Lecture 7 & Chapter 8 Notes:
○ these particles are referred as the colloidal fraction because of their extremely
small size and colloid-like behaviour. they behave like colloids if they are less
than about 1 micrometer in diameter.
○ have a huge large surface area, externally, and internal surface area between
the platelike layers.
○ overall negative charge; act as a huge anion that causes attraction and a swarm
of positively charged cations are held
■ exchangeable ions refers to when a cation is held, it can freely
exchange w/ other ions in the soil solution
■ also such thing as exchangeable anions
● there are four groups of important colloids
○ crystalline silicate clays - dominant, two to four sheets of tightly packed and
bonded oxygen, silicon, and aluminum atoms. various particle shapes, intensity
of charge, stickiness, plasticity, and swelling behaviour.
○ non-crystalline silicate clays - do not exhibit order like crystalline silicate clays
○ iron and aluminum oxides - from highly weathered soils, low in plasticity and
stickiness. can be slightly negative to moderately positive.
○ organic (humus) - found in upper parts of the soils, they are amorphous chains,
rings of carbon atoms bonded to H, O, and N.
■ high capacity to absorb water, almost no plasticity or stickiness.
● silicate clay formation (1+2) - are products of chemical weathering
○ primary mineral undergoes weathering, the ions in the the solution undergo
crystallization producing clay mineral
● clay minerals have two distinct structural units:
○ silicon tetrahedron - requires 4 oxygen ions
○ aluminum (or magnesium) octahedron - requires 6 oxygen ions
● kaolinite is a mineral that has a (1:1) ratio of a tetrahedral sheet stacked on an
octahedron sheet; non-expanding (no swelling) bc being held by hdrogen bonds
● a silicate clay has a (2:1) ratio; non expanding clay minerals such as fine grained mica,
and chlorite do not swell because ions such as K, Al, Mg, Fe, bind the 2:1 layers tightly.
Maximum interlayer expansion w/ smectite, and somewhat less expansion in
vermiculite because of moderate binding power of Mg ions.
● silicate clays are developed from the weathering of minerals (1) physical and chemical
alteration of the primary minerals (2) decomposition of primary minerals with the
subsequent recrystallization of their products into the silicate clay
○ fine grained micas, chlorite, and vermiculite are formed w/ mild weathering
○ kaolinite and oxides of iron and aluminum are products of intense weather
○ smectite form w/ moderate conditions
○ genesis occurs w/ removal of elements such as K, Na, Ca, & Mg (bases)
● tetrahedral sheets hold ions that have smaller ionic radii ● isomorphic replacement of Al3+ with Mg2+ - occurs during crystallization, resulting in
a permanent negative charge that can be balanced by cations from the soil solution
absorbed to the crystal surface
○ can also be replaced w/ a cation w/ a higher charge.
● two major sources of charges on soil colloids
○ the hydroxyl groups and other functional groups; by releasing or accepting H
ions can provide either negative or positive charges
○ the charge imbalance brought by the isomorphous substitution
● pH dependent charge - all colloids have surface charges associated with OH- group;
1:1 type clays, oxides of iron and aluminum, and allophane are of this type.
● permanent or constant charges - created by isomorphous substitution of one cation,
and are not dependent on pH.
● net charge is the balance between the negative and positive charges
● organic colloids - noncrystalline, contains C, O, H and N, negative charge developed
from loss of H+ ions from dissociation
○ influence of pH on the CEC of smectite and humus.
○ below a pH of 6 the charge is considered permanent; past 6.0, the charge
increases slightly because of ionization of hydrogen from exposed hydroxyl