FOR 345/545 REVIEW for Exam 3
28 November 2012
This is an outline of the topics. You should know the details that we covered in lecture and lab.
I COLLOIDS & CEC
Soil Colloids Overview
small size, high specific surface area, net negative charge expressed at surface
implications for cation exchange
Chemistry reviewed
moles, equivalents, and conversion of mass (g) to charge (cmolc/kg)
Colloid Types
A. Organic – chemical structure and source of net negative charge
- dissociation of H+ from carboxyl, phenyl hydroxyl, alcohol hydroxyl groups
B. Mineral
i. crystalline silicate clays (phyllosilicates)
ii. noncrystalline silicate clays - short range order:
allophane, imogolite (high phosphate adsorption)
iii Fe and Al oxides and hydroxides (crystalline & amorphous)
Goethite, ferrihydrite, hematite, gibbsite
C. Chemical Structure of mineral colloids -source of net negative charge
i Phyllosilicates Si tetrahedron and Al octahedron –
ratio of Si tetrahedral : Al octahedral sheets - understanding phyllosilicate properties
1:1, 2:1, 2:1:1 clay types, CEC, shrink swell potential
dioctahedral vs. trioctahedral
charge and isomorphous substitution
ii noncrystalline silicate clays imogolite, allophone
iii Fe and Al oxides and hydroxides (crystalline & amorphous)
charge and broken edges
Cation exchange CEC
unit of measure
balancing cation exchange reactions
measuring/estimating CEC
remove soluble salts, replace exchangeable cations with single cation (e.g., NH4+),
quantitatively determine how much cation was adsorbed
buffered vs. unbuffered methods
Anion exchange (when and what colloids can exhibit a net positive charge?)
Strength of attraction of various cations to colloidal surfaces
Charge density- fn(valance, hydrated radius)
Genesis of silicate clays – relation to weathering intensity
II SOIL ACIDITY
Soil pH vs. nutrient availability, bacterial and fungi activity
pH definition and conversion to [H+]
Source of [H+] – weak inorganic acids (high pKa, i.e. carbonic acid), stronger organic acids (low
pKa), Al
Hydrolysis of water generates acidity (Al+3, Al(OH)+2, Al(OH)2+1 how they change with pH
Conceptual pools of acidity: active, salt replaceable, residual
Acid saturation - concept & calculation (related to base saturation)
Factors influencing soil reaction (how and why): acid saturation (graph of pH vs base saturation),
FOR 345/545 REVIEW for Exam 3
28 November 2012
colloid type, adsorbed cations, neutral salts in solution (lab procedure influence), parent
material, vegetation, precipitation , depth in profile, season, fire
pH dependent charge
Soil as a Buffer (resists change in pH)
dynamic equilibrium among active, salt replaceable, and residual pools
buffer curves- coming to a classroom near you
III. Nutrition
Fertilizer Guarantee (N, P2O5, K2O), calculate elemental N, P, K in fertilizers
Context of Nutrition (Six Requirements for Plant Growth)
Composition of The Average Plant ... (dry matter and mineral composition)
18 Essential elements - know them
Criteria to be classified as essential
Role of essential elements in the plant
macro vs. micro, uptake forms & sources (i.e. air, water, specific minerals, organic matter)
Nutrient availability model- FOUR conceptual pools
N cycle components – organic pool, SON, etc.
Details (what are they, what factors influence these) Transfers & Processes
e.g., mineralization, ammonification, immobilization and the C/N ratio,
nitrification, denitrification, N fixation, chemical reduction & volatilization,
fertilization, clay fixation, leaching
P in Ecosystem – relative amount, available forms,
P fixation
Availability of applied P fertilizer
End Point as of Wednesday 28 Nov. Material presented week of Dec 3 will be covered on the
Final.