the chemistry of salinity in soils
TRANSCRIPT
The Chemistry of Salinity in SoilsThe Chemistry of Salinity in Soils
Naomi Waissman AssadianTexas A&M Research and Extension
Center at El PasoSeptember 4, 2002
2
Introduction
Salinity concentrations in soils have profound effects on soil properties
Poor drainage, nutrient deficiencies, and crop toxicity
Factors in soil salinity include:Arid climatesErosion of primary mineralsSaline irrigation and drainage watersHigh water tableHigh potential EvapotranspirationAdditions of inorganic and organic fertilizersSludges and sewage effluents
3
“The term Salinity refers to the presence of the major dissolved inorganic solutes, essentiallyNa+, Mg2+, Ca2+, K+,Cl‐, SO2
4‐, HCO‐3, CO2
3‐ “
SSSA BOOK SERIES: 5, METHODS OF SOIL ANALYSIS PT 3 – CHEMICAL METHODS.
(It could also include NH4
+, NO3, PO4, and charged organic particles in aqueous (soil) samples when determined by electrical conductance)
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Dissolved Solids (Salts)Dissolved Solids (Salts)
Intensity Factor – salt concentration in soil solution Capacity Factor – the ability of solid phases to replenish elements as it is depleted from solution
6
Salt Measurement Salt Measurement –– Intensity Intensity FactorFactor
Electrical Conductance – EC – Electrical current carried by a salt solution. EC increases as salt concentrations increase.
Soil CharacteristicsSoil Characteristics
pH is generally below 8.5pH is generally below 8.5
EC is more than 4EC is more than 4 dSdS/m/m
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Calculations
Total Dissolved Solids=(~640)(EC)Ionic Strength= (0.013)(EC)=ΣciZ2
Single-Ion Activityαj=γjmj m=molar concentration
Extended Debye Huckle Equation (estimate activity coefficients)
Logγ=-AZ2[(I)1/2/(1+Bå(I)1/2)]A=0.512 for water @ 25oC,B~0.33 in water @ 25oCå=adjustable parameter corresponding to ion size and charge (6)
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Factors Affecting Salt Factors Affecting Salt DissolutionDissolution
Salt Effect or Ionic Strength Effect – Addition of an inert salt can increase the solubility of an ionic compound– Added NaCl can increase CaSO4 dissolution– Increased ionic strength affects pH
Ion pairing effect– Weak bonds between hydrated ions
Common Ion Effect – a salt will be less soluble if one of its constituent ions is already present in the solution
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+++
++
The Physiochemical EffectTheElectric Double Layer
(Soil solution)
+
+
+
+
+
+
+
+
+
Ca2+
Na
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Sodium Adsorption Ratio (SAR): Defined as Sodium Adsorption Ratio (SAR): Defined as being “related to the adsorption of sodium by the being “related to the adsorption of sodium by the
soil” soil” SAR=NaSAR=Na++/ (Ca/ (Ca+++++Mg+Mg++++)/2)/2NaNa++, Ca, Ca++++, and Mg, and Mg++++ inin milliequivalentmilliequivalent/L (Allison, 1954)/L (Allison, 1954)
SodicitySodicity occurs when the ratio of sodium is much occurs when the ratio of sodium is much higher to that of other soluble salts.higher to that of other soluble salts.
Soil CharacteristicsSoil Characteristics
pH ranges between 8.5 to 10, pH ranges between 8.5 to 10, ESP>15ESP>15
Plastic and sticky when wet, forming Plastic and sticky when wet, forming hard clods and crustshard clods and crusts
Impermeable to all water.Impermeable to all water.•Specific Ion Effect – Na ion can be inhibitory to certain plant processes
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Surface Salt Pattern
Low saltaccumulation
Moderate salt accumulation
High salt accumulationVery high salt accumulation
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Salt Conc. vs Extraction Ratios-Anerobically Digested Sludge
0
500
1000
1500
2000
2500
paste 1:5 1:25 1:50
ratio
salt
conc
. (m
g/kg
)
CaKMgNa
Salt conc. vs. Extraction Ratios-Aerobically Digested Sludge
0
10000
20000
30000
40000
paste 1:5 1:10 1:25 1:50Extraction ratios
Salt
conc
. (m
g/kg
)
Ca
K
Mg
Na
Study #1
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Extraction Ratio's impacts on EC-Aerobically Digested Sludge
02468
1012
paste 1:5 1:10 1:25 1:50
Extraction Ratio
EC (d
S)
Sodium Adsorption Ratio-Aerobically Digested Biosolids
01234567
paste 1:5 1:10 1:25 1:50
Extraction Ratio
SAR,
rat
io
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Log of Soluble (Ca) y = -0.5248x + 6.7474R2 = 0.6891
7.60
7.70
7.80
7.90
8.00
8.10
8.20
8.30
8.40
8.50
8.60
-3.300 -3.100 -2.900 -2.700 -2.500 -2.300 -2.100 -1.900 -1.700 -1.500
Log (Ca)pH
Study #2
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Study #3 - Objectives
Determine if minimum tillage is possible on irrigated soils with a high potential for salt hazard.Determine if biosolids
remediate accumulation of salts at the soil surface.Evaluate the impact of
ammonium on soil salinity.
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Treatment Design
TillageMaximumMinimum
Soil TextureGe – Glendale siltyclay loam. Gs – Glendale siltyclay.Hs – Harkey siltyclay loam.Tg – Tigua siltyclay.
Subplots – Bed Row PositionFurrowSideTopCenter
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Minimum versus Maximum Tillage
NH4+ Content - Min v Max Tillage
0
20
40
60
80
100
120
J u n e J u l y A u g u s t D e c e m b e r
Tillage Max.Tillage Min.
EC Min v Max Tillage
0
1
2
3
4
5
6
7
June July August December
TillageMax.
TillageMin.
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Soil Texture
EC- Soil Texture
0
2
4
6
8
10
12
June July August December
EC, d
S/m
Ge Gs Ha Tg
NH4+ Content - Soil Texture
0
20
40
60
80
100
120
J une J uly A ug us t D e c e m be r
Ge GsHa Tg
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Furrow Position
NH4+ Content - Furrow Position
0
20
40
60
80
100
120
June July August December
FurrowSideTop CenterTop Depth
nh4+, mg/kg
EC Furrow Position
0
1
2
3
4
5
6
7
June July August December
EC, d
S/m
FurrowSideTop CenterTop Depth
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Potential Salt Management Potential Salt Management StrategiesStrategies
Develop a consistent and detailed salt monitoring program – Miyamoto and Iglesias
Crop Selection – Halophytes – Miyamoto and USSL
Pretreatment of alternative water sources for irrigation– Relatively new, site-specific
Irrigation delivery systems– Enciso
Integrated Soil Management to maintain soil structure and permeability (organic enrichment)