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)

4

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

5

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

7

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)

8

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

9

Osmotic Potential (kPa) =40EC

10

+++

++

The Physiochemical EffectTheElectric Double Layer

(Soil solution)

+

+

+

+

+

+

+

+

+

Ca2+

Na

11

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

12

Surface Salt Pattern

Low saltaccumulation

Moderate salt accumulation

High salt accumulationVery high salt accumulation

13

Current ResearchCurrent Research

14

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

15

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

16

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

17

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.

18

Treatment Design

TillageMaximumMinimum

Soil TextureGe – Glendale siltyclay loam. Gs – Glendale siltyclay.Hs – Harkey siltyclay loam.Tg – Tigua siltyclay.

Subplots – Bed Row PositionFurrowSideTopCenter

19

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.

20

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

21

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

22

The Future in the Upper Rio The Future in the Upper Rio Grande BasinGrande Basin

23

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)