Soil Structure: The Roles of Sodium and Salts

The University of Arizona Cooperative Extension

Soil Structure: The Roles of Sodium and Salts

Dr. Jim WalworthDepartment of Soil, Water and Environmental Science

University of Arizona

AZ 1414Revised 10/2011


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


Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.


In all but the sandiest soils, excess sodium causes clays to disperse. Dispersed clay plugs soil pores and impedes water infiltration and soil drainage.


Negatively charged clayparticle


Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another.




+

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).


If we add the right cations to a dispersed soil

the soil will flocculate and form aggregates


Flocculating Cations

• We can divide cations into two categories– Weak flocculators

• Sodium– Strong 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


Flocculating Power of CationsCations in water attract water molecules because of their charge, and become

hydrated.

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 1 0.53 1.7

Magnesium 2 1.08 27.0

Calcium 2 0.96 43.0

Water molecule is polar: (+) on one end, (-) on the other end

(+)

(-)

(+) Hydrated cation surrounded by water molecules

+


Sodium Adsorption Ratio

The ratio of ‘weak’ to ‘strong’ 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+]


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 (cmolc/kg)

ESP = Na+

Cation Exchange Capacity

- - ---

- -- -

+++

+

++

++

++

++ ++

SAR and ESP are approximately equal numerically


Measuring Soil SaltsIons in solution conduct electricity, so the total amount of

or soluble soil ions (salts) 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

High EC = salty soil Low EC = non-salty soil

Soil salts can also be expressed as Total Dissolved Salts (TDS) in ppm

TDS = EC (dS/m) x 640+

-+-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

+

-

---

-

+

-

-

-+

+


Ca2+ and Mg2+Na+

SAR

EC

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.

Flocculated soil

Dispersed soil

++++++

++++

++

+ + +++++

+

+

Lower EC Higher EC


Na+

SAR

EC

Soil particles will flocculate if concentrations of (Ca2+ + Mg2+) are increased relative to the concentration of Na+ (SAR is decreased).

Flocculated soil

Dispersed soil

+

++

Ca2+ and Mg2+

++

++++

++++

++++

++++

++


Na+

SAR

EC

Flocculated soil

Dispersed soil

++

+

Ca2+ and Mg2+

++++++

Soil particles will disperse if concentrations of (Ca2+ + Mg2+) are decreased relative to the concentration of Na+ (SAR is increased).

++

+

+


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.

Flocculated soil

Dispersed soil

Na+

SAR

EC

Ca2+ and Mg2+

Lower EC Higher EC

++

++

+

+

+

++

++++

++++

++++

++++

++


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+

SAR

EC

Lower EC

Flocculated soil

Dispersed soil

Higher 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.

Ca2+ and Mg2+

Na+

SAR

EC

Lower EC

Dispersed soil

Higher EC

++++

++

+

++

Higher EC

Flocculated soil

Na+

SAR

EC

Ca2+ and Mg2+

Lower EC

++

++

+

+

+

++++

++ ++++

++++

++++

++

++++

++

If soils are irrigated with clean water (with low EC), soil EC will decrease, which can

destabilize aggregates. Irrigation water will infiltrate slowly.

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”.


Soil Classification EC SAR ConditionNormal <4 <13 Flocculated

Saline >4 <13 Flocculated

Sodic <4 >13 Dispersed

Saline-Sodic >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.


Observe your soil - sodic soils absorb water slowly and often crack when dry


EC

Controlling Na+ requires increasing concentrations of soluble Ca2+ to decrease soil or water SAR, thus favoring soil flocculation.

Flocculated soil

Dispersed soil

Na+

Ca2+ and Mg2+

SAR

++

+

++ ++++

++++

++++

++++

++


Na+

SAR

EC

Increasing soluble calcium improves aggregate stability in soils with poor structure.

Flocculated soil

Dispersed soil

+

++

Ca2+

++

++++

++++++++

++++

++

GypsumCaSO4

SO42-


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+

Na+

Na+

Na+

Na+

Na+

- - ---- -- -

Ca++ Ca++

Ca++Ca++

Ca2+ SO42-

- - ---- -- -

Na+Na+

Na+

Na+

Na+ Na+

Na+

Na+

Na+

Na+


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!

H SO CaCO CO H O CaSO2 4 3 2 2 4

*Sulfuric acid is extremely dangerous and should only be handled by trained personnel.


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

S O CO H O H SO CH O ½ 2 2 2 2 4 22


Alternatives to Gypsum:Sulfur burners

S + O2 → SO2

SO2 + H2O → H2SO3 (sulfurous acid)

Sulfur burners eliminate the need for handling dangerous sulfuric acid


Relative effectiveness of amendments for supplying calcium

Amendment Chemical composition

Solubility in water (lbs/100 gal)

Amount (lbs) equivalent to 1 lb of gypsum

Gypsum CaSO4.2H2O 2.0 1.0

Calcium chloride CaCl2.2H2O 810 0.9

Sulfuric acid H2SO4 Very high 0.6

Sulfur S 0 0.2

Ammonium thiosulfate (NH4)2S2O3 850 0.5 - 1.4

Potassium thiosulfate K2S2O3 1290 1.1

Aluminum sulfate Al2(SO4)3.18H2O 725 1.3


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.


Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Kirk A. Astroth, Interim Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona.The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

cals.arizona.edu/pubs/crops/az1414

Soil Structure: The Roles of Sodium and Salts

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