Chemical Properties of Secondary Phyllosilicates
Isomorphous substitution
•‘replacement’ of an ion by another of similar size, but
differing charge
•Creates net negative charge on mineral structure
Cation Exchange Capacity
•Measure of ability of soil to retain positively charged ions
(meq/100 g)
•Measured on basis of cations retained per 100 g soil
Base Saturation
•Fraction of total CEC that is counter balanced by ‘base
cations’ (Ca, Mg, Na, K)
•Remaining charge neutralization by H, Al is refered to as
‘exchangable acidity”
Estimating soil clay mineralogy from CEC
CEC/100 g soil x 1/clay% x 100 = CEC/100g clay
(meq/100g soil)(100gsoil/g clay)(100)
Organic matter correction
CEC/100gsoil x C% x CEC/g C = corrected CEC
(insert into equation above)
(meq/100gsoil)(gC/100gsoil)(meq/gC)
Example from Brazil
Correction of A horizon
CEC= 6.7 meq/100 g soil
C = 2.76% (x2 = OM)
Clay = 34.7%
CEC/100g clay =19.3
Mineralogy=kaolinite and geothite (~5 meq/100g clay)
6.7 - (2.76x2)(1 meq/g SOM) = 1.94meq/100 soil (corr)
(1.94)(100/34.7)(100)= 5.6 meq/100 clay
Method of calculating amounts (mass) from
concentrations
•Data sheets give horizon concentrations of various
componds in a given horizon (clay%, C%, CEC/100 g,
etc)
•Common to ask what is mass per unit area (m-2) per
horizon or entire soil profile.
•To do calculation, need horizon thickness,
concentration, gravel content and bulk denisty.
Calculation of mass of compounds in soils
A
Bt
BC
•Most concentration data given on < 2mm
fraction (“fine earth”). Therefore:
Mass/horizon = (horizon vol - rock
vol)(BD)(conc/100)
Volume= cm3
BD= g/cm3
Mass/soil= all horizons
Rock volume adjustment
1. Volume adjustment
- useful only if gravel given in volume values
- subtract directly from horizon volume
- most gravel given in weight percentages….
2. Weight adjustment [mass = (vol)(BD)(FE)(conc/100), where FE=
100  %rock
BD
%rock 100  %rock

2.65
BD
=%vol of horizon occupied by
non-rock
= (vol fines/100g soil)/(vol total
soil/100g soil)
Clay Dispersion and Flocculation: mechanisms and
soil impacts
•Clay formation can occur througout soil, though clay is usually
concentrated below surface
–Implies some sort of transport
•The suspension of clays in downward moving water is related to their
electrical properties and the chemistry of the surrounding waters
Role of Clay Mineral
Type
Mobility Requires:
1. CEC
2. expandability
Concentration vs.
Composition
Basics of Clay
Mobility
ESP= ratio of Na/Ca+Mg on
clays
Ratios > 15 produce undesirable
features (from irrigation)
SAR~ ratio of Na/Ca+Mg in
soln.
SAR easier to measure than ESP
The combination of SAR and
solute conc of soil water (or
irrigation) determines clay
mobility
Effect of Na in soils
Leads to:
•Rapid downward transport
•Development of Btn horizons
•Columnar structure
Sodic Soils of San
Joaquin Valley
Btn horizon formation in < 10,000
yrs due to:
1. High pH (9-10) which rapidly
dissolves silicates and increase
Si solubilty
2. High Na, in combination with
dilute rain, disperse clays near
surface
3. High salt content rapidly
increase soln. Conc. With
depth, flocculating clay
Topographic Transect of east
SJ Valley
The toposequence
•Granitic alluvium
•~10,000 yrs
•Depth to H2O table primary
variable
–Causes increase in salt/Na
content
–Increases weathering
–Increases clay dispersion
Fresno soil: highest water
table and Btn
A
E
Btnk1
Btnk2
Bqnkm
Bqnk1
Bqnk2
BC
Hesperia: moderate depth
and no Bt but high CaCO3
A
Bk1
Bk2
etc
Hanford: no Bt or salts
A
Bw
C
Basin-rim landscape