1. Soil Structure: The Roles of Sodium and Salts
Dr. Jim Walworth
Department of Soil, Water and Environmental Science
University of Arizona
AZ 1414
July 2006

2. 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

3. 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

4. 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

5. 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.

6. A cation is a positively charged molecule
A cation is a positively charged molecule. 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

7. 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

8. 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

9. 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+]

10. 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.

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Observe your soil - sodic soils often crack when dry

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

20. 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.
Na+ -
Ca++
Ca2+
SO42-
Here is a schematic representation of sodic soil reclamation.

21. 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.

22. 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

23. Manage soil structure
Be aware of the quality of irrigation water. Water with high levels of sodium (high SAR) will tend to destabilize soil.
Have irrigation water analyzed for SAR and EC or ask your water provider for analyses.
If you have high sodium irrigation water, the water and/or the soil may need amendments such as gypsum or sulfuric acid.
Observe your soil.
If water infiltrates very slowly, or if rain water infiltrates more slowly than irrigation water, the soil may have a sodium problem.
Sodium impacted soils may noticeably crack when dry.
Analyze your soil.
Laboratory analysis can tell you the soil EC and SAR or ESP.

24. cals.arizona.edu/pubs/crops/az1414
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona.
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