Calculating wet topsoil pile weight Calculate the moisture content (w): w = [(g water) / (g dry soil)] x 100 = % Calculate dry topsoil weight using Db.

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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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