1. Soil Salinity/Sodicity/Alkalinity and Nutrients
Section E
SWES 316

4. Definition
Salt-Affected Soil: Any soil containing sufficient quantities of soluble salts or sodium to cause adverse effects to plants or soil
Saline
Sodic
Saline-Sodic

5. Definition Alkaline Soil: A soil with pH >7.0.
Commonly, soil alkalinity is found in areas with limited soil weathering.
Moderately alkaline pH Often but not always associated with the presence of CaCO3 in soils (calcareous)
Highly alkaline pH >8. Often associated with the presence of excess exchangeable Na in soils.

6. Soil Salinity
Soils in arid regions commonly have “excessive” concentrations of soluble salts because:
Lack of leaching to remove salts
Poor Drainage
Salts added in irrigation water

7. Soluble Salts Common soluble cations found in saline soils:
Mg2+
Na+ K+
Common soluble anions found in saline soils:
Cl-
SO42-
HCO3- /CO32-
NO3-

8. Saline Soils
Definition: Have an electrical conductivity of a saturated paste extract (ECe) of >4 dS/m and an exchangeable sodium percentage (ESP) of <15%.
So, to classify a soil as saline, the EC, exchangeable Na, and CEC must be known.
Note: these numbers are somewhat arbitrary.

9. Measurements
EC is measured in a saturated paste extract. The soil is saturated, extracted and the EC is measured with a conductivity meter.

12. Properties of Saline Soils
Saline soils typically:
Are well-aggregated (salts flocculate clays)
Have a pH from 7 to 8 (usually occur in areas of limited soil weathering)
Are often calcareous (contain calcium carbonate)

13. Soil clay particles can be unattached to one another (dispersed) or clumped together (flocculated) in aggregates. Soil aggregates are cemented clusters of sand, silt, and clay particles.
Dispersed Particles
Flocculated Particles

14. Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.
In A horizons, where organic matter levels are high and there is a lot of biological activity (earthworms, ants, termites, microbes, etc.) particles tend to be arranged in small, round aggregates or granules.
This type of structure is common in the surface horizons of many forest and prairie soils

15. In all but the sandiest soils, dispersed clays plug soil pores and impede water infiltration and soil drainage.
In A horizons, where organic matter levels are high and there is a lot of biological activity (earthworms, ants, termites, microbes, etc.) particles tend to be arranged in small, round aggregates or granules.
This type of structure is common in the surface horizons of many forest and prairie soils

16. Most clay particles have a negative electrical charge
Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another.
Negatively charged clay particle
Negatively charged clay particle
Here is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) contains dissolved cations and anions. The concentration of cations is much greater close to the particle surface than in the bulk soil solution. The cations are not bonded to the clay, but just attracted to the surface.
Conversely anions are repelled by negatively charged clays, so the concentration of anions is greater in the bulk soil solution than close to a clay particle.

17. Common soil cations include sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+). Cations can make clay particles stick together (flocculate). +
Here is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) contains dissolved cations and anions. The concentration of cations is much greater close to the particle surface than in the bulk soil solution. The cations are not bonded to the clay, but just attracted to the surface.
Conversely anions are repelled by negatively charged clays, so the concentration of anions is greater in the bulk soil solution than close to a clay particle.
Negatively charged clay particle
Negatively charged clay particle

18. Relative Flocculating Power
Flocculating Cations
We can divide cations into two categories
Poor flocculators
Sodium
Good flocculators
Calcium
Magnesium
Ion
Relative Flocculating Power
Sodium
Na+
1.0
Potassium K+
1.7
Magnesium
Mg2+
27.0
Calcium
Ca2+
43.0
Sumner and Naidu, 1998

19. Flocculating Power of Cations
Cations in water attract water molecules because of their charge, and become hydrated.
Water molecule is polar: (+) on one end, (-) on the other end
(+)
(-)
Hydrated cation +
Cations with a single charge and large hydrated radii are the poorest flocculators.
Cation
Charges per molecule
Hydrated radius (nm)
Relative flocculating power
Sodium 1
0.79
1.0
Potassium
0.53
1.7
Magnesium 2
1.08
27.0
Calcium
0.96
43.0

20. Dispersion Clay with only exchangeable Ca2+,
clay particles can approach closely,
promoting flocculation
Clay with exchangeable Ca2+ and Na+,
clay particles cannot approach closely,
causing dispersion
Ca2+
Na+

21. Sodium Adsorption Ratio
The ratio of ‘bad’ to ‘good’ flocculators gives an indication of the relative status of these cations: +
Na+ ++
Ca2+ and Mg2+
Mathematically, this is expressed as the ‘sodium adsorption ratio’ or SAR:
where concentrations are expressed in mmoles/L
SAR =
[Na+]
[Ca2+] + [Mg2+]

22. Exchangeable Sodium Percentage
AN alternative to SAR is ESP, Exchangeable Sodium Percentage + + ++
Na+ ++ - + - - - - - - ++ - - ++ +
Ca2+ and Mg2+ ++ + ++
Mathematically, this is expressed as the percentage of the CEC (cation exchange capacity) that is filled with sodium in units of charge per mass (cmol(+)/kg)
Na+
ESP =
Cation Exchange Capacity
SAR and ESP are approximately equal numerically

23. Electrical Conductivity
Ions in solution conduct electricity, so the total amount of soluble soil ions can be estimated by measuring the electrical conductivity (EC) of a soil water extract.
EC is measured in units of conductance over a known distance:
deci-Siemens per meter or dS/m
Soil with a high EC is salty; soil with a low EC is not.

24. Aggregate stability (dispersion and flocculation) depends on the balance (SAR) between (Ca2+ and Mg2+) and Na+ as well as the amount of soluble salts (EC) in the soil.
Na+
Ca2+ and Mg2+ + + + ++ ++ ++ + + + + ++ ++ ++ ++
SAR EC
Lower EC
Higher EC
Flocculated soil
Dispersed soil

25. Soil particles will flocculate if concentrations of (Ca2+ + Mg2+) are increased relative to the concentration of Na+ (SAR is decreased).
Na+ +
Ca2+ and Mg2+
SAR ++ EC
Flocculated soil
Dispersed soil

26. Soil particles will disperse if concentrations of (Ca2+ + Mg2+) are decreased relative to the concentration of Na+ (SAR is increased).
Ca2+ and Mg2+ ++ ++ ++
Na+
SAR + + EC + + +
Flocculated soil
Dispersed soil

27. Soil particles will flocculate if the amount of soluble salts in the soil is increased (increased EC), even if there is a lot of sodium.
Na+
SAR EC
Ca2+ and Mg2+
Lower EC
Higher EC + ++
Flocculated soil
Dispersed soil

28. Soil particles may disperse if the amount of soluble salts in the soil is decreased (i.e. if EC is decreased).
Ca2+ and Mg2+ ++ ++ ++
Na+ EC
SAR + + +
Lower EC
Higher EC
Flocculated soil
Dispersed soil

29. If soils are close to the “tipping point” between flocculation and dispersion, the quality of irrigation water will influence aggregate stability. If irrigation water infiltrates, and rain water does not, this indicates that the soil is close to the “tipping point”.
Na+ +
If soils are irrigated with clean water (with low EC), soil EC will decrease, which can destabilize aggregates. Irrigation water will infiltrate slowly. + + + + + +
Ca2+ and Mg2+
SAR ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ EC
Lower EC
Higher EC
Ca2+ and Mg2+
Flocculated soil ++ ++ ++
Na+
SAR + EC +
Soils irrigated with saline water (with high EC) will generally have good structure, and water will infiltrate rapidly. However, salts can accumulate and damage plants unless properly managed. +
Lower EC
Higher EC
Dispersed soil

30. Soil Classification EC SAR Condition Normal <4 <13 Flocculated
Soils can be classified by the amount of soluble salts (EC) and sodium status (SAR). This classification can tell us something about soil structure.
Soil Classification EC
SAR
Condition
Normal
<4
<13
Flocculated
Saline
>4
Sodic
>13
Dispersed
Saline-Sodic

31. Sodic Soils Are adversely affected by the presence of excess Na
Definition: Have an electrical conductivity of a saturated paste extract (ECe) of <4 dS/m and an exchangeable sodium percentage (ESP) of >15%.
So, to classify a soil as sodic, the EC, exchangeable Na, and CEC must be known.
Note: these numbers are somewhat arbitrary.

32. Properties of Sodic Soils
Sodic soils typically:
Are poorly-aggregated (sodium disperses clays)
Have slow rates of water infiltration
Have a pH of 8 or above . This is due to the presence of soluble Na2CO3.

33. Saline-Sodic Soils
Definition: Have an electrical conductivity of a saturated paste extract (ECe) of >4 dS/m and an exchangeable sodium percentage (ESP) of >15%.
So, to classify a soil as saline-sodic, the EC, exchangeable Na, and CEC must be known.

34. Salts Affect Soil Structure
Based on irrigation water analysis
Sodium level (SAR)
Poor soil
structure
Good soil
structure
This is an illustration of the ability of sodium to destabilize soil. On the upper and left side of the graph, there is lots of sodium (measured by sodium adsorption ratio) but not enough total salts (represented by electrical conductivity) to keep soil particles aggregated. At the lower right, there are lower levels of sodium and more total salts, so soil structure is stable. Soil aggregates are stable if sodium levels are low and/or if total salt levels are high.

35. Salt and Sodium Risks Salinity is mostly harmful to plant growth.
Most plants, especially crop plants, are sensitive to salts. The properties of the soils themselves can be improved by the presence of salts (flocculation).
Sodium is harmful to plants and soils
Sodium causes soils to have undesirable physical and chemical properties. Sodium can also cause toxicities to plants.
Alkaline pH (esp. in sodic soils) can limit nutrient availability to plants.

36. Salt Effects on Plants
Excess soluble salts can be harmful to plant growth because:
Salts lower the osmotic potential energy of soil water. Water is less available to plants.
Some soluble salt ions can have specific toxic effects on plants, such as:
Na+, Cl- , H3BO3

39. Soil Salinity and Nutrients
Some specific effects of salinity on nutrients:
High Na concentrations can inhibit Ca, Mg uptake by roots.
Ion toxicity limits nutrient uptake, lowering nutrient requirements.
High HCO3- can limit Ca availability.

41. Soil Alkalinity and Nutrients
Soil pH >7.5
Alkalinity is specifically associated with:
Sodic soils
Calcareous soils
Soils high in soluble carbonates
Saline soils may or may not be alkaline.

42. Soil Alkalinity and Nutrients
Specific Effects:
pH dependent AEC decreases, and CEC increases as pH increases.
Nintrogen: NH3 volatilization increases as pH increases.
Phosphorus: P availability decreases at pH>6 due to Ca-P reactions.
Fe, Mn, Cu, Zn: solubility decreases x for every 1 pH unit increase.
B: availability decreases at pH >7.

43. Treatment for Saline Soils
Amendments for removing salts from soils:
Nothing
Management Practices
Adequate Leaching
Maintain soil drainage through proper tillage

45. Soil Amendments for Salinity and Sodium Control
Soil amendments will not help with salinity control unless a sodium problem also exists.
Amendment additions are necessary to correct sodium problems. Leaching alone is not enough.

46. Should Alkaline Soils be Acidified?
It is rarely advisable to acidify soils to significantly lower pH:
Amounts required may be enormous:
A soil with 2% CaCO3 in the top 30 cm will contain kg CaCO3/ha. This would require about 93 tons H2SO4/ha to neutralize the CaCO3 .
There is rarely an economic benefit to such large application rates.

47. So, what to do about alkaline soils?
If soils are sodic and highly alkaline, use of gypsum and leaching will usually lower pH to <8.4.
When pH is <8.4, micronutrient deficiencies in most crops are rare and manageable with foliar applications.

49. Soil Amendments (1) Gypsum (CaSO4.2H2O)
the amendment most commonly used for controlling sodium problems. Can be soil-applied or water-run.
Gypsum application rates for removing sodium are commonly 1 to 10 tons/acre, depending on soil and irrigation water properties.
Gypsum will normally lower the pH of sodic soils, by replacing exchangeable Na+ and allowing Na2CO3 to be leached from soils.

50. Increasing soluble calcium improves aggregate stability in soils with poor structure.
Gypsum
Na+
CaSO4 +
Ca2+
SO42-
SAR ++ ++ EC ++ ++ ++ ++ ++ ++ ++ ++
Flocculated soil
Dispersed soil

51. Apply gypsum before leaching salts out of soils susceptible to dispersion (the amount of gypsum needed can be determined by a soil test). Replacing sodium with calcium before leaching will stabilize soil structure.
Ca2+
SO42-
Ca++
Ca++ - - - - - - - - -
Ca++
Ca++
Na+
Na+
Na+ -
Here is a schematic representation of sodic soil reclamation. - - -
Na+ - - - - -
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+

52. Gypsum Application

53. Soil Amendments (2) Sulfuric Acid (H2SO4)
In soils with free lime (calcareous), sulfuric acid is an effective amendment for correcting or preventing sodium problems: CaCO3 + H2SO4 --> Ca2+ + SO42- + H2O + CO2
Can be applied to soil or water-run
Rates are commonly 1-3 tons/acre

54. Sulfuric acid* can be used instead of gypsum on calcareous (CaCO3 containing) soil only.
Sulfuric acid dissolves calcium carbonate in the soil
and makes gypsum!
*Sulfuric acid is extremely dangerous and should only be handled by trained personnel.

55. Constant H2SO4 injection
keeps water pH low and
prevents formation of CaCO3
in the drip lines, and also
dissolves some CaCO3 in
the soil, helping to maintain
high exchangeable Ca2+ and
low exchangeable Na+.

56. Soil Amendments (3) Elemental Sulfur 97% Sulfur Reaction:
Effective acid-forming amendment: soil microorganisms use S to produce sulfuric acid. The sulfuric acid reacts with CaCO3 to release Ca.
Requires microbial activity to react. May take months to react completely.
Reaction:
2 S + 3 O2 + 2 H2O  2 H2SO4

57. Soil microbes convert sulfur into sulfuric acid
Elemental sulfur can also be used as an alternative to gypsum on calcareous soils
Soil microbes convert sulfur into sulfuric acid
H2SO4 dissolves calcium carbonate and makes gypsum
Conversion to sulfuric acid takes time
several weeks
faster in warm soils

58. Soil Amendments (4)
Nitro-Sul (Ammonium Polysulfide) 20% NH4-N, 40-45% sulfur
Causes release of acidity after microbial oxidation
In some, but not all, cases applications increase rate of water infiltration
Relatively expensive as a source of N or S

59. Soil Amendments (5) Thio-Sul (Ammonium Thiosulfate) 12% NH4-N, 26% S
Releases small amounts of acidity. Is used mostly as a fertilizer.
Remember--a need for S is rare in Arizona soils

60. Soil Amendments (6) N-Phuric (urea + sulfuric acid) 10-28% N, 9-18% S
Safer way to use sulfuric acid
Releases acidity
Good for drip or microsprinkler systems