CSES 2040-Exam 2 UPDATED ACTUAL Questions and CORRECT Answers
• ALL soils have negative and positive charge sites.
• MOST soils have an overwhelmingly negative charge.
• These negative charge sites develop because of the clay content, presence of
What Makes CEC Happen?
Fe/Al oxides, soil pH and organic matter content. We will discuss how/where these
specific negative charges come from later.
• For now just know that most soils have, to some degree,negative charge.3
The degree to which a soil has negative charges is measured by the Cation
Exchange Capacity (CEC).
• The degree to which a soil has positive charge is theAnion Exchange Capacity
Cation Exchange Capacity (AEC).
• Defined as the sum total of exchange able cations that a soil can adsorb.
• Cations are the positively charged ions - K+,Ca+2, Mg+2, Na+, NH4+, Al+3, H+• Anion
exchange capacity (AEC) - attraction of negatively charged ions (NO3-, SO42-)
• Calcium (Ca+2)
• Magnesium (Mg+2)
• Potassium (K+)
Cations (have a positive charge) in the soil:
• Sodium (Na+)
• Ammonium (NH4+)
Their total is the Cation Exchange Capacity (CEC) of the soil.
• We will discuss this more when we get to clays and OM.
• And when we get to soil pH.
• There is some relationship between soil pH and the development of negative
What is 'Fixed' and 'Variable' Charge? charges in soil. So, as soil pH goes up or down (in a certain range only) CEC may
change. Thus, this is 'variable' charge.
• Other negative charges are developed when the clays are formed, and thus are
'fixed'.
, • This is important in acid, humid soils.
• When we have pH dependent charge (variable charge).
-Al-OH + OH- <---→ Al-O- + H2O
The Difference Between CEC and
Increasing pH ------------ →
EffectiveCEC (called ECEC)
• The reaction shown above can happen when we add NH4-acetate at a pH of 7. It
creates more negative sites, so more NH4 gets adsorbed on those negative sites.
• So, in acid soils we may overestimate the CEC of the soil.
• We calculate the Effective CEC (ECEC)
• Do the CEC determination as shown before, but don't measure the NH4 collected.
• Instead, determine the individual amounts of Ca, Mg, K,Na collected from the first
How do you fix overestimate CEC?
filtering.
• Then, extracted with an unbuffered salt solution (KCl), and then analyze for the Al
and H content
• Do the CEC determination as shown before, but don't measure the NH4 collected.
• Instead, determine the individual amounts of Ca,Mg, K, Na collected from the first
ECCE filtering.
• Then, extracted with an unbuffered salt solution(KCl), and then analyze for the Al
and H content.
Ca + Mg + K + Na + Al +H
• In acid weathered soils the ECEC will always be less than the CEC.
CEC versus ECEC
• Really - in most of our soils it's not an issue. Just be aware of it as some soil tests
note the difference.
CEC can get a little wonky if you've just CEC measurement matters
limed orif you have free calcium carbonate
- then the method of
nonacid cations Ca+2, Mg+2, K+, Na+
acid cations H+, Al+3
Base saturation (nonacid cations/ CEC(acid+ nonacid cations))X100
• Related to soil acidity/alkalinity
• Should not be used for fertilizer recommendations
Base Saturation
• Often used as a rationale to sell more fertilizer than probably needed.
• If your soil pH is low liming fixes it.
• Most soils have some anion exchange capacity.
• Contributes to phosphate retention.
• Most common in soils that:
A Brief Drive By of AEC(Anionic Exchange
- Have soils high in 1:1 clays
Capacity)
- Hydrous oxides of iron and aluminum
- Heavily weathered soils
- Volcanic soils
• We express CEC in terms of moles of positive charge adsorbed per unit of mass -
this is centimoles of positive charge per kilogram soil
- cmolc/kg
Older textbooks or writings may refer to something called a'milliequivilant' (meq),
but the number is the same as a cmolc/kg.
Meq is just an old system.
How do we express CEC?
Note - this seems to be making a comeback - the use of meq
• CEC is expressed on a charge basis to reflect that it is the number of charges, and
not the number of ions that is important.
• Ions have different charges (K+, Ca+2 etc.) so that is why the number of charges is
most important.
• To do this we use the molar concept.
• ALL soils have negative and positive charge sites.
• MOST soils have an overwhelmingly negative charge.
• These negative charge sites develop because of the clay content, presence of
What Makes CEC Happen?
Fe/Al oxides, soil pH and organic matter content. We will discuss how/where these
specific negative charges come from later.
• For now just know that most soils have, to some degree,negative charge.3
The degree to which a soil has negative charges is measured by the Cation
Exchange Capacity (CEC).
• The degree to which a soil has positive charge is theAnion Exchange Capacity
Cation Exchange Capacity (AEC).
• Defined as the sum total of exchange able cations that a soil can adsorb.
• Cations are the positively charged ions - K+,Ca+2, Mg+2, Na+, NH4+, Al+3, H+• Anion
exchange capacity (AEC) - attraction of negatively charged ions (NO3-, SO42-)
• Calcium (Ca+2)
• Magnesium (Mg+2)
• Potassium (K+)
Cations (have a positive charge) in the soil:
• Sodium (Na+)
• Ammonium (NH4+)
Their total is the Cation Exchange Capacity (CEC) of the soil.
• We will discuss this more when we get to clays and OM.
• And when we get to soil pH.
• There is some relationship between soil pH and the development of negative
What is 'Fixed' and 'Variable' Charge? charges in soil. So, as soil pH goes up or down (in a certain range only) CEC may
change. Thus, this is 'variable' charge.
• Other negative charges are developed when the clays are formed, and thus are
'fixed'.
, • This is important in acid, humid soils.
• When we have pH dependent charge (variable charge).
-Al-OH + OH- <---→ Al-O- + H2O
The Difference Between CEC and
Increasing pH ------------ →
EffectiveCEC (called ECEC)
• The reaction shown above can happen when we add NH4-acetate at a pH of 7. It
creates more negative sites, so more NH4 gets adsorbed on those negative sites.
• So, in acid soils we may overestimate the CEC of the soil.
• We calculate the Effective CEC (ECEC)
• Do the CEC determination as shown before, but don't measure the NH4 collected.
• Instead, determine the individual amounts of Ca, Mg, K,Na collected from the first
How do you fix overestimate CEC?
filtering.
• Then, extracted with an unbuffered salt solution (KCl), and then analyze for the Al
and H content
• Do the CEC determination as shown before, but don't measure the NH4 collected.
• Instead, determine the individual amounts of Ca,Mg, K, Na collected from the first
ECCE filtering.
• Then, extracted with an unbuffered salt solution(KCl), and then analyze for the Al
and H content.
Ca + Mg + K + Na + Al +H
• In acid weathered soils the ECEC will always be less than the CEC.
CEC versus ECEC
• Really - in most of our soils it's not an issue. Just be aware of it as some soil tests
note the difference.
CEC can get a little wonky if you've just CEC measurement matters
limed orif you have free calcium carbonate
- then the method of
nonacid cations Ca+2, Mg+2, K+, Na+
acid cations H+, Al+3
Base saturation (nonacid cations/ CEC(acid+ nonacid cations))X100
• Related to soil acidity/alkalinity
• Should not be used for fertilizer recommendations
Base Saturation
• Often used as a rationale to sell more fertilizer than probably needed.
• If your soil pH is low liming fixes it.
• Most soils have some anion exchange capacity.
• Contributes to phosphate retention.
• Most common in soils that:
A Brief Drive By of AEC(Anionic Exchange
- Have soils high in 1:1 clays
Capacity)
- Hydrous oxides of iron and aluminum
- Heavily weathered soils
- Volcanic soils
• We express CEC in terms of moles of positive charge adsorbed per unit of mass -
this is centimoles of positive charge per kilogram soil
- cmolc/kg
Older textbooks or writings may refer to something called a'milliequivilant' (meq),
but the number is the same as a cmolc/kg.
Meq is just an old system.
How do we express CEC?
Note - this seems to be making a comeback - the use of meq
• CEC is expressed on a charge basis to reflect that it is the number of charges, and
not the number of ions that is important.
• Ions have different charges (K+, Ca+2 etc.) so that is why the number of charges is
most important.
• To do this we use the molar concept.
