1. Soil Chemistry

2. Colloids Clay minerals and humus and complexes
The most chemically active part of soil
Large surface area
Electrical charge (usually net negative)
Some +, some –
Nutrient cations (+ions) and anions (- ions) are held on colloid surfaces, in reserve for plants

3. Why are colloids negative?
1.Clay:
Oxygen ions along edges of micelles

4. 2. humus: H+ ions tend to migrate away from organic compounds in humus, to soil solution, leaving net negative charge (OH-)

5. Ion Exchange What is the exchange?
exchange of ions on soil surfaces with ions in soil solution.
Cations and anions are involved

6. Where does exchange take place?
Organic colloids (humus) and inorganic micelles (clays) and complexes of both
Where do ions in soil come from?
Release from organic matter
Rain
Weathering of parent material

7. Leaching ions (on soil surfaces) cannot be removed by leaching.
ions (in solution)
can be removed by leaching.

8. When soil is dried…
…the ions on soil surfaces STAY ON adsorption sites
…the soluble ions (in soil solution) precipitate or crystallize (come out of solution) as salts.

9. Examples of soluble cations precipitating

10. Ion exchange
Exchangeable ions on soil surface trading places with ions in solution.

11. On soil surfaces, there are exchangeable and nonexchangeable ions
Exchangeable: weakly held, in contact with soil solution, ready for quick replacement, available for plants “outer sphere complex”

12. adsorbed by strong bonds or held in inaccessible places
Nonexchangeable:
adsorbed by strong bonds or held in inaccessible places
(e.g., the K+ between layers of illite)
“inner sphere complex”
not part of ion exchange !

13. Cation exchange capacity (CEC)
Sum total of exchangeable cations that a soil can adsorb.
If a soil has a high CEC,
it prevents nutrients from
being leached away from roots

14. CEC
Expressed in:
milliequivalents per 100 g (meq/100g)

15. Dynamic equilibrium
Strive for equivalent proportions of solution and exchangeable ions.
Upset equilibrium by:
removal by plants
leaching
fertilization
weathering
Initiate ion exchange

16. Ion exchange example Add K fertilizer
Ca+2
Ca+2 K+
Ca+2 + K+ + K+ K+
Ca+2 K+ K+ K+
exchangeable
exchangeable
solution
solution

17. Rules of ion exchange Process is Reversible Charge by charge basis
Ratio Law:
ratio of exchangeable cations will be same as ratio of solution cations

18. K+
Ca+2
Ca+2 K+
Ca+2 + K+ + K+ K+
Ca+2 K+ K+ K+
1 Ca : 2 K
1 Ca : 2 K
Same ratio

19. Energy of adsorption
The more strongly a cation is attracted to the exchange surface, the greater the chance of adsorption.
Depends on:
1. charge
2. hydrated radius

20. Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+
Strong Weak
Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+
Radius
Unit
Na+ K+
Mg2+
Ca2+
Al3+
Non-hydrated nm
0.095
0.133
0.066
0.099
0.050
Hydrated
0.360
0.330
0.430
0.410
0.480

21. The less tightly held (lower energy of adsorption) ions are the ones furthest from the soil surfaces and can be leached more easily and are further down the soil profile.
The strongly held ones are closer to the soil particle surfaces and tend to move more slowly down profile

22. How do plants get nutrients?
Nutrients on the colloids are kept within root zone of plants.
H+ from roots exchange with cations on a charge-by-charge basis

23. Two videos:
CEC
CEC

24. Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+
Treating a sodic soil
Sodic: too much sodium (Na)
Add gypsum (CaSO4) : increases calcium concentration; Ca is adsorbed at expense of sodium
Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+

25. Base saturation
% of exchange sites occupied by basic cations (cations other than H+ and Al+3) Base saturation + H+/Al ion saturation should equal 100%

26. Base saturation and pH relationship (for midwest US soils)
Notice neutral pH (7.0) requires a base sat of 80%.

27. Soil pH

28. Soil pH importance Determines solubility of nutrients
Before plants can get nutrients, they must be dissolved in soil solution
Microbial activity also depends on pH

29. pH negative log of the hydrogen ion concentration
(also a measure of OH- concentration)
If H+ concentration > OH- : acidic
If OH- > H+ : basic
Soil pH is pH of solution, NOT exchange complex

30. General soil pH conditions:
“Slightly acid”
6.0 – 6.6
“Moderately acid”
5.0 – 6.0
“Strongly acid”
< 5.0
“Slightly basic”
7.4 – 8.0
“Moderately basic”
8.0 – 9.0
“Strongly basic”
> 9.0

31. In soil, both H+ and Al+3 ions produce acidity
Al+3 produces H+ ions when it reacts with water.
(when pH below 6: Al+3 is the cause of acidity)

32. Causes of soil basicity
Hydrolysis of basic cations
Hydrolysis of carbonates

33. 1. Hydrolysis of basic cations: (especially Ca+2, Mg+2, K+, NH4+, Na+)
(also called exchangeable bases)
Extent to which exchangeable bases will hydrolyze depends on ability to compete with H+ ions for exchange sites. Na Na Na Na +
H2O
+ OH- Na H Na + Na Na Na Na

34. K+ and Na+ are weakly held compared to Ca+2 and Mg+2.
Recall energy of adsorption
So, K+ and Na+ are hydrolyzed easily and yield higher pHs .

35. 2. Hydrolysis of carbonates (especially CaCO3, MgCO3, Na2CO3)
As long as there are carbonates in the soil, carbonate hydrolysis controls pH.
Calcareous soils remain alkaline because H+ ions combine with OH- to form H2O.
For those soils to become acid, all carbonates must be leached.
Basic cations replaced by Al+3 and H+
CaCO3 + H2O
Ca+2 + HCO3- + OH-
Na2CO3 + H2O
Na + HCO3- + OH- (higher pH because Na more soluble)

36. Causes of soil acidity Accumulation of soluble acids
Exchangeable acids (Al+3, H+)

37. Accumulation of soluble acids at faster rate than they can be neutralized or removed
Carbonic acid
(respiration and atmospheric CO2)
b. Mineralization of organic matter
(produces organic, nitric, sulfuric acids)
Precipitation increases both a and b

38. 2. Exchangeable acids Dissociation of exchangeable H+ or Al+3
Al+3 ties up OH- from water, releases an equivalent amount of H+ ions.
Al H2O
AlOH H+

39. CEC and pH Only 2:1 silicate clays do not have pH-dependent CECs.
Others are pH-dependent:
1:1 kaolinite:
low pH: low CEC
high pH: high CEC
Oxidic clays
pH dependence of humus