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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 (m3) x Db (Mg/m3) = 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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Page 156
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)
MgIsomorphicsubstitution
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 chargedAs 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+ = NH4+ > 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)
http://www.physchem.co.za/Atomic/Graphics/GRD50005.gif
http://boomeria.org/chemlectures/textass2/table10-9.jpg
http://www.gly.fsu.edu/~salters/GLY1000/6_Minerals/6_Minerals_index.html
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 cmolc/kg);
(CEC of OM = 200 cmolc/kg);
CEC associated with clay = 0.2 * 80 cmolc= 16 cmolc
CEC associated with OM = 0.04*200 cmolc = 8 cmolc
Total CEC = 16 + 8 = 24 cmolc per kg soil
Estimating CEC based on soil components
cmolc for
Colloid each 1% colloid
2:1 Silicate Clay ------------------- 0.5
1:1 Silicate Clay ------------------- 0.1
Fe or Al oxide Clay --------------- 0.1
Organic Matter (humus)----------- 2.0
Common estimate values