Chapter 8: Acidic Soils and SaltAffected Soils

1
Chapter 8 Acidic Soils and Salt-Affected
Soils
Homework 1, 2, 3, 7, 9, 11, 12 Due 26 Oct 2007
2
Why we care about acidity 1.The availability
of many nutrients is strongly affected by acidity
(Fig 8-7) 2.Al3 is toxic to many plant roots
and fish and impedes Ca, K, or Mg uptake by
plants
3
Review cation exchange capacity, pH, and base
saturation
Base Cation Saturation Percentage (BCSP) (often
stated as simply base saturation) BCSPis defined
as the sum of exchangeable base cations (Ca2,
Mg2, K, and Na) divided by CEC. It is usually
expressed as a percentage of CEC thus BCSP ()
(or BS) Ca Mg K Na CEC x100
4
(No Transcript)
5

• II. Soil Acidity, Base Saturation
• As noted earlier, exchangeable H on clay
  minerals is short-lived, and is rapidly replaced
  by Al3 from within octahedral layers.
• Active acidity is defined as the acidity
  immediately released into soil solution, that is,
  soil pH.
• Potential acidity is defined as the total
  acidity of the soil which includes all H
  pH-dependent CEC sites.

6

• II. Soil Acidity, Base Saturation
• Potential acidity can be determined by titrating
  the soil with base (e.g., NaOH).
• Potential acidity buffers active acidity (i.e.,
  "tries" to maintain constant pH), and must be
  taken into account when trying to raise soil pH
  by liming.
• Potential acidity is nearly always much greater
  than active acidity, and constitutes the great
  proportion of total soil acidity.

7

• Total acidity on solid phase gt 10,000 x that in
  soil solution

8
II. Soil Acidity, Base Saturation Active
acidity pH Potential, acidity exchangeable
Al3, H, adsorbed SO42- Total acidity
9

• II. Soil Acidity, Base Saturation
• Very important note on soil pH measurement
  methods!
• pH in water in most non-tropical soils is higher
  than pH in salt solution (e.g., 0.01 M CaCl2).
• This is because the cation in the salt solution
  (e.g., Ca2 in CaCl2) displaces a certain amount
  of all cations on the exchanger, including the
  acid cations, H and Al3.
• Thus, salt pH is almost always lower than water
  pH, and care must be taken to keep methods
  constant if measurements of different soils or
  soils at different times are to be compared.

10

• II. Soil Acidity, Base Saturation
• Aluminum
• Aluminum is a weak acid. Al3 takes on waters of
  hydration and then dissociates them to become
  divalent and monovalent, depending on pH.
• At very high pH, it can even become an anion.
• See Insight 1.

11
Al3 is a weak acid and combines with water to
form various ions depending on pH pH lt 4.5
pH 4.5-6.5 (mostly monovalent
form) Al(H2O)63 Al(H2O)5(OH)2 H
Al(H2O)4(OH)2 H pH 6.5-8 (gibbsite)
pH 8-11
Al(H2O)3(OH)30 H
Al(H2O)2(OH)4-
12
(No Transcript)
13

• Relationships among CEC, active acidity,
  potential acidity and base saturation.
• The greater the CEC, the greater the potential
  for the soil to retain both base and acid
  cations.
• Thus, it is possible for soil A in the following
  figure to have lower base saturation than soil B,
  yet soil A actually has a greater reserve of
  exchangeable base cations.

14
5 meq/100 g BSCP 25

• Soil A has a greater total amount of base
  cations, but lower BSCP and is more acidic
  because it has higher CEC (mostly organic CEC)
• Which soil has the greatest active acidity, given
  what is shown?
• Which has the greater potential acidity?

CEC 20 meq/100 g
4.0 meq/100 g BSCP 80
CEC 5 meq/100 g
H , Al

Soil B
Soil A
Organic matter 0.8
Organic matter 4
15

• Why some soils are acidic
• Leaching by carbonic acid
• Carbonic acid forms when water enters the soil,
  whose
• atmosphere typically has 100 to 500 times greater
  CO2 concentrations than the atmosphere.
• CO2 H2O ---------gt H2CO3 ---------gt HCO3-
  H
• Carbon dioxide water ---gtcarbonic
  acid------------gt bicarbonate hydrogen ion

16

• Why some soils are acidic
• Leaching by carbonic acid
• The hydrogen ion produced from carbonic acid then
• displaces an exchangeable base cation
• X K H ----------gt X H K
• Where x exchange site.
• The example shows K, but could be any cation.

17

• Why some soils are acidic
• Leaching by carbonic acid
• The overall reaction is, then
• CO2 H2O X K --------gt X H K HCO3-
  
• A bicarbonate salt leaches and the soil is
  acidified.
• In extremely acidic soils, the carbonic acid
  system does not function because carbonic acid is
  a weak acid and does not dissociate below pH 4.5.

18
Why some soils are acidic 2.
Nitrification Nitrification of NH4 from
fertilizers (or from any other source) to nitric
acid by nitrification 2NH4 4O2 --------gt
2NO3- 4H 2H2O Nitrosomonas and
Nitrobacter
19
Why some soils are acidic 3. Oxidation of
elemental S 2S 3O2 2H2O -------gt 2H
SO42- Thiobacillus thiooxidans
20
Why some soils are acidic 4. Excretion of H
from plant roots in exchange for cations
21

• Why some soils are acidic
• 4. Excretion of H from plant roots in exchange
  for cations
• The net acidifying effect is equal to the uptake
  of cations minus anions
• Acidification (Ca2 Mg2 K NH4) -
  (H2PO4-, SO42- , NO3- ).
• Since N is taken up in greatest quantities and
  can be taken up as either a cation or an anion,
  the form of N uptake is usually the most
  important factor in acidification by plant
  uptake.
• Uptake is very acidifying in some species with
  high Ca concentrations in their tissues (Aspen,
  oaks as opposed to pines)

22

• Why some soils are acidic
• 5. Acid rain
• H2SO4 and or HNO3 from precipitation.
• Also, ammonium deposition is acidifying because
  it is either taken up or nitrified.
• More on acid rain later in the course.

23
(No Transcript)
24
Reasons some soils become acidic - not in the
book Organic matter As noted before, organic
matter is acidifying, just as a weak organic acid
is. The same mechanism by which organic matter
can provide pH-dependent CEC sites also makes it
acidifying, because it releases a proton. This
is a major contributor to soil acidity in some
forest soils
25
Liming Why apply lime? See nutrient
relationships to acidity, Fig. 8-7 Note do not
apply too much lime or lime to already alkaline
soils, or you will tie up P, Fe, Cu, Zn, Mn, and
mobilize B
26
(No Transcript)
27
8.3 Composition of Lime 1. Calcic limestone
(CaCO3) ground fine 2. Dolomitic limestone
CaMg(CO3)2. Runs about 50 Ca, 34 Mg. 3.
Quicklime (CaO) burned limestone (lost a CO2) 4.
Hydrated quicklime (Ca(OH)2)) quicklime hydrated
with water 5. Marl CaCO3 from freshwater
ponds. 6. Chalk CaCO3 from ocean deposits. 7.
Blast furnace slag (CaSiO3, CaSiO4). Also
contains P. 8. Oystershells, woodash, etc. 9.
Fluid lime suspension
28
Gypsum (CaSO4) is also added to acid soils to tie
up Al into aluminosulfate minerals. It does not
raise pH much.
29
Chemical Guarantees of Lime Ways of
expressing this 1. Calcium carbonate
equivalent Equals 100 if pure. What is the
calcium carbonate equivalent of 100 g of CaO? CaO
weighs 401656 g/mole, CaCO3 weighs
4012(3x16) 100. The reactions are CaO H2O
2H ------gt Ca2 2 H2O CaCO3 CO2 H2O
2H -------gt Ca2 H2CO3
30

• Chemical Guarantees of Lime
• Thus, both CaO and CaCO3 are equally effective
  in consuming H and the calcium carbonate
  equivalent of 100 g of CaO equals 100/56 179.
• In short, you have to carry less weight when you
  use CaO for the same effect.
• Similar calculations can be made for Ca
  equivalent.

31

• Physical guarantees of lime
• The finer the texture, the more quickly lime
  dissolves.
• Limestone in coarse particles can stay intact for
  decades.

32
Reactions of Lime with Soils Crop plants and
some forest plants grow poorly on acidic soils
because of 1. Al toxicity (most common
reason) 2. Reduced micro-organism activity
(especially bacteria) 3. Mn toxicity 4. Fe
toxicity 5. Ca deficiency (often exascerbated by
Al impeding Ca uptake) 6. Mg deficiency (often
exascerbated by Al impeding Mg uptake)
33

• Reactions of Lime with Soils
• Crop plants and some forest plants grow poorly on
  acidic soils because of
• 7. Mo deficiency (especially for legumes and
  cabbage family)
• 8. N, P, or S deficiency (see Figure 8-6 on p.
  256).
• Lime is added to raise pH of soils, not to
  fertilize with Ca
• It does so by displacing exchangeable H and Aln
  by mass action

34
Not in the book but may be useful the lime
potential. A way of getting need for lime or
acidifying potential of waters with very simple
pH measurement in CaCl2
35

• Acidifying Soils
• On occasion, it may be necessary to acidify
  soils. Some plants (azeleas, spruce trees,
  rhododendrons) like acidic soils.
• This is commonly done by adding elemental S (see
  reactions above).
• Acidification also takes place by N fertilization
  (see above).

36

• Reactions of Gypsum with Soils (not in chapter
  8)
• Gypsum is often used to reclaim sodic soils
  (Chapter 11). Gypsum is also used on occasion to
  remove Al toxicity in acidic soils.
• It does so by causing aluminosulfate minerals
  (jurbanite, alunite, basaluminite) without
  raising pH by much. For example
• 2XAl3 3CaSO4.2H2O 6H2O --------gt
• 3XCa2 2H SO42- 2AlOHSO4.5H2O
  (jurbanite).

37
Reactions of Gypsum with Soils (not in chapter
8) Then the 2H may exchange for Ca2
2H X Ca2 ----------gt 2X H Ca2
Sulfate itself can do this by forming
alunite 3XAl3 2SO42- K 6H2O --------gt
6XH KAl3(OH)6(SO4)2 (alunite).
38
Reactions of Gypsum with Soils (not in chapter
8) Thus, Al is removed and the soil is
still acid. You can see a natural example of
this in the hydrothermally altered soils at
Steamboat and Peavine
39

• Salt-affected soils
• Soluble salts
• Those more soluble than gypsum (CaSO4), 2.4 g L-1
  at 0oC.
• Common anions Cl-, SO42-, HCO3- (sometimes
  CO32-, NO3-)
• Common cations Na, Ca2, Mg2 (sometimes K,
  NH4)
• NaCl is 150 times more soluble

40

• Salt-affected soils
• Soluble salts
• Measurement electrical conductivity, paste or
  water
• Reclamation
• 1) leach away
• 2) remove Na from sodic soils

41

• Salt-affected soils
• Excess salts negatively affect plant growth by
  creating large osmotic potential - may cause
  exo-osmosis pulling water from plants.
• Visual indicators
• Summer snow surface white accumulation
• Slick spots sodic (pH 9)

42

• Measuring soluble salts
• Conductance p. 245
• Approximate conversions to molarity, TDS, osmotic
  potential

43

• Saline and sodic soils
• Saline high salts, high osmotic potential hard
  for plants to take up water
• Sodic high exchangeable sodium percentage (ESP)
  (gt15).
• Saline-Sodic have both properties

44

• Saline and sodic soils
• Exchangeable Sodium Percentage (ESP) a measure
  of sodicity from the soil exchange phase
• Defined in eq. 8-1, p.247 (units are centimoles
  of charge per kilogram)

45
Saline and sodic soils Sodium Activity Ratio
(SAR) A measure of sodicity from solution
values. If units are in moles L-1
46
SAR When SAR gt 13, permeability is greatly
reduced because Na causes clays to repel and
line up, greatly reducing infiltration and
porosity (structure).
SAR gt13 Clays repel each other and line up Poor
aeration, infiltration Sodic soils
SAR lt13 Clays attract each other and stack on
end Good aeration, infiltration
47
(No Transcript)
48
SAR of 13 equals ESP of 15 by this empirical
relationship
49
Not in the book SAR is derived from the Gapon
equation Q XM2 (Na)
XNa (M2)1/2 Where X exchanger, M2
(Ca2 Mg2) Thus, if Ca2 and Mg2 dominate the
exchanger, then SAR (Q)( XNa/XM2) (Q
)(ESP)
50
(No Transcript)
51
Saline and sodic soils Definitions
8-5
52

• Salt balance exists when salt input salt
  output
• Irrigation salt rainfall salt - salt removed in
  drainage 0
• (Di)(Ci) (Dr)(Cr) - (Dd)(Cd) 0
• or (Da)(Ca) (Dd)(Cd)
• Dd/Da Ca/Cd ECa/ECd Lr
• D depth of water (cm)
• C concentration
• iirrigation
• rrain
• ddrainage
• a added in irrigation plus rain.
• Lr leaching requirement

53
Salt balance exists when salt input salt
output Leaching requirement (Lr) "the
minimum leaching fraction that the crop can
tolerate without yield reduction" Effectively,
when salt balance occurs, and that can be
measured by electrical conductivity, and it is
approximated as the ratio of EC in applied water
to that in drainage water.
54
Equation 9-5, page 266
Thus, the leaching fraction (L) can be defined as
the ratio of electrical conductivities of the
applied water to that of the drainage water. L is
the amount of additional water (in addition to
that needed to wet the soil) that must be added
for leaching to maintain salt balance For
example, if L 0.2 and 3.2 cm of irrigation is
planned, then 0.2 x 3.2 0.64 cm of additional
water must be applied to maintain salt balance
55
(No Transcript)
56

• Reclaiming salty soils
• Three basic methods
• Establish internal drainage maybe tiles allow
  water to flow out.
• Replace exchangeable Na (sodic soils)
• Leach out soluble salts

57

• Reclaiming salty soils
• Establish internal drainage maybe tiles allow
  water to flow out. But, if you are in a low spot,
  you have troubles

58

• Reclaiming salty soils
• 2. Replace exchangeable Na (sodic soils)
• Gypsum (CaSO4) or elemental S
• Gypsum adds Ca to exchanger, and soils flocculate
• Elemental S oxidizes to sulfuric acid and has
  same effect
• S 1.5O2 H2O--gt H2SO4 ? 2H SO42-
• H either releases Ca from CaCO3
• Or exchanges for Na
• XNa H ? XH Na

59
8-7
60

• Reclaiming salty soils
• 3. Leach out soluble salts
• Note must use lower salinity water!
• See Leaching requirement above

61

• Managing Salty Soils
• Water Control
• Simply add adequate water.
• However, this could result in higher water table
  and salt problems from when it evaporates

62
(No Transcript)
63
Managing Salty Soils Planting position
Plant crops away from where salt accumulated Fig
8-17
8-17
64

• Managing Salty Soils
• Plant Salt tolerant crops
• What makes plants salt-tolerant?
• Exclusion
• High NaCl in shoots as organics
• Excretion (halophytes)

65

• Monitoring salts in the field
• Vacuum Extractors remove soil water at near FC
• In-place sensors electrodes
• Electomagnetic induction (EMI) measures
  conductivity without touching soils.