1. Soil Acidity and Nutrients
Section D
Soil Fertility and Plant Nutrition

2. Soils Soils behave as weak acids or bases
Some soils behave like a buffered weak acid:
are acid, but resist increases in pH
have a “buffering capacity”
Some soils behave like a buffered weak base
are basic, but resist decreases in pH

3. Titration Curve - Acid Soil
Alkaline “ pH
“Buffering”
Acid
Base added

4. Soils Many soils behave like weak acids because: production of CO2
organic acids in SOM, root exudates
EXCHANGEABLE CATIONS
Exchangeable cations are the most important influence on soil pH. The relative proportion of acid or basic cations on the cation exchange sites determine the soil pH in almost all cases.

5. Titration Curve - Alkaline Soil“ pH
“Buffering”
Acid
Acid added

6. Soils Some soils behave like weak bases because: presence of CaCO3
it is a weak base that will buffer soil solutions against decreases in pH

7. Formation of Soil Acidity
“Young soil”
Time
“Old soil”
Al3+ dissolves from minerals
Ca2+, Mg2+, K+ are leached from soil
Clay minerals
with exchangeable
Ca2+, Mg2+, K+ and
Al 3+, H+
Clay minerals
with exchangeable
Ca2+, Mg2+, K+

8. Exchangeable Cations
Ca2+, Mg2+, Na+, and K+ do not produce H+ in soils, and so are called the “basic cations”
Al3+ and Fe3+ react to produce H+ in soils, and so are called the “acid cations”
The “base saturation” is the percentage of the CEC occupied by the basic cations. It is highly related to soil pH.

9. “Base Saturation Percentage”8
Low CEC Soil
Soil pH
High CEC Soil 2
100
Percent base saturation

10. Why is Al3+ called “acidic”?
Al3+ ions in solution are surrounded by water molecules (octahedral coordination).
The high positive charge of Al3+ causes it to pull electrons from O on H2O.
This makes H2O more acidic.
This is called “Al hydrolysis”
Fe3+ behaves similarly.

12. 3+
+ H +
[Al(H 2 O) 5
OH] 2+
Al(H O) 2 6 3+ Al H O 2

13. 2+ 2+
+ H +
[Al(H 2
O)4(OH) ]
[Al(H O)
OH] 2 5 3+ Al H O 2

14. +
[Al(H O)
OH]
+ H +
[Al(H 2 O) 3
(OH) ] 2 5 3+ Al H O 2

15. Titration Curve—Acid Soil
Alkaline “
The amount of buffering capacity
will determine the lime requirement.
Soils with higher CEC will have
a higher L.R. pH
Buffering due to
Al hydrolysis
Acid
Lime added

16. Soil Acidity
Al3+
Ca2+ H+
Clay
Solution

17. Al 3+ H+

19. Sulfide Oxidation
Mine tailings (spoils) may become acidic because of the
oxidation of pyrite (FeS2):
FeS2 + H2O + 7/2O2  FeSO4 + H2SO4

21. Al Toxicity
Below a pH of ______, Al3+ may be present in concentrations that are toxic to plants. Al solubility is inversely proportional to pH.
Severely inhibits root growth, interferes with P uptake.
Al toxicity is the #1 problem in acid soils. Mn and Fe may be toxic as well, because they become more __________ under acid conditions.
5.5
soluble

24. Plants and pH Soil pH Ranges Plant preference for pH Strongly Acidic
<4
5.5
6.5
7.5
8.5
Strongly
Acidic
Moderately
Slightly
Neutral
Alkaline
Highly
Plant preference for pH
Most crop
plants and
plants of
temperate regions
Most tropical
plants. Tea,
Coffee, azaleas
Alfalfa
Halophytes

25. Plants and pH Differing plant tolerance to acidity is mostly due to:
Different nutrient requirements
Ability to immobilize Al3+ in the rhizosphere or to detoxify Al3+ within the plant.

26. Correcting Soil Acidity
Remember the two “components” of soil acidity: __________ acidity, which is ____ in the soil solution, and ____________ acidity which is _______ on the soil clay cation exchange sites.
Both of these have to be neutralized to correct soil acidity. To do this, ________ is added to acid soils.
active H+
reserve
Al3+,H+
lime

27. Al 3+ H+

28. Liming Acid Soils Lime Sources:
CaO (quicklime), Ca(OH)2 (hydrated lime), CaCO3 (ag lime), CaMg(CO3)2 (dolomitic lime)
The amount of lime to add to a soil (lime requirement) depends on: pH
Cation exchange capacity
L.R. is usually determined by a soil test

29. Lime Rates

30. Reactions of Lime in Soils
Lime reacts to neutralize solution acidity:
CaCO3 + 2H+  H2O + CO2 + Ca 2+
Lime also removes exchangeable Al3+ from clay cation exchange sites (X is a cation exchange site)
3CaCO3 + 2AlX + 3 H2O  3CaX + 2Al(OH)3 + 3CO2