1. Soil Acidity and Alkalinity ILMU TANAH 2016
The Soil Reaction
Soil Acidity and Alkalinity
ILMU TANAH 2016

2. Soil pH
* pH - the negative log of the hydrogen ion(H+) concentration in the soil water solution.
pH = - log [ H+]
* the pH scale is how we measure acidity and alkalinity of solutions.
at neutral (pH =7) the number of H+ = OH-
Remember –
at pH of 6 there are 10x more H+ ions than at a pH 7
and there are 100x more H+ ions between pH 7 & 5

3. Soil pH… This gives a measure of the acidity or basicity of a soil.
0-7 = acidic; 7-14 = basic.
Acidity is measured by determining the concentration of Hydrogen (H+) ions in the soil.
Higher concentration of H+ ions = high acidity, higher concentration of OH- ions = high basicity.
In general, the ideal pH for plant growth is about 5.5 in organic soils and about 6.5 in mineral soils.

4. Soil pH Ranges Neutral Acidity Alkalinity 7 6 5 4 3 8 9 10 11 10x 100x
Soil acidity increases as the pH level drops. The amount of acidity increases by 10 fold for each point on the scale. A pH of 4.0 is 100 times more acid than a pH of 6.0. At very low pH levels, more lime is needed to increase the pH one point on the scale compared to the amount needed at pH 5.5. Most Arkansas soils have a pH range of 4.0 to 7.0. Most forages grow best between a pH of 5.5 and 7.5.

5. Sources of acidity in Soil
* Hydrogen and Aluminum cations are responsible for soil acidity
* Exchangeable Hydrogen is the main source of H+ at pH 6 and above.
* Below pH 6 Aluminum is the main source of H+ due to dissociation of Al from clay minerals. Aluminum becomes more soluble at lower pH’s
Al3+ + H > Al(OH) H+
Al(OH) H2O ---> Al(OH) H+
Al(OH) H20 ---> Al(OH)3 + H+

6. The Colloidal Control of Soil Reaction
Source of Hydrogen Ions
Al under very acid conditions Al
Soil Solution
Al H2O Al(OH) H+
The Al(OH)3 is not ionized so the H ions thus released give low pH value in the soil solution H
H Soil Solution Hydrogen
The effect of both adsorbed hydrogen and aluminum is to increase the H ion concentration in the soil solution
Micelle =Al+++
Al in solution
Hydrolize
Micelle =H+

7. The Colloidal Control of Soil Reaction
Source of OH ions
Ca H
Under natural conditions the reaction to furnish H and OH ions to the soil solution occur simultaneously
Micelle H2O Micelle Ca++ + 2OH

8. factor of Acid forming in Soil
1. Nitrification: Ammonium to Nitrate (oxidation of NH4+)
NH O2 ---> NO3- + H2O + 2 H+
2. O.M. decomposition
organic acids ionized :
R-COOH---> R-COO- + H+
respiration:
CO2 + H2O ----> H2CO3 = H+ +HCO3-

9. 3. Acid rain Acid rain is caused by the burning of fossil fuels.
Burning oil, gas and coal in power stations releases Sulfuric Dioxide (SO2) into the atmosphere.
Burning oil and gasoline in motor vehicles puts nitrogen oxides (NOX) into the atmosphere.
These gases mix with water droplets in the atmosphere creating weak solutions of nitric and sulfuric acids.
When precipitation occurs these solutions fall as acid rain.

10. Acid Rain 3. Acid Rain SO2 +OH --> H2SO4 --> SO4- + 2 H+
NO2 + OH --> HNO3--> NO3- + H+

11. Sources of acidity in Soil
4. Uptake of basic cations by plants. Basic cations are sources of OH- to the soil solution.
Ca++, Mg++, K+, =
Basic cations that are taken up by plants no longer contribute OH- to the soil solution.
H+ ions are released to the soil solution.

12. Leaching
5. Leaching of basic cations as basic cations are removed from the soil solution by leaching they no longer contribute the OH- ions to neutralize the ever increasing amounts of H+
Ca H20 ---> Ca(OH)2 + 2H+
-----> Ca OH-

13. Back to Soil pH
Active Acidity - due to the H+ ion activity in the soil solution at any given time
Reserve Acidity - represented by the H+ and Al3+ that are easily exchanged by other cations (positively charged ion)
H H H H H H+
H Ca++ H+
Mg Mg H+
Ca Ca H H+
H H H Na
soil
Reserve /PotentialAcidity Active Acidity -Soil solution

14. Active Versus Exchangeable Acidity
Two types of pH in Soil pH aktual/Active- Soil Solution pH potensial/ Potential – Exchangable/ adsorbed Adsorbed H ions Soil Solution H+ (Reserve) (Active)

15. Buffer pH
Organic matter
Soil pH

16. Pools of acidity in soils
Active acidity
Exchangeable acidity
Residual acidity
Pools of acidity in soils

18. Determination of soil pH
suspension
Combination electrode in soil suspension
Determination of soil pH
Millivolt
meter Ag Ag
AgCl2
Sat’d KCl
Calomel
Reference
electrode
Glass membrane
H+ electrode
Glass
membrane
Fiber wick H+ H+ H+ H+ H+ H+ H+ H+ H+ H+
colloid

19. Combination pH electrode

20. Comparison of Methods of Measuring Soil pH
Soils from Tennessee, Malawi, & Honduras
Methods of pH determination:
Soil:solution ratio
1:1
1:2.5
Solution used to suspend soil
Distilled water (pHH2O)
0.02M CaCl2 (pHCaCl2)
1M KCl (pHKCl)
General, approx. relationship:
pHCaCl2 = pHH2O
pHKCl = pHH2O
From Brady and Weil, 2002

21. Soil Buffering Capacity…
The tendency of soils is to resist changes of the pH of the soil solution.
This resistance is termed “buffering”.
Soils have different buffering capacities.
Generally, higher CEC = greater buffering capacity.
Buffering capacity indicates dynamic equilibrium of soil solution.
Changes of all types tend to be resisted by the system.

22. The ability of soil to resist change in pH.
Buffering Capacity
The ability of soil to resist change in pH.
The amount of H+ in the soil solution is small compared with the “H+, Al 3+” adsorbed on the soil colloids (reserve)
Neutralization (by the addition of bases) of the solution H+ (H+ is removed from the system) results in a rapid replacement of H+ from the exchangeable H+ on the soil colloid.
CaCO3 when added to soil will neutralize H+.
CaCO3 = Lime (dolomitic = MgCO3 & CaCO3

23. Coffee urn analogy for active and reserve acidity
Soil buffering capacity resists change in pH.
Related to CEC and exchange and residual acidity
Sandy or low CEC soil
 High CEC soil 
exchange and residual acidity
Brady and Weil, 2002

24. Exchangeable Al (cmol(+)/kg)
Soil + peat
Exchangeable Al (cmol(+)/kg)
Mineral soil alone
Soil pH
From Hargrove and Thomas, 1981

25. Why is soil pH important?
Affects solubility of minerals.
Affects type, numbers and activity of microorganisms.
Fungi tolerate acidity better than bacteria. Bacteria often negatively affected by high acidity (i.e. low pH).
Indirectly affects aggregate stability.
Determines what happens to many soil pollutants.
CEC increases with soil pH.

26. Acid Soil “Headache” Aluminum toxicity Manganese toxicity
Calcium deficiency
Fe deficiency induced by Al
Molybdenum deficiency
Magnesium deficiency

27. Soil pH affects many chemical and physical reactions in soil
Availability of most essential elements
Activity of microorganisms
Ability of soil to hold cations
Solubility of non-essential elements such as heavy metals
Herbicide performance

28. pH and nutrient availability
As the soil pH increases from an acidic condition to pH 6.5
Macronutrients (N,P,K) increases in solubility
Secondary nutrients (Ca, Mg, S) increases in solubility
Micronutrients (except Molybdenum) decreases in solubility
Al decreases in availability (very important)

29. Soil Reaction Correlation
Available Phosphorous
- Acid - deficient pH % available
- Alkaline - deficient
Soil Organisms
- Low pH fungi
- Nuetral - Bacteria, Actinomycetes, Fungi
- Nitrate production, pH 5.5 and higher

30. pH influence on microorganisms
Bacteria and actinomicete are reduced at low pH
Nitrification occurs at pH range 6.0 to 9.0, optimum pH 7
Denitrification (biological loss of N) occurs at a minimu of pH 5.5, bellow this point chemical denitrification occurs
Nitrogen fixation by Rhyzobium (legume-bacteria symbiosis) optimum occurs between a pH of 6.0 to 6.5
Organic matter decomposition: optimum pH 7.0

31. Relationship of Plants to pH
Very Acid Soils - pH 4.5 and lower 1. Low exchange Ca and Mg 2. High Sol. of Fe, Al, and Mn 3. Presence of organic toxins 4. Low available N and P

32. Relationship of Plants to pH
Alkaline soils pH 7.5 and over 1. Plenty exchangeable Ca and Mg 2. Active humus N is available 3. Minor elements unavailable Problems of Acidity Fertility Plant growth

34. Breeding to overcome Al toxicity
Al-Tolerant variety
pH 4.4
pH 5.7
Breeding to overcome Al toxicity
Photo by C. D. Foy, USDA

36. TERIMA KASIH

37. Increase in Soil pH is Attained by
1. Metallic cations
Calcium Ca++
Magnesium Mg++
Anion also Important
H Ca
Micelle CaSO Micelle H+ SO4= H

38. Forms of Lime
Calcium and Magnesium Carbonate CaCO3 Calcium and Magnesium Oxides and Hydroxides CaO, Ca (OH)2 H Ca Micelle + CaCO3 Micelle + CO2 + H2O H

39. Why apply lime ? 1. helps nutrients become available to plants
2. improves soil structure
3. provides nutrients for plant growth
4. promotes growth of beneficial microorganisms
5. overcomes acidifying effects of fertilizers
6. reduces metal toxicity to plants (solubility vs. pH)

40. Lime needs and Soil Texture
% OM CEC 7 6 5 4
Sands
sandy loams
loams
SiCl
Soil pH

41. Reaction of Saline and Alkali Soils
Halomorphic -
Salt accumulation in surface horizon
a. Saline soils pH below 8.5
(White Alkali) Na< 15% of CEC
- High in Ca, Mg
- Leaching won’t change pH

42. Reaction of Saline and Alkali Soils
b. Saline - alkali soils
- Na > 15% of CEC
- pH < 8.5
- Leaching will increase pH
c. Nonsaline - alkali soils
(Black Alkali) - pH > 8.5
- Low Soluble Salts
- Toxic effect of Na+ and OH-

43. TERIMAKASIH