ACID SOILS

1
ACID SOILS

• Assigned reading Sparks Chapter 9 (Skip
  polymeric Al section pp. 273-277) and McBride
  1.2f, 5.1-5.4d.
• Additional Essington Chapt 10 through 10.1.2

2
Measuring soil pH

• Use pH sensitive dyes
• Use pH electrode

3
Response of glass electrode to H Ion Specific
Electrodes to Activity

• Glass electrode is one type of ion specific
  electrode
• Electrode materials are specific ion absorbers
  whose adsorption is proportional to ion activity.
  Also, they are semiconductors.
• The potential across a ion specific membrane is
  translated into a usable potential by an Internal
  electrochemical half cell.

4
The pH Electrode Membrane

5
Internal Electrode

• Silver/silver chloride (Ag/AgCl) internal
  electrode
• AgCl e- Agmetal Cl-

6

• The chemical potential across a H electrode is
  described by a difference in Gibbs free energy
  inside and outside the electrode.
• H external gt H internal
• At equilibrium ?Go 0

7

• In electrochemical terms
• At 25 C0

8

• For H (and other monovalent ions)
• EH 0.059 (pHinternal - pHexternal)
• For a divalent ion n 2 and
• EH 0.0295log(M2)internal - log
  (M2)external

9
A Reference Electrode is needed to Complete the
Circuit

• Ag/AgCl Reference Electrodes
• AgCl e- Agmetal Cl-
• Voltage, E, is a function only of (Cl-) which can
  be controlled by using a high concentration of
  KCl.

10
Reference Electrodes
11
Overall Cell

• AgCl/Ag reference
• Glass electrode reference
• Ag(s) AgCl, HCl Test solution Ag(s) AgCl,
  KCl
• Phase Glass Liquid Junction
• Boundary Membrane
• Potential of the Cell responds to H activity.
• Measure pH by adjusting the meter to pH
  standards.

12
Error Analysis The precision is low with
electrodes

• The determination of (H) by measuring pH

13
Error Analysis (cont.)

• 0.1 unit error in pH is a 23 error in (H)
• For Cu2 electrode a 1 millivolt error is a 7.8
  relative error in (Cu2)

14
Comparison of 0.01 M CaCl2 and water pH
(Essington Fig 10.1)
15
Salt pH is lower that water pH

• K of Ca2 displace acidity from surface sites.
• Essington shows that for some Tennessee soils
  0.01 M CaCl2 pH in 11 soil to water is about 0.5
  units lower

16
Soil pH Measurement

• 11 H2O (SoilSolution) (US)
• I5 in Australia
• 15 for peats (US)
• 12 or 11 0.01M CaCl2
• For surface soils, this value is generally about
  0.3 to 0.5 units lower than pH(H2O).
• 11 1.0M KCl
• Lower than pH(H2O) and pH(CaCl2).

17
Importance of Acidity in Soil
18
Plant growth inhibited (species and variety
dependent)

• 1) H, inhibits growth of common bean (Phaseolus)
  below pH 5 and alfalfa below pH 6.5
• Not a problem for most plants at soil pH values.
• 2) In most soils for most acid sensitive crops
  Al3, not H-, is the toxic ion. It inhibits
  root growth by interfering with cell division in
  the meristen.

19

• 3) Mn2 (reduced Mn) is toxic to some plants.
  It is kinetically stable at low pH. (Will not
  oxidize rapidly to MnO2 under aerobic conditions
  at low pH).
• Reducing conditions are needed to produce Mn2.
• 4) Mn, Fe, Co, Ni, Cu, and Zn mobility and plant
  availability are increased at low pH.
• 5) Decreased mobility of anions. Important in Mo
  deficiency in low Mo soils.

20
Acid Rain

• In forested ecosystems interaction of acidic
  rainwater with soil is important in determining
  pH of lakes.
• Al3 in acid lakes can be toxic to fish.

21
Forms of Soil Acidity

• Active Acidity
• Titratable Al3 and H in soil solution. This is
  very small compared to the other types acidity.
• Reserve Acidity
• Total titratable, solid phase, acidity
• This the quantity of base neutralized to raise
  the pH to a given value.
• Includes exchangeable and and non exchangeable
  acidity.
• Total acidity active reserve

22

• Exchangeable Acidity - (1M KCl)
• i) Exchangeable Al3 on clay permanent charge
  sites
• Al3 3OH- Al (OH)3
• ii) A small fraction of the Al3 and H on
  organic matter is extracted by KCl.
• Predominantly Al3 in mineral soils and H in
  peats.

23

• ) Non-exchangeable acidity
• i) H and Al3 on organic matter not greatly
  displaced by 1M KCl.
• Organic acid groups
• RCOOH RCOO- H
• Organic bound Al
• (RCOO)3Al 3OH- 3RCOO- Al(OH)3
• ii) pH dependent charge sites on silicate clay
  edges and hydrous oxides of Fe and Al

24
Buffering in Soils (McBride Fig 5.10)
25

• Note 1/ slope of the titration plot of an acid
  soil with a strong base represents the buffer
  capacity

26
Review of reporting of CEC

• Ammonium Acetate pH 7.0
• Saturate exchange sites with NH4.
• Exchangeable bases Total acidity determined at
  pH 8.0 or 8.2
• Use BaCl2TEA at pH 8.0 or 8.2
• TEA is triethanolamine, a pH buffer
• Exchangeable bases KCl extractable acidity.
  This is effective CEC ECEC.

27
Base Saturation (correlates with pH)

• Effective CEC definition
• Exchangeable bases
• B.S. -----------------------------------
  ----------
• Exchangeable bases KCl extractable acidity
• Exchangeable bases
• B.S. ----------------------
• ECEC

28
Base Saturation (cont.)

• CEC at pH 7 or 8.2 (or 8.0)
• Exchangeable bases
• B.S. -----------------------------------------
  --------------------------
  Exchangeable bases Total acidity (pH 7.0 or
  8.2)
• Exchangeable bases
• B.S. -----------------------------------------
  --------------------------
  CEC at pH 7 or 8

29
In Class Exercise

• What is the BaCl2TEA acidity for soil B if the
  starting pH is 6.
• Use BaCl2TEA at pH 8.0 or 8.2

30
Answer

• Total acidity
• 13 - 6 cmolckg-1

31
At Low pH Al3 Becomes Very Important

• Al(OH)3 solubility("soil Al-hydroxide")
• Al(OH)3 3H Al3 3H log K 8.5
• log(Al3) 8.5 - 3pH
• At low pH Al3 also dissolves from
  aluminosilcates, especially clays

32
Hydrolysis increases the Al in solutions in
equilibrium with aluminous solids

• Hydrolysis
• Log K
• Al3 H2O AlOH2 H - 5.0
• AlOH2 H2O Al(OH)2 H - 4.9
• Al(OH)2 H2O Al(OH)3o H - 5.7
• Al(OH)3o H2O Al(OH)4- H - 7.4

33
Effect of hydrolysis on solubility

• log K
• Al(OH)3 3H Al3 3H2O 8.5
• Al3 H2O AlOH2 H -5.0
• --------------------------------------------------
  -----------
• Al(OH)3 2H AlOH2 2H2O 3.5

34
Effect of hydrolysis on solubility(cont.)

• log(AlOH2) 3.5 - 2 pH

35
Al solubility in equilibrium with Al(OH)3
(McBride Fig 5.3)
36
At low pH Al3 is an important exchangeable
cation (Fig 5.5)
37
Liming

• Net reaction of lime with an acid soil
• 2/3Al3-soil CaCO3 H2O
• Ca2 Soil 2/3Al(OH)3 CO2

38
Liming of Soils ( Fig 5.10)
39
Liming (cont)

• Buffer plot are really plots of B.S. (referenced
  to CEC pH 7.0 or 8.0) vs. pH.
• Buffer plots show soil pH is essentially a linear
  function of base saturation (not a log function).

40
Liming (cont)

• Recommendations for liming in agricultural
  production
• Lime to pH 6.5 if alfalfa is grown
• In highly weathered soils liming to 6.5 can
  induce micronutrient deficiencies.
• Lime to 1.5 times exchangeable Al (Southeast of
  US and Brazil)

41
In Class Exercise

• Using Fig 5.10 predict the quantity of lime
  needed to raise the pH of soils A and B to 6.5.
• Calculate in mg kg-1 and kg ha-1 assuming a 20 cm
  plow depth. (BD 1.3)

42
Answer

• Soil A 2 cmolc kg-1
• Soil B 7 cmolc kg-1
• 1 ha 10000 m2 Mass of 1 ha of soil
  (104)(.2)(103 )1.3) 2.6 x 106 kg
• 1 mol of CaCO3 100 g
• I mol of charge 50 g
• 1 cmolc 0.5 g 5 x 10-4 kg/cmolc
• Soil A 2 (2.6 x 106 kg)(5 x 10-4) 2600 kg
  2.6 T

43
Acidification of Soils

• Nitrification of NH4 (natural or from
  fertilization)
• NH4 202 NO3- 2H H2O
• But this acidity is partially neutralized if NO3-
  is taken up by plants because uptake releases one
  HCO3- for every NO3 taken up.

44
Natural acidification by carbonic acid

• CO2 produced by biological activity PCO2 in
  soils can be 100 times that in the atmosphere.
• log K
• CO2 H2O H2CO3 - 1.5
• H2CO3 H HCO3- - 6.3
• --------------------------------------------------
  ------------
  
• CO2 H2O H HCO3- - 7.8
• Important down about pH 5

45
Natural acidification (cont.)

• Soluble organic acids (stronger acids than CO2)
• RCOOH RCOO- H pK 4 - 5
• If an organic acid is oxidized by microbes there
  is no acidification.
• RCOOH CO2 H2O
• If an acid anion leaches out with a metallic
  cation in leaves H behind
• Can produce pH values as low as 3.5.

46
Acidification by pyrite and P fertilizer

• Pyrite (acid mine drainage) or acid sulfate
  soils.
• FeS2 7/2 H2O 15/16 O2 gt 4 H 2SO42-
  Fe(OH)3
• Can produce pH values less than 3
• Acid phosphate fertilizers
• Triple superphosphate Ca(H2PO4)2
• As it goes to hydroxyapatite, (Ca5OH(PO4)) ,
  it produces H ions.

47
Acidification by alum and S

• Alum KAl(SO4)2 (Added to acidify soils)
• KAl(SO4)2 3H2O Al(OH)3 3H 2SO42-
  K
• Elemental S (added to acidify soils)
• S 3/2 O2 H2O gt H2SO4
• Can produce pH values less than 3.

48
Acidification by acid precipitation

• Acidic precipitation
• HNO3 and H2SO4 at dilute concentrations
• (NH4)2SO4 - Neutralization product of sulfuric
  acid in the air with NH3
• Nitrification of NH4 in soil will make sulfuric
  acid.

49
Precipitation of Smectite or Vermiculite
Interlayer Al(OH)3

• Hydroxy interlayer vermiculite (HIV) or hydroxy
  interlayer smectite (HIS)
• In acid soils pH increases with depth
• In highly acid soils Al3 mobilized in the
  surface horizon can be precipitated in
  interlayers of smectite or vermiculite when pH
  reaches about 5.0 to 5.5.

50

• Hydroxyinterlayers block access to exchangeable
  ions. Can slow the reaction with lime.
• Because of hydroxyinterlayers Al3 toxicity can
  be worse in Ultisols than Oxisols.

51
Short Summary

• Ion specific electrodes are used to measure the
  activity of some ions including H.
• Ion specific electrodes utilize ion specific
  membranes to generate differences in protection
  with activity.
• Acidity in soils can be toxic to be to plants.
• Al3 is toxic to root growth.
• Most of the extractable acidity in acid soils is
  in the form of Al3.

52
Summary (cont.)

• Reserve acidity involves the exchangeable acidity
  and pH dependent charge sites.
• High SOM and high clay soil are more highly
  buffered.
• Soils acidification
• Carbonic acid
• Organic acids
• Oxidation of ammonium
• Other compounds added to soil.
• Acid rains can very slowly acidify soils