Soil Salinity/Sodicity/Alkalinity and Nutrients

Section ESWES 316

Definition

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

SalineSodicSaline-Sodic

Definition

Alkaline Soil: A soil with pH >7.0. Commonly, soil alkalinity is found in areas with limited soil weathering.Moderately alkaline pH 7-8. 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.

Soil Salinity

Soils in arid regions commonly have “excessive” concentrations of soluble salts because:

Lack of leaching to remove saltsPoor Drainage Salts added in irrigation water

Soluble Salts

Common soluble cations found in saline soils:

Ca2+

Mg2+

Na+

K+

Common soluble anions found in saline soils:

Cl-

SO42-

HCO3- /CO3

2-

NO3-

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.

Measurements

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

0

2040

60

80100

120

0 5 10 15Soil Salinity (dS/m)

Rela

tive

Yiel

d (%

)

Lettuce

Cotton

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)

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, dispersed clays plug soil pores and impede water infiltration and soil drainage.

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

Common soil cations include sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+).

Cations can make clay particles stick together (flocculate).

Negatively charged clay particle

Negatively charged clay particle

+

Flocculating Cations

We can divide cations into two categoriesPoor flocculators

• Sodium

Good flocculators• Calcium• Magnesium

IonRelative

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 Cations

Cations 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 +

Dispersion

Ca2+ Na+

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

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+]

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)

ESP = Na+

Cation Exchange Capacity

- - ---

- -- -

+++

+

++

++

++

++ ++

SAR and ESP are approximately equal numerically

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.

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.

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 Condition

Normal <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.

Sodic SoilsAre 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.

Properties of Sodic Soils

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

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.

0

5

10

15

20

25

0.5 1.0 1.5 2.0 2.5

Electrical conductivity (dS/m)

Salts Affect Soil Structure

Poor soilstructure

Good soilstructure

Sod

ium

leve

l (S

AR

)Based on irrigation water analysis

Salt and Sodium RisksSalinity 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.

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

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.

Soil Alkalinity and Nutrients

Soil pH >7.5Alkalinity is specifically associated with:

Sodic soilsCalcareous soilsSoils high in soluble carbonates

Saline soils may or may not be alkaline.

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 10-100x for every 1 pH unit increase. B: availability decreases at pH >7.

Treatment for Saline SoilsAmendments for removing salts from soils:

Nothing Nothing Nothing

Management PracticesAdequate LeachingMaintain soil drainage through proper tillage

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.

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 84000 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.

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.

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.

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+

Gypsum Application

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+ + SO4

2- + H2O + CO2

• Can be applied to soil or water-run• Rates are commonly 1-3 tons/acre

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 C aC O C O H O C aSO2 4 3 2 2 4

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

Constant H2SO4 injectionkeeps water pH low andprevents formation of CaCO3

in the drip lines, and alsodissolves some CaCO3 inthe soil, helping to maintainhigh exchangeable Ca2+ and low exchangeable Na+.

Soil Amendments (3)• Elemental Sulfur

97% Sulfur• 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

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 C O H O H SO C H O ½ 2 2 2 2 4 22