Chemistry of Sodium-Affected Soils

Dr. Tom DeSutter and Yangbo ‘Kathy’ He

Associate Professor of Soil Science Program Leader for Soil Science North Dakota State University

2nd Annual Reclamation Conference

25 February 2014

DeSutter et al, 2014 unpublished data

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

8/21/12 12:00 AM 8/22/12 12:00 AM 8/23/12 12:00 AM 8/24/12 12:00 AM 8/25/12 12:00 AM

flux

(cm

/h)

Flux (cm/h)

Start Brine

Stop Brine

DeSutter et al, 2014 unpublished data

Why does this happen?

• Na inflicts two different possibilities: – Dispersion – Internal swelling

• Both of these are a function of the concentration of Na and the EC of the soil solution

Photo by Tom DeSutter

Theory of dispersion

• Guoy-Chapman theory

( )IZ

k10

1 10042.3 −− =

At low EC and monovalent cations, thickness of double layer is large, Double layers of adjacent particles will overlap, causing electrostatic repulsion.

Effect of ionic strength (I)

Effect of ion charge (Z)

Ca Na

(Ess

ingt

on, 2

004)

2MgCa

NaSAR22e ++

+

+=

CECNaESP 100*=

Lab data of dispersion

EC= 0.5, 2, 4, 6, 8, 12, and 16 dS/m

Montmorillonite SAR =24

SAR=24R

elat

ive

%of

dis

pers

ed c

lay

(%)

0

20

40

60

80 SAR=12

0

20

40

60

80

SAR=5

EC (dS m-1)0 2 4 6 8 10 12 14 16

0

20

40

60

80 SAR=1

0 2 4 6 8 10 12 14 160

20

40

60

80

Montmorillonite

Clay dispersion decreases with a decrease in SAR and an increase in EC

He, Y

., T.

DeS

utte

r, an

d D.

Cla

y. 2

013.

Dis

pers

ion

of p

ure

clay

min

eral

s as i

nflu

ence

d by

Ca

to M

g ra

tios,

SAR

and

EC.

Soi

l Sci

. Soc

. Am

. J. 7

7:20

14-2

019.

Lab data of dispersion

SAR= 24 Montmorillonite

0 2 4 6 8 10 12 14 16 18

Dis

per

sed

cla

y co

nce

ntr

atio

n (

g/L

)

0

5

10

15

20SAR=24 Illite

EC (dS/m)0 2 4 6 8 10 12 14 16 18

0

5

10

15

20SAR=24 Kaolinite

0 2 4 6 8 10 12 14 16 180

5

10

15

20

He, Y., T. DeSutter, and D. Clay. 2013. Dispersion of pure clay minerals as influenced by Ca to Mg ratios, SAR and EC. Soil Sci. Soc. Am. J. 77:2014-2019.

Lab data of dispersion • Dispersion is as a function of both amount of Na

and EC • Dispersion increase with SAR and decrease with

increase of EC for 2:1 type clays (Mont. & Illite)

• Adverse effects of dispersion can be alleviated by increasing the solution EC (but plants may suffer)

EC0 1 2 3 4 5 6

SAR

0

1

2

3

4

5

6

Dispersion

Swelling theory • Na+ has little interruption on water net in interlayer, the

water is retained and fluidity is reduced • Ca2+ disrupts water net allowing water to more mobile,

reduces overall swelling

Na+ Na+ Na+ Na+ Na+ H2O

H2O H2O H2O

H2O H2O

Ca2+ Ca2+

Interlayer space H2O enter in and cations hydrated

(Grim, 1968)

Repulsion > attraction force, interlayer space increase, clay swell

Wyndmere CIG Field SoilSAR = 14

EC (dS/m; mmhos/cm)0 2 4 6 8 10 12 14 16

% W

ater

(1/3

bar

)

16

18

20

22

24

26

28

30

32

Reference

He et al, 2014 unpublished data

Adverse effects on water movement

Na and EC

Control swelling and dispersion

Reduced soil pores and clogged soil pores by clay particles

Slow down water movement (Ksat)

Adverse effects on water movement • Ksat (Saturated hydraulic conductivity) with water

Exline soil series

SAR7 14 24 28H

ydra

ulic

con

duct

ivity

(cm

/h)

0.000

0.005

0.010

0.015

0.020

0.025

He et al, 2014 unpublished data

Now what? • Steps to remediate/mitigate a Na-affected soil

– Need Ca amendment (other cations may work too) – Need water for leaching – Need drainage to get Na out of the system

When Ca and EC are provided, water movement improves

No treatment

High Ca and EC

SAR7 14 24 28H

ydra

ulic

con

duct

ivity

(cm

/h)

0.00

0.05

0.10

0.15

0.20

0.25

0.30Ksat Gypsum solution Ksat FGDG (10 tons/acre)

He et al, 2014 unpublished data

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

8/21/12 12:00 AM 8/22/12 12:00 AM 8/23/12 12:00 AM 8/24/12 12:00 AM 8/25/12 12:00 AM

flux

(cm

/h)

Flux (cm/h)

Start Brine

Stop Brine

DeSutter et al, 2014 unpublished data

Summary • Swelling and dispersion are the major

mechanisms controlling soil structure and H2O movement

• Closely related with soil Na and solution EC • SAR between 0-8 should be acceptable for

H2O movement • If SAR >8, EC will be need to be ≥2

– This may only hold true for SAR values less than 15

– The higher the SAR the higher the solution EC values need to be

Acknowledgements

• ND Water Resources Research Institute • USDA-NRCS Conservation Innovation Grant

– Reducing sodification in high risk northern Great Plains soils

Chemistry of Sodium-Affected Soils

Dr. Tom DeSutter

Associate Professor of Soil Science Program Leader for Soil Science North Dakota State University

2nd Annual Reclamation Conference

25 February 2014

Summary

• Gypsum (tons/acre-foot) required for SAR to be reduced to 8

Summary

CEC\SAR 13 18 30

Quantity of gypsum

Quantity of gypsum

Quantity of gypsum

10 0.9 1.7 3.5

20 1.7 3.4 6.9

30 2.5 5.1 10.2

CEC

SAR