Calculating wet topsoil pile weight Calculate the moisture content (w): w = [(g water) / (g dry soil)] x 100 = % Calculate dry topsoil weight using Db (g dry soil/bulk volume): vol (m 3 ) x Db (Mg/m 3 ) = dry weight of pile (Mg) Rearrange the first eqn to solve for wet soil wt. g wet soil = Dry soil wt x (1 + w/100)

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Ch. 8 continued Estimating and Calculating CEC

What is the source of charge on colloids? Isomorphic substitution (2:1 clays) Iso (same) morph (shape) –an ion of similar size, but not necessarily the same charge, can replace another during formation of the crystal and result in a net charge without disrupting the crystal. Deprotonation (remove H + to get negative) or protonation (add H + to get positive) Humus, 1:1 clays, Fe & Al oxides

Cation exchange on negatively charged sites (Mg substituting for Al in Octahedral sheet) Mg Isomorphic substitution deprotonation

Note all the potential sites for ‘deprotonation’, or removing a H + which will give you a negative site to attract cations.

Broken edges of minerals can “de-protonate” or “protonate” and become charged As pH increases, CEC increases

Characteristics of Ion Exchange Electrostatic (charge) interactions Rapid Exchange requires nearby proximity of one ion for another Reversible Stoichiometric - ions on surface are exchanged with equivalent (number of charges) amounts of other ions (not molar amounts). 2 Na + exchange for 1 Ca +2 3 Na + exchange for 1 Al +3 Selective - some ions are preferred (more tightly held) over others.

The predominant cations on the exchange complex and the order of strength of adsorption include: Al +3 > Ca +2 > Mg +2 > K + = NH 4 + > Na + The strength of adsorption is dependent on the charge of the cation and the size of the hydrated cation Usually, higher charge and smaller hydrated radius results in stronger adsorption Less tightly held cations oscillate farther from colloid surface Therefore, more likely to be displaced into solution or leached Multivalent cations help flocculate soils; sodium disperses soils (large radius, low valence)

CEC range for common soils and materials at pH 7 Note: Very high CEC of humus (soil organic matter); High CEC for 2:1 clays; Low CEC for sandy soils, 1:1 clays, Fe & Al oxides

E.g., estimate the CEC of a soil with pH = 7.0; 20% clay; 4% organic matter; assume: (CEC of clay = 80 cmol c /kg); (CEC of OM = 200 cmol c /kg); CEC associated with clay = 0.2 * 80 cmol c = 16 cmol c CEC associated with OM = 0.04*200 cmol c = 8 cmol c Total CEC = = 24 cmol c per kg soil Estimating CEC based on soil components

cmol c for Colloid each 1% colloid 2:1 Silicate Clay :1 Silicate Clay Fe or Al oxide Clay Organic Matter (humus) Common estimate values