance or neutralize the net negative charge generated by clay particles
Adsorbing cations
It refers to the amount of charge associated per unit area of the clay colloid.
surface charge density
if the change in pH is positive, what is the charge of the colloid
negative
If the change in pH is negative, what is the charge of the colloid?
positive
Implications of charge colloids to soil management
increasing cation retention
liming acid soil
What will happen to phosphate applied to an oxisol decreased pH0?
increase CEC
Mg-dominated octahedral sheets minerals
vermiculite
chlorite
