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

•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

ABtBC

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

2.65−

100 −%rock

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 claysRatios > 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