Soils Exam 2 - Lectures 14, 15, and 16

2 units for CEC

1. centimoles of positive charge per kilogram soil (cmolc/kg)

2. “milliequivalent” (meq)

both are the same number, just different units.

calculating soil CEC

Step 1 — know the charge and weight of each ion

Step 2 — convert each ion from grams of soil to cmolc/kg of soil

YOU MUST KNOW:

• How much soil was extracted

• Atomic weight and charge on each ion measured (these do not change – from the periodic table)

• Only NONACID cations are used (disregard H or Al!)

calculating the equivalent weight of ions

molar mass of the ion (periodic table) / charge of the ion

What are characteristics of soil colloids?

• Extremely small size (only can see with an electron microscope)

• Large external surface area per unit of mass

• Have both negative and positive surface charges

• These charges lead to adsorption of ions and water

4 types of soil colloids

1. Layer silicate clays – crystalline silicate clays

2. Iron and aluminum oxide clays (the minerals gibbsite and

3. Allophane – non-crystalline silicate clays (volcanic)

4. Humus (organic matter – well decomposed)

isomorphic substitution

when cations of a similar size substitute for the Al and Si in the sheets – this occurs during weathering; this is the part of CEC we call Permanent Charge.

Which ions can sub for which in each layer?

• Si, Al and Fe3+ are the only ions that can substitute in the tetrahedral sheet.

• Octahedral sheet: Al, Fe3+, Mg, Fe2+

• Exchange sites: Al, Fe3+, Mg, Fe2+, Zn, Na, Ca, K

• Both sheets: O and OH

basic properties/possible uses (if given), arrangement of Si tetrahedrals/Al octahedrals of kaolinite, smectite, vermiculite, illite and chlorite

weathering (the degree of) and the genesis of layer silicate clays and Fe/Al oxides

‘Low,’ ‘Medium’ and ‘High’ ranges of CEC of layer silicate clays, humus, allophane and Al/Fe oxides

low: Al/Fe oxides, kaolinite

medium: illite, chlorite, allophane

high: humus, vermiculite, smectite

Which colloids have a pH dependent charge?

humus, kaolinite, allophane and Fe/Al oxides

How does pH dependent charge develop?

• We get it from the presence of OH-’s or other groups that accept or release H+ ions to create negative or positive charges.

• These charges are a function (depend upon) the pH of the soil.

• Thus, these are called pH-dependent charge.

Which colloids have AEC?

kaolinite, allophane, and Al/Fe oxides

equation for which a changing pH affects in Al compounds

-- Al – OH (Al with attached OH)

when pH increases…OH- added

-- Al – OH + OH- -→ Al-O- + H2O

equation for which a changing pH affects in OM

-CO-OH + OH- → - CO – O- + H2O

Al/Fe oxides and their formulas

Goethite — (FeOOH)

Hematite — (Fe2O3)

Gibbsite — (AlOH)3