Exam 3 Flashcards
Soil Colloids characteristics
- small size
- high specific surface area
- net negative charge expressed at surface with adsorbed cations
Implications of adsorbed cations in colloids
-adsorbed cations exchange with cations in soil solution
Cation Exchange Capacity (CEC)
- expressed as cmol(c)/kg
- the sum of exchangeable cations soils can absorb
- fn(amount and type of colloid)
- organic matter and clay are the 2 types
what is a mole
-weight of element that contains 6.02 x 10^23 atoms (molecular weight
What is a centimole?
= to .01 mole
Equivalent weight
- molecular weight/valance
- the amount needed to generate a charge
How to convert mass to charge
- find the equivalent weight
- convert to cmol(c) by multiplying .01
Organic colloids
- carbon based
- primarily C, H, O
What are the 2 major types of colloids
- mineral
- organic
What is the source of the net negative charge in organic colloids
- H+ dissociation from:
- carboxyl group
- phenol hydroxyl group
- alcohol hydroxyl group
Dissociation is ________
pH dependent
-high pH, low acidity
Mineral colloids (3 types)
- Crystalline silicate clays = phyllosilicates
- Noncrystalline silicate clays (short range order and paracrystalline)
- Fe and Al oxides and hydroxides (silicate missing, crystalline and amorphous)
What is the source of the net negative charge in for mineral (inorganic) colloids
- Isomorphous substitution
- replacement of 1 atom by another of similar size
acid saturation
(H+Al)/CEC x 100
-percentage of CEC accounted for by H and Al ions
Cation saturation
- Proportion of CEC satisfied by specific cation (expressed as %)
- cation/CEC
Genesis of silicate clays
- alteration of primary minerals
- breakdown and recrystallization of primart minerals
- weathering sequence
Strength of attraction of cations____
-depends on valance, hydrated radius
CEC determination
- Buffer methods (extraction solution buffered)
- Unbuffered methods (extraction solution at pH of soil with neutral salt)
ECEC
- effective cation exchange capacity
- the total amount of exchangeable cations, which are mostly sodium, potassium, calcium and magnesium
Soil acidity (pH) is correlated with _
- nutrient availability
- organism activity
-one pH unit means ____ difference in H+
10^-4
Source of H+ dissociation
-wear inorganic acids (high pKa, ex. carbonic acid), stronger organic acids (low pKa)
Al and Al hydroxides generate acidity by ____
hydrolyzing water
hydrolysis of water
Three conceptual pools of acidity
1) Active acidity (pool)
2) Salt replaceable acidity (pool)
3) Residual acidity (pool)
Active pool
-the samllest pool,
H+ activity in soil solution
Salt replacable pool
- the mid-sized pool
- ions of H+ and Al+ are semi-readily available as exchangeable compounds
residual pool
- the largest pool
- Bounded ions are help up in silicate clays and organic matter
- the H+ ions are not readily available to exchange with
why is soil pH relatively resistant to change
-the three pools are in dynamic equilibrium and act as a buffer
What is a buffer
- a system that resists change
- resists change in pH for soils
Fact influencing soil pH or reaction
1) Acid Saturation
2) Type of colloid
3) adsorbed cations (Na results in higher pH)
4) parent material (limestone vs sandstone)
5) vegetation
6) precipitation
7) depth in the profile (going deeper increases alkaline)
8) season
9) fire
10) in the lab
As acid saturation _____ pH ____
increases, decreases
Which colloids can have a net positive charge?
- Kaolinite
- Fe and Al hydroxides and oxides
- noncrystalline silicate clays
Buffer curve
pH vs Base saturation
how to intensify soil acidity
- organic matter addition
- addition of FeSO4, S2, H2SO4
How to reduce soil acidity
- lime addition
- add manure
6 requirements for plant growth
1) light
2) favorable temp
3) water
4) aeration
5) absence of toxic substances
6) nutrient elements (18 essential)
Criteria for an element to be essential
1) required to complete life cycle
2) requirement is direct
3) cant be substituted by another element
The average plant
- 75% water and 25% solid
- C is 42% of solid
- H is 8% of solid
- O is 42% of solid
Macronutrients
- required by plants in relatively large amounts
- S, P, Mg, K, Ca, N, O, C, H
Micronutrients
- required by plants in relatively small amounts
- Mo, Ni, Co, Cu, Zn, Mn, Fe, B, Cl
Macronutrients C, H, O
- structural role
- no shortage due to being in the air and water
Macronutrients N, P, S
- OM is the major source
- S responsible for ATP and DNA
- N plays a role in amino/nucleic acids
Macronutrient K
-osmoregulator, protein synthesis
Macronutrient Ca
-structural role, membrane regulation
Macronutrient Mg
-chlorophyll, protein synthesis
What is the general role of micronutrients
-to recycle enzyme cofactors
Mineralizations
-conversion of organic to inorganic
Immobilization
-conversion of inorganic to organic
If the carbon to nitrogen ration is greater than 25:1 then _______ if less than 25:1 then ______
- greater than the net effect is immobilization
- less than the net effect is mineralization
Percentage of K in K2O (potash)
83%
Percentage of P in P2O5 (soluble phosphate)
43.7%
