Soil Chemistry 13- lect (1)_b3c02146ef2619bf4a99009ba2649eaf

1. Soil Chemistry

2. Brief Contents

Soil pH importance
General soil pH conditions
Causes of soil basicity. Hydrolysis of basic
cations
Causes of soil acidity. Accumulation of
soluble acids
Problems associated with acidity
Alkaline, Saline, and Sodic of Soils

3. Ion Exchange

Ions adsorbed to soil surfaces can be
exchanged with ions in soil solution.
Cations and anions

4. Ion exchange

Organic colloids and inorganic micelles
(clays) are sites of ion exchange
Where do ions in soil come from?
Release from organic matter
Rain
Weathering of parent material

5. Ion exchange

Exchangeable cations (on soil surfaces)
cannot be removed by leaching.
Soluble cations (in solution)
can be removed by leaching.

6.

When soil is dried…
…exchangeable cations hold to adsorption sites
on soil surfaces.
…soluble cations (and anions) precipitate or
crystallize as salts.

7. Examples of soluble cations precipitating

8. Ion exchange

Exchangeable ions on soil surface trading
places with ions in solution.

9. On soil surfaces, there are: Exchangeable and Nonexchangeable Ions :

Exchangeable: weakly held, in contact with soil
solution, ready for quick replacement.
“outer sphere complex”
Nonexchangeable:
“inner sphere complex”
adsorbed by strong bonds or held in
inaccessible places
(e.g., the K+ between layers of illite)
not part of ion exchange !

10. Cation exchange capacity (CEC)

Sum total of exchangeable cations that a
soil can adsorb. ( prevents nutrients from
leaching away from roots)

11. CEC

Expressed in:
milliequivalents per 100 g (meq/100g)

12. Base saturation

% of exchange sites occupied by basic
cations
Basic cations are cations other than H+ and
Al+3
Base saturation
+ H+ ion saturation
should equal 100%

13. For midwest US soils

Notice neutral pH (7.0)
requires a base sat
of 80%.
(neutral pH is not 50%
because most base
cations have a + charge
of 2)

14. equilibrium

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

15. Ion exchange example: Add H+ ions to soil :

Ca+
Ca+
H+
Ca+
H+
+
Ca+
Ca+
soil
Ca+
exchangeable
Ca+
Ca+
Ca+
H+
H+
solution
solution
H+
Ca+ Ca+
+
H+
Ca+ H+
H+
Ca+
Ca+
Ca+
Ca+
exchangeable
solution

16. Rules of ion exchange

Process is Reversible
Ratio Law:
ratio of exchangeable cations will be
same as ratio of solution cations

17. Add K fertilizer…

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

18. Energy of adsorption

Strong --------------------------------------Weak
Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+
(based on charge and hydrated radius)

19. 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

20. 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

21. General soil pH conditions:

“Slightly acid”
6.0 – 6.6
“Slightly basic”
7.4 – 8.0
“Moderately acid”
5.0 – 6.0
“Moderately basic”
8.0 – 9.0
“Strongly acid”
< 5.0
“Strongly basic”
> 9.0

22.

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)

23. Causes of soil basicity

1.
2.
Hydrolysis of basic cations
Hydrolysis of carbonates

24. 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
Na
Na
+
Na
H2O
H
Na
Na
+
Na
+ OH-

25.

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 .

26. 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)

27. Causes of soil acidity

1.
2.
Accumulation of soluble acids
Exchangeable acids (Al+3, H+)

28. Accumulation of soluble acids at faster rate than they can be neutralized or removed

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

29. Distribution of acid soils

Acidic soils usually occur where rainfall
leaches the cations out of the soil over
time. In the U.S. there is a fairly strong
correlation between precipitation and pH,
with soils receiving more than about 30
inches of annual precipitation having a pH
<6

30. Problems associated with acidity

Aluminum toxicity: Aluminum becomes
more available when pH is pH < 5
Manganese toxicity: This may occur in soil
that are high in Mn and that have a pH < 5.

31. 2. Exchangeable acids

Exch. H+ or Al+3 dissociate
Al+3 ties up OH- from water, releases an
equivalent amount of H+ ions.
Al+3 + H2O
AlOH+2 + H+

32.

Acid soils and liming Lime (calcium
carbonate) is added to acid soils to raise
the pH. Calcium (Ca2+) replaces hydrogen
and aluminum on the exchange sites. For
a good reference on liming, see the Soil.

33. 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

34. Alkaline, Saline, and Sodic Soils

Alkalinity and acidity: Soils that vary from a
neutral pH have varying degrees of
alkalinity (pH > 7) or acidity (pH < 7). The
mean soil pH in the U.S. is around 6.4.

35. Salinity:

Soils that have excess soluble salts in the
soil solution have varying degrees of
salinity

36. Sodicity

Soils that specifically have excess sodium
in the soil solution are called sodic

37. Alkalinity

Alkalinity Soils in arid and semi-arid areas
can lack enough rainfall to leach cations,
especially calcium (Ca2+), magnesium
(Mg2+), potassium (K+) and sodium (Na+),
from the soil. These cations bind many of the
CEC sites, blocking hydrogen (H+) ions from
binding and making the soil alkaline. This can
also happen if irrigating with water high in
calcium bicarbonate or magnesium
bicarbonate.

38. Salinity

Salinity A soil containing sufficient soluble
salts (these salts include Mg2+, Na+, Ca2+,
chloride (Cl- ), sulfate (SO4 2-), bicarbonate
(HCO3 -) and carbonate
(CO3 2-). Saline
Sodicity
soils mainly occur in dry areas, again, where
there is not enough precipitation to leach the
salts from the soil, so the salts build up over
time. In order for there to be salts in the soil,
there must be a source for them.

39. Sodicity

A soil containing sufficient exchangeable
sodium to adversely affect crop production
and soil structure under most conditions of
soil and plant types. Many saline soils are
also sodic, although not necessarily. Sodium
is toxic to plants. It also causes soil particles
to disperse (separate), which causes
cracking and sealing of the soil surface,
leading to poor soil structure and decreased
water intake.

40.

Sodic soils can be reclaimed with a two-
step process. First the sodium is flushed
from CEC sites by adding amendments
high in calcium (such as lime, gypsum, or
dolomite) or by adding sulfur followed by
lime. (The sulfur is converted to sulfuric
acid by microbial activity.

41.

The sulfuric acid then reacts with lime to
free calcium.) In either case, the Ca2+
ions replace the Na+ cations, freeing the
Na+ in the soil solution. The second step
is to leach out the sodium ions by irrigating
in excess of what the plant needs.

42. Quantitative definitions

Specifically, alkaline, saline, and sodic
soils are defined as such:
a) Alkaline soil: Has a pH of > 8.5 or with
an exchangeable sodium percentage
(ESP, that is, the percent of the CEC
occupied just by sodium) greater than
15%. Soils at this ESP contain sufficient
sodium to interfere with the growth of most
crop plants.

43.

b) Saline soil: Soil salinity is determined by
measuring the electrical conductivity (EC)
of a saturated paste of soil: if the EC is
greater than 4 dS/m (decisiemens per
meter), the soil is classified as saline.
However this is a rough range: saltsensitive plants can be affected at half this
EC and highly tolerant plants can handle
up to about twice this EC.

44. Conclusion

Soil chemistry plays a fundamental role in determining
soil fertility, plant nutrition, and overall ecosystem health.
Understanding ion exchange processes, cation
exchange capacity, and base saturation helps explain
how nutrients are retained and made available to plants.
Soil pH strongly influences nutrient solubility and
microbial activity, while the balance between acidity and
basicity is shaped by factors such as rainfall, mineral
weathering, organic matter decomposition, and the
presence of carbonates.

45.

Acidic soils often present challenges such as aluminum
and manganese toxicity, whereas alkaline, saline, and
sodic soils can restrict plant growth through excess salts
or sodium-induced structural problems. Effective
management strategies—including liming acid soils and
reclaiming sodic soils with calcium
amendments—are essential for maintaining soil
productivity. Overall, knowledge of soil chemical
properties is crucial for sustainable agriculture,
environmental protection, and long-term soil health.