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Communications in Soil Science and Plant Analysis

ISSN: 0010-3624 (Print) 1532-2416 (Online) Journal homepage: http://www.tandfonline.com/loi/lcss20

Determining soil salinity from measurements ofelectrical conductivity

J. D. Rhoades

To cite this article: J. D. Rhoades (1990) Determining soil salinity from measurements of electricalconductivity, Communications in Soil Science and Plant Analysis, 21:13-16, 1887-1926, DOI:10.1080/00103629009368347

To link to this article: https://doi.org/10.1080/00103629009368347

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COMMUN. IN SOIL SCI. PLANT ANAL., 21(13-16), 1887-1926 (1990)

DETERMINING SOIL SALINITY FROM MEASUREMENTS OF ELECTRICAL

CONDUCTIVITY

J. D. Rhoades

USDA-ARSU.S. Salinity Laboratory

4500 Glenwood DriveRiverside, CA. 92501

ABSTRACT

This paper summarizes the principles of soil electrical

conductivity, the equipment and methods used for measuring it,

and the means of interpreting field soil salinity without need

for soil sampling and laboratory analysis. A new technique for

determining the salinity of soil samples from the electrical

conductivity of the saturated-paste is also described.

INTRODUCTION

The proper management of saline soils requires knowledge of

the concentration and distribution of soluble salts in the

rootzone of the soil. In the past, the diganosis of soil

salinity has required analyzing soil samples brought into the

laboratory, although less precise measurements may be made in the

field with portable field kits (1,2). In either case the many

1887

Copyright © 1990 by Marcel Dekker, Inc.

1888 RHOADES

samples required, because of typically high variability, demand

much time and effort (3). Further, to evaluate the effects of

farm management practices and assess time trends, soil salinity

levels must be monitored periodically. The extensive time and

labor requirements for adequately sampling with conventional soil

analysis procedures tend to reach the point of impracticality,

especially for purposes of mapping.

The measurement of bulk soil electrical conductivity (ECa)

using four-electrode and electromagnetic-induction (EM)

techniques can be used to great advantage for these needs of

salinity appraisal. Soil salinity can be determined from ECa

directly in the field without requiring soil sampling, laboratory

analysis, or numerous expensive in situ devices. These

measurement techniques are rapid, simple, inexpensive and

practical.

Instrumentation for measuring ECa has been substantially

advanced since 1971 when ECa was first shown applicable to the

determination of soil salinity in the field (4). Theories of the

measurement and inter-relations among the various soil parameters

involved have been advanced and subsequently refined, improved

instrumentation and circuitry have been developed, and commercial

units have become available for measuring ECa using both

four-electrode and electromagnetic induction (EM) methodology.

It has been shown that ECa and soil salinity (in terms of either

the electrical conductivity of the soil solution, ECw, or of the

saturation-paste extract, ECe) are closely related. Accurate and

DETERMINING SOIL SALINITY 1889

simple methods have been developed for calibrating soil salinity

and ECa. Methods for predicting such calibrations have also been

developed. Applications of the method for measuring, monitoring

and mapping field salinity, detecting the presence of a shallow

water table, detecting saline seeps, determining leaching

fraction, and scheduling and controlling irrigations have also

been developed and demonstrated. Reviews of some of the above

are given elsewhere (5-11).

This paper summarizes the principles of soil electrical

conductivity, the equipment and methods used for measuring it,

and the means of interpreting soil salinity, in terms of ECW and

ECe. A new technique for determining ECe from the electrical

conductivity of the saturated-paste, ECp, is also described. Its

use speeds the determination of salinity using soil samples; it

may be used in the field, as well.

INSTRUMENTAL FIELD METHODS OF SALINITY APPRAISAL

A. Saturation Paste Conductivity

1. Principles

ECe may be estimated from measurement of the electrical

conductivity of the saturated soil-paste (ECp) and estimates of

saturation percentage (SP). The measurement of ECp and the

estimate of SP are made using an EC-cup of known geometry and

volume. The method is suitable for both laboratory and field

applications, especially the latter, because the apparatus is

inexpensive, simple and rugged and because the determination of

ECp can be made much more quickly than ECe.

1890 RHOADES

Rhoades et al. (12) have shown that the following relation

describes the electrical conductivity of saturated soil pastes,

I (we + Ome)2 ECp ECc I . /a ft \ t?r 11 1

l(8s) ECg + (6Ws!> ECSJ (8« " e«s) ECe, H I

ECP l(8s) ECg + (6Ws!> ECSJ

where ECp and ECe are as defined previously, 8W and 8S are the

volume fractions of total water and solids in the paste,

respectively, 9WS is the volume fraction of water in the paste

that is coupled with tne solid phase to provide a series-coupled

electrical pathway through the paste, ECS is the average specific

electrical conductivity of the solid particles, and the

difference (By - 9^) is 8^, which is the volume fraction of

water in the paste that provides a continuous pathway for

electrical current flow through the paste (a parallel pathway to

G w s ) . Assuming the average particle density (ps) of mineral

soils to be 2.65 g/cm3 and the density of saturation soil-paste

extracts (pw) to be 1.00, 0S and 8W are directly related to SP as

follows:

6w = SP/ Kwfe + sp' • m

e s = i - ew. t3]

As shown by Rhoades et al. (12, 13), saturation percentage of

mineral soils, generally, can be adequately estimated in the

field for purposes of salinity appraisal from the weight of the

paste-filled cup. Figure 1 may be used for this purpose; for

details of the relations inherent in this figure see Wilcox (14).

ECe can be determined from measurement of ECp and SP (using

equations 1-3), if values of ps, 8 W S and ECS are known. These

DETERMINING SOIL SALINITY 1891

ill

10090

eo

70

60

50

40

30

crP 205

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

! , , , , !• Q I 1 I I I I I I I I I I t I fl I | t I

60 70 80 90 100 110

GRAMS PASTE

Figure 1. Theoretical relation between saturation percentage(SP) and weight (in grains) of 50 cm3 of saturatedsoil paste, assuming a particle density of 2.65g/cm3.

1892 RHOADES

parameters can be adequately estimated, as demonstrated by

Rhoades et al. (12), for typical arid land soils of the

Southwestern United States. ps may be assumed.to be 2.65 g/cm3.

ECS may be estimated from SP as: ECS = 0.019 (SP) - 0.434. The

difference (8W - 0WS) may be estimated from SP as: (8V - 8WS) =

0.66570.0237 (SP)

2. Apparatus

For this determination use any suitable conductivity meter

and cup-type conductivity cell. Examples are shown in Figure 2

and 3.

a. Conductivity meter, temperature compensating type

b. Conductivity cell of 50 cm3 volume, such as the "Bureauof Soils" cup (2).

c. Portable balance capable of weighing accurately to thenearest 1 gram.

3. Reagents

a. Standard potassium chloride (KC1) solutions, 0.010 and0.100N solution: For 0.010 N solution (EC = 1.41 dS/mat 25*C), dissolve 0.7456 g of KC1 in distilled water,and add water to make 1 liter at 25*C. For 0.100Nsolution (EC = 12.900 dS/m at 25'C), use 7.456 g of KC1.

4. Procedure

Rinse and fill the conductivity cup with KC1 solution.

Adjust the conductivity meter to read the standard conductivity.

Rinse and fill the cup with the saturated soil-paste; tap the cup

to dislodge any air entrapped within the paste. Level off the

paste with the surface of the cup. Weigh the cup plus paste;

DETERMINING SOIL SALINITY 1893

Figure 2. Picture of portable balance used in the field todetermine the weight of the saturated soil-pastefilling the "Bureau of Soils" cup.

subtract the cup tare weight to determine the grams of paste

occupying the cup. Obtain the SP value from Figure 1

corresponding to this weight. Connect the cup electrodes to the

conductivity meter and determine the ECp, corrected to 25"C,

directly from the meter display. Obtain ECe from Figure 4 from

ECp using the curve corresponding to the SP value or as

calculated from Equations 2 - 4 (see below).

1894 RHOADES

Figure 3. Picture of "Bureau of Soils Cup" filled withsaturated soil paste connected to conductance meter.

DETERMINING SOIL SALINITY 1895

CO

•a

oUJ

un

xUJIc_ora

raCO

o•ocoO

75u

uui

SP: 20 30 40 50 60 70110

I 6

40-

35-

30

25-

20

15

10

5

0-

80

90

100

2 3 4 S

SP-. 20 30 40 50 60 70 80

i f -—Z-..Z---'.

.i. .i t - -.-7--7-• j ?-5"Z"£;

2 . . 1.. I. ~f--X -

' ; ' ' Z ';2 . ^. ^ j _ . _ . 27 y 2 ,

l-l~*-1.tS-~&tr---

.. A *. .*.ii\,'.

ill!!::::::::::::::::

"" X

^ t - - 2

;r±p

90100

0 2 4 6 8 10 12 14 16 18 20

Electrical Conductivity of Saturation-

Paste, EC , dS/m

Figure 4. Relations between electrical conductivity ofsaturated soil-paste (ECg), electrical conductivityof saturation extract (ECe) and saturation percentage(SP), for representative arid-land soils.

1896 RHOADES

5. Comments

Sensitivity analyses and tests have shown that the estimates

used in this method are generally adequate for salinity appraisal

purposes of typical mineral arid-;iand soils of the Southwestern

United States (13). For organic soils or soils of very different

mineralogy or magnetic properties, these estimates may be

inappropriate. For such soils, appropriate values for ps, ECS

and 8 W S will need to be determined using analogous techniques to

those used by Rhoades et al. (12). The accuracy requirements of

these estimates may be evaluated using the relations given in

Rhoades et al. (13).

The curves relating ECp, ECe and SP were developed by solving

Equation 1 using the quadratic formula as follows:

ECe - (-b + Vb2 - 4ac)/2a, [4]

where a - [6S (6W - 6WS)], b - [(6S + 6ws)a(ECs) + (6W - 6WS)

(6WSECS) - (6S) ECp] and c = -(6ws)(ECs)(ECp).

A. Bulk Soil Electrical Conductivity

1. Principles

Because most soil minerals are insulators, electrical

conduction in moist, saline soils is primarily through the large

water-filled pores, which contain the dissolved salts

(electrolytes). There is also a relatively small contribution of

exchangeable cations (associated with the solid phase) to

electrical conduction in soils, the so-called surface conduction

(ECS), because these electrolytes are more limited in their

DETERMINING SOIL SALINITY 1897

amounts and mobilities. The value of ECS is assumed, for

practical purposes, to be essentially constant for any given

saline soil. ECS is coupled in series with the electrolyte

present in the water films associated with the solid surfaces and

in the small water-filled pores which bridge adjacent particles

to provide a secondary pathway for current flow in moist soils.

This pathway acts in parallel with the major, continuous flow

pathway (large water-filled pores). The relative flow of current

in the two pathways depends on the solute concentration of the

soil water, the magnitude of ECS and the contents of water in the

two different categories of pores.

A mathematical description of the above model of electrical

current flow in soils is given in Equation 5 after Rhoades et al.

(15):

l(Oe + Oue) E&jc E C C I . / ft

I J + (Bft r r i

(8s) EC*,s + (6ws) ECSJ + (B« " Bws> E C ^ [5]

where ECa, 8S, 6y and ECS are as previously defined, 6WS and (0WC

= 0 W - 0WS) are the volumetric soil water contents in the

series-coupled pathway (the fine water-filled pores) and the

separate continuous liquid pathway (large water-filled pores),

respectively, and ECws and ECwc are the specific electrical

conductivities of the soil water in the two corresponding

pathways, respectively.

The relation between ECWS and ECWC and ECe is, after Rhoades

et al. (15):

6WC + ECWS 6ws)/pb * ECe SP/100 [6]

1898 RHOADES

where />b is 'the bulk density of the soil. For practical purposes

of salinity appraisal, it is assumed that EC^ s ECWS and,

therefore, that (ECW 6W) • (ECWC 8 W C + ECWS 6 W S). Data exist to

support the general validity of this assumption for typical field

soils (Rhoades et al. 13, 16).

The other relations used in the practical application of ECa

measurements to appraise soil salinity are (after Rhoades et al.

16):

SP = 0.76 (%C) +27.25, [7]

p b = 1.73 - 0.0067 (SP), [8]

8S = Pb/2.65, [9]

9wfc = SP.pb/200, [10]

ew = ewfC'Fc/ioo, Hi]

e w s = 0.639 6W + 0.011, [12]

and ECS = 0.019 SP - 0.434 [13]

where %C is clay percentage as estimated by "feel" methods, 0wfc

is the estimated volumetric water content at field capacity, and

FC is the percent water content of the soil relative to that at

field capacity, as estimated by "feel" methods. Use of the above

relations permits ECe to be estimated in the field sufficiently

accurately for salinity appraisal purposes from the measurement

of ECa and the estimates of %C and 8wfc made by "feel" methods.

That such procedures are generally adequate for typical and land

mineral soils, of the Southwestern United States has been

demonstrated by Rhoades et al. (16).

DETERMINING SOIL SALINITY 1899

Figure 5. Photograph of four electrodes positioned in a surfacearray and a combination electric generator andresistance meter.

2. Apparatus

In situ or remote devices capable of measuring electrical

conductivity of the bulk soil can be used advantageously for

purposes of soil salinity appraisal. Two kinds of field-proven,

portable sensors are now available, each with its own advantages

and limitations: (i) four-electrode sensors and (ii)

electromagnetic induction sensors. Both measure the electrical

conductivity of the bulk soil (ECa).

1900 RHOADES

Figure 6. Photograph of a "fixed-array" four-electrodeapparatus and commercial generator-meter.

a. Four-electrode Sensors

A combination electric current source and resistance meter,

four metal electrodes, and connecting wire are needed for large

soil volume (surface array) measurements (Figure 5). The current

source-meter unit may be either a hand-cranked generator type

(Figure 5) or a battery-powered type (Figure 6). Units designed

for geophysical purposes generally read in ohms and, if used for

general soil salinity measurement need, should measure from 0.1

to 1000 ohms.

DETERMINING SOIL SALINITY 1901

Table 1 - Equations for predicting ECa within different soil depthincrements from electromagnetic measurements made withthe EM-38 device placed on the ground in the horizontal

and vertical (EMy) configurations.

depth, cm Equations for Electrical Conductivity^/

for EMH S EMy

0-30 EC^ = 3.023 EMJ - 1.982 EMy

0-60 ECa" = 2.757 EMH " ^539 EMy - 0.097

0-90 ECa = 2.028 EMg - 0.887 EMy

30-60 EC^ = 2.585 EMH - 1-213 EMy - 0.204

60-90 EC^ = .958 E % + 0.323 Ehty - 0.142

for EMH *• EMy

0-30 EC^ = 1.690 EMH " °-591

0-60 EC^ = 1.209 EMJJ - 0.089

0-90 ECa = 1.107 EMH

30-60 EC^ = .554 E % + .595 EMy

60-90 ECa " -0.126 EM^ + 1.283 EMy - 0.097

}J EC^, EMjJ and EMy are the fourth roots of ECa, EMH a n d

1902 RHOADES

Figure 7. Photograph of commercial four-electrode conductivityprobe and generator-meter.

Electrodes used in surface arrays are made of stainless

steel, copper, brass, or almost any other corrosion-resistant

metal. Array electrode size is not critical, except that the

electrode must be small enough to be easily inserted into the

soil, to not tip over and to maintain firm contact with the soil,

when inserted to a depth or 5-cm less. Electrodes 1.0 to 1.25 cm

in diameter by 45 cm long are convenient for most array purposes,

although smaller electrodes are preferred for determination of

DETERMINING SOIL SALINITY 1903

t;CURRENT LOOPS

T - TRANSMITTER COILR- RECEIVER COIL

INDUCED CURRENT FLOW IN GROUND

Figure 8. Diagram showing the principle of operation ofelectromagnetic induction soil conductivity sensor.

ECa within shallow depths (less than 30 cm). Any flexible,

well-insulated, multi-stranded, 12 to 18 gauge wire is suitable

for connecting the array electrodes to the meter.

For survey or traverse work, the array electrodes may be

mounted in a board with a handle (see Figure 6) so that soil

resistance measurements can be made quickly for a given

inter-electrode spacing (17). These "fixed-array" units save the

time involved in positioning the electrodes. For most purposes,

1904 RHOADES

an inter-electrode spacing of 30 or 60 cm is adequate and

convenient (wider spacings require lengthy, cumbersome units).

A four-electrode salinity probe, in which the electrodes are

built into the probe (18) is needed for small soil volume

measurements (Figure 7). Current source-meter units specifically

designed for use with the four-electrode salinity probe are much

smaller and more convenient (19). One such commercial unit,

Martek S c W , reads directly in ECa corrected to 25°C (Figure 7).

b. Electromegnatic Induction Sensors

The basic principle of operation of the EM soil electrical

conductivity meter is shown schematically in Figure 8. A

transmitter coil located in one end of the instrument induces

circular eddy current loops in the soil. The magnitude of these

loops is directly proportional to the conductivity of the soil in

the vicinity of that loop. Each current loop generates a

secondary electromagnetic field which is proportional to the

value of the current flowing within the loop. A fraction of the

secondary induced electromagnetic field from each loop is

intercepted by the receiver coil and the sum of these signals is

amplified and formed into an output voltage which is linearly

related to a depth-weighted soil ECa, EC^.

1/ Mention of trademark or proprietary products in thismanuscript does not constitute a guarantee or warranty of theproduct by the U.S. Department of Agriculture and does notimply its approval to the exclusion of other products thatmay also be suitable.

DETERMINING SOIL SALINITY 1905

Figure 9 shows the commercially available EM soil salinity

sensor (Geonics EM-38J/) being held in the vertical (coils)

position. This device has an inter-coil spacing of 1 meter,

operates at a frequency of 13.2 kHz, is powered by a 9 volt

battery, and reads ECg directly. The coil configuration and

inter-coil spacing were chosen to permit measurement of ECg to

effective depths of approximately 1 and 2 meters when placed at

ground level in a horizontal and vertical configuration,

respectively. The device contains appropriate circuitry to

minimize instrument response to the magnetic susceptibility of

the soil and to maximize response to ECg.

3. Procedures

a. Large Volume Measurements

For the purpose of determining soil salinity of entire

rootzones, or some fraction thereof, it is desirable to make the

measurement when the current flow is concentrated within the soil

depth. This is accomplished with the four-electrode equipment by

selecting the appropriate spacing between the two current (outer)

electrodes which are inserted into the soil surface to a depth of

about 5 cm. In this arrangement, four electrodes are placed in a

straight line. With conventional geophysical resistivity

measurements the electrodes are equally spaced in the so-called

Wenner array (4). With the Martek SCT meter each of the

inner-pair of electrodes is placed inward from its closest

outer-pair counterpart a distance equal to 10% of the spacing

1906 RHOADES

between the outer-pair. The inner-pair is used to measure the

potential while current is passed between the outer-pair. The

effective depth of current penetration for either configuration

(in the absence of appreciable soil layering) is equal to about

one-third the outer-electrode spacing, y, and the average soil

salinity is measured to approximately this depth (4, 20). Thus,

by varying the spacing between current electrodes, one can

measure average soil salinity to different depths and within

different volumes of soil. Another advantage of this method is

the relatively large volume of soil measured compared with soil

samples. The volume of measurement is about (iry/3)3. Hence,

effects of small-scale variations in field-soil salinity on

sampling requirements can be minimized by these large-volume

measurements.

For measurements taken in the Wenner array (electrodes

equally spaced) using geophysical type meters which measure

resistance, the soil electrical conductivity is calculated, in

dS/m, from:

ECa = 159.2 ft/a Rt [14]

where a is the distance between the electrodes in cm, Rt is the

measured resistance in ohms at the field temperature t, and f^ is

a factorf/ to adjust the reading to a reference temperature of

2/ Ft = (0.0004)(T2)-(0.043)(T) + 1.8149; based on data given on

page 90 in (2).

DETERMINING SOIL SALINITY 1907

25*C. For measurements made with the Martek SCT meter, a factor

is supplied in chart form for each spacing of outer electrodes;

this factor is dialed into the meter and the correct soil ECa

reading is directly displayed in the meter readout.

Large volumes of soil can also be measured with the

electromagnetic induction technique. The volume and depth of

measurement can be increased by increasing the spacing between

coils, by reducing the current frequency, and by varying the

orientation of the axes of the coils with respect to the soil

surface plane. The effective depths of measurement of the

Geonics EM-38 device are about 1 and 2 meters when it is placed

on the ground and the coils are positioned horizontally and

vertically, respectively. The EM-38 device does not integrate

soil ECa linearly with depth. The 0 to 0.30, 0.30 to 0.61, 0.61

to 0.91, and 0.91 to 1.22 m depth intervals contribute about 43,

21, 10, and 6 percent, respectively, to the Ec| reading of the EM

unit when positioned on homogeneous ground in the horizontal

position (21). Thus, the weighted bulk soil electrical

conductivity read by the EM device in this configuration is

approximately:

EC* = 0.43ECa,0-0.3 + 0.21ECa>0.3-0.6

+ 0.H>ECa,0.6-0.9

+ 0.06ECa)0.9-1.2 + 0«2ECa>>1>2 [15]

where the subscript designates the depth interval in meters.

It is desirable to determine soil ECa by depth intervals for

calculating soil salinity within various parts of the rootzone as

1908 RHOADES

Figure 9. Photograph of electromagnetic induction soilconductivity sensor.

needed for making assessments and management decisions. Since

the proportional contribution of each soil depth interval to ECa,

as measured by the EM unit, can be varied by raising it above

ground to higher heights, it is possible to calculate the

ECa-depth relation from a succession of EM measurements made at

various heights above ground (21). The ECa values of each soil

depth interval are simply correlated with the succession of EM

DETERMINING SOIL SALINITY 1909

J&L

Figure 10. Photograph of commercial seedbed four electrodeconductivity probe and generator-meter.

readings (0-4) as:

ECa, 0-0.3 " POEMO + PlEMi + P2EM2 + P3EM3 + B4EM4 [16aJ

ECa, 0.3-0.6 " roEMo + TlEMi + 72^2 + 73EM3 + 74EM45 etc., [16b]

where EM represents the reading obtained with the EM-38 unit held

in the horizontal position and 0, 1, 2, 3 and 4 represent height

above ground in feet. The values of the coefficients of Equation

(16) reported by Rhoades and Corwin (21) are reasonably general,

though exceptions have been found.

Another series of equations and coefficients have been

derived to obtain ECa within discrete soil depth intervals from

1910 RH0A.DES

Figure 11. Photograph of burial-type four electrodeconductivity probe and generator-meter.

just two measurements made with the magnetic coils of the EM

instrument positioned at ground level, first horizontally and

then vertically (22, 23, 24). For the depth increment xl-x2 the

equations are of the form:

ECa, xl-x2 = kH EMH - k v EMV + k [17]

DETERMINING SOIL SALINITY 1911

o3

"aCO

——

3TJ

OO

"5o

"o

EV.CO• D

a>aUJ

-ooX

UJ

l±J

30

25

20

15

10

5

%Clay=5 /os=2.

SP=22 c9s=O.6O

2z2iY<-

^P.20.0.25

^-0.30"0.35-0.40

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure 12a. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (8W)and soil clay content (% clay), for representativearid-land soils.

where EMy and EMH are the readings of the EM-38 device obtained

at the soil surface in the vertical and horizontal positions,

respectively; xl-x2 is the soil depth increment in cm and kjj, ky

and k3 are empirically determined coefficients for each depth

increment. Equation (17) is more easily solved than (16) and is

almost as accurate for the two depth intervals 0-30 and 30-60

cm. Values of the coefficients for Equation [17] are given in

1912 RHOADES

1

ionur

atS

ati

"o>>

ivi

o

•ao

O

"a

ric

75a>

E• —

• o

a>OUl

uo

Ul

3 0

25

20

15

10

5

Ul

%Clay=IO /5S = 2.65SP = 28 0S=O.58

(0.)-0.10

-0.15

-0.20

J^O.250.30.35

0.40

^v N

\

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure 12b. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (8W)and soil clay content (% clay), for representativearid-land soils.

Table 1, after Rhoades et al. (24). For more discussion of the

theory and calibration of EM and ECa see Corwin and Rhoades (10).

b. Small Volume Measurements

Sometimes information on salinity distribution within a

small, localized volume of the whole rootzone is desired, such as

that within the seedbed or under the furrows. For such

conditions, the four-electrode salinity probe (18) and burial

DETERMINING SOIL SALINITY 1913

cooZJ

oCO

*o

—

o

•ocoo

"5o

oCD

E•—<o-o

0)OUJ

oo"><UJ

30

25

20

15

10

5

O.lOv 0.15/ ^0.20

LU

%Clay=l5 /?s=2.65

SP = 34 t9s=0.57 /Vl -50

/r / AY(r

///'A

/

/

>

W

s

ty

\ j

J

/

A

/

/ /

If

J

x

>

Vy

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).4O

Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure 12c. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (0W)and soil clay content (% clay), for representativearid-land soils.

type probe (25) are recommended. The seedbed probe (see Figure

10) is designed to be directly inserted into the soil. In the

larger probes (see Figures 7 and 11), four annular rings are

molded in a plastic matrix that is slightly tapered so that it.

can be inserted into a hole made to the desired depth with a

coring tube. In the portable version (Figure 7), the probe is

1914 RHOADES

(0J

o

"5

o•ogoou

oUl

E-s.

OUJ

uaKui

30

25

20

15

10

5

% Clay =20 Ps=2.650 0.15

.20^ - 0 . 2 52 V-0.30- V0.35

N0.40

Electrical Conductivity of Bulk Soil, EC Q ,dS/m

Figure 12d. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (0W)and soil clay content (% clay), for representativearid-land soils.

attached to a shaft (handle) through which the electrical leads

are passed and connected to a meter. In the burial unit (Figure

11), the leads from the probe are brought to the soil surface.

The volume of sample under measurement can be varied by changing

the spacing between the current electrodes. The commercial unit,

Martek SCT, has a spacing of 6.6 cm and measures a soil volume of

about 2350 cm3.

DETERMINING SOIL SALINITY 1915

.10 (0J

Io

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oo

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30

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Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure 12e. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (8W)and soil clay content (% clay), for representativearid-land soils.

To determine soil ECa with the four-electrode probe (Figure

7), core a hole in the soil to the desired depth of measurement

using a Lordj/ soil sampling tube (or sampler of similar

diameter). Insert the four-electrode probe into the soil and

record" the resistance, or the displayed value of ECa, depending

on the meter used. When using meters which display resistance,

1916 RHOADES

co

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DO

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Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure 12f. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (6W)and soil clay content (% clay), for representativearid-land soils.

ECa in dS/m is calculated as:

ECa = k ft/Rt [18]

where k is an empirically determined geometry constant (cell

constant) for the probe in units of 1000 cur1, Rt is the

resistance in ohms at the field temperature, and f^ is a factor

to adjust the reading to a reference temperature of 25*C (see

footnote2/).

DETERMINING SOIL SALINITY 1917

1co

urat

Sat

>»

ivi

o3

• a

oO

•5

ric

UJ

Eto•0

a>OUJ

OO

Ul

3 0

25

20

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^0.10

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Electrical Conductivity of Bulk Soil, EC-,dS/m

Figure 12g. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (6W)and soil clay content (% clay), for representativearid-land soils.

4. Calculations

is calculated from the solution of equations (5) and

(6-13) using the quadratic formula:

ECW = (-b ± ,/b2 - 4ac)/2a, [19]

where a = -[(6S)(6W - 6 W S)], b = [(9sECa) - (6S + 9 W S )2 (ECS) -

(6W - 6WS)(0WSECS)J, and c = [(6w)(ECs)(ECa)]. Then ECe can be

1918 RHOADES

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Electrical Conductivity of Bulk Soil, EC ,dS/m

Figure l'!h. Relations between electrical conductivity of bulksoil (EC a), electrical conductivity of saturation-extract (EC e), soil volumetric water content (8W)and soil clay content (% clay), for representativearid-land soils.

solved from Equation (6). Alternatively obtain EC e, given

measurements of EC a and reasonable estimates of %C and 8 W C , using

Figures (12a-l).

5. Comments

Sensitivity analyses and tests have shown that the estimates

used in this method are generally adequate for salinity appraisal

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DETERMINING SOIL SALINITY 1921

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30

25

20

15

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0.200.250.300.350.40

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Electrical Conductivity of Bulk Soil, EC Q fdS/m

Figure 12k. Relations between electrical conductivity of bulksoil (ECa), electrical conductivity of saturation-extract (ECe), soil volumetric water content (0W)and soil clay content (% clay), for representativearid-land soils.

certain minimum water content is required in the soils for the

measurements of ECa and the model calculations to be valid; this

water content is about 10 percent on a gravimetric basis, though

it may be somewhat higher for very sandy soils.

The ratio SP/100 in Equation (6) may be replaced by the ratio

(©e/z'p)' where p p is the bulk density of the saturated paste and

1922 RHOADES

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"5"aCO**-o>.

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o"oo

o

LJ

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30

2o

20

15

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% Clay = 60 Ps = 2.65SP = 89 0S =0.43 PB = U3

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I 2 3 4 5 6 7 8

Electrical Conductivity of Bulk Soil, EC. ,dS/m

Figure 12<. Relations between electrical conductivity of bulksoil (ECo), electrical conductivity of saturation-extract (ECe), soil volumetric water content (6W)and soil •• clay content (Z clay), for representativearid-land soils.

0e is the total volumetric content of water in the saturated

paste. It should be noted that (ECe 6e) is not equivalent to

(ECW 9W) because different amounts of soil are involved in the

two measurements. The relation between these two products is

given in Equation (6). 6e is related to SP as follows:

[20]

DETERMINING SOIL SALINITY 1923

where pe is the density of the saturation extract (~1.00 g/cm3).

Pp (soil dry weight basis) is related to SP as follows:

These relations are described in more detail elsewhere (13, 15,

16).

If devices are available to measure 0W, or if other more

appropriate values for any of the other estimated parameters are

available, then, of course, they should be used in place of the

estimates obtained by the methods given here. If more accurate

measurements of ECe, or ECy, are required than can be obtained by

the estimation procedures provided, quantitative measurements of

6W, ECS, p\y, etc. should be made using appropriate methods.

The ECa value, as obtained from the EM-38 placed on the

ground in the horizontal position, may be appropriate to use as a

single index of soil salinity in some cases, as it roughly

corresponds to the water extraction behavior of plants.

Irrigated crops tend to remove the soil approximately in the

proportions 40:30:20:10 by successively deeper quarter-fractions

of their rootzone, which is about 1 meter in depth for many

crops, and to respond to water uptake-weighted salinity (26, 27).

Diagnosis guidelines for judging soil salinity problems of

plant growth, etc., from ECe are discussed in Rhoades (28),

Rhoades (29) and Rhoades and Miyamoto (30).

1924 RHOADES

Literature Cited

1. Bover, C. A. 1963. Diagnosing soil salinity. U.S. Dept.Agr. Inf. Bull. 279. 11 p.

2. U.S. Salinity Laboratory Staff. 1954. L. A. Richards (ed.)Diagnosis and improvement of saline and alkali soils.U.S. Dept. of Agri. Handbook No. 60.

3. Sayegh, A. H., L. A. Alban and R. G. Petersen. 1958. Asampling study in a saline and alkali area. Soil Sci.Soc. Am. Proc. 22: 252-254.

4. Rhoades, J. D. and R. D. Ingvalson. 1971. Determiningsalinity in field soils with soil resistancemeasurements. Soil Sci. Soc. Amer. Proc. 35: 54-60.

5. Rhoades, J. D. 1976. Measuring, mapping and monitoringfield salinity and water table depths with soilresistance measurements. FAO Soils Bulletin. 31:159-186.

6. Rhoades, J. D. 1984. Principles and methods of monitoringsoil salinity, pp. 130-142. In: Soil salinity andirrigation - processes and management. Springer Verlag,Berlin.

7. Rhoades, J. D. and J. D. Oster. 1986. Solute content, pp985-1006. In: A. Klute (ed.) Methods of Soil Analysis,Part 1, 2nd Ed. American Society of Agronomy, Madison,WI.

8. Rhoades, J. D. and D. L. Corwin. 1984. Monitoring soilsalinity. J. Soil and Water Conservation. 39: 172-175.

9. Rhoades, J. D. and D. L. Corwin. 1989. Soil electricalconductivity: Effects of soil properties and applicationto soil salinity appraisal. Aust. J. ScientificResearch. (Submitted).

10. Corwin, D. L. and J. D. Rhoades. 1989. Establishing soilelectrical conductivity - Depth relations fromelectromagnetic induction measurements. Aust. J.Scientific Research. (Submitted).

11. Rhoades, J. D., D. L. Corwin and P. J. Shouse. 1988. Useof instrumental and computer assisted techniques toassess soil salinity, pp. 50-103. In: SymposiumProceedings of Int'l Symposium in Solonetz Soils, Osijek,Yugoslavia.

DETERMINING SOIL SALINITY 1925

12. Rhoades, J. D., N. A. Manteghi, P. J. Shouse and W. J.Alves. 1989a. Estimating soil salinity from saturatedsoil-paste electrical conductivity. Soil Sci. Soc. Am.J. 53: 428-433.

13. Rhoades, J. D., B. L. Waggoner, P. J. Shouse and W. J.Alves. 1989b. Determining soil salinity from soil andsoil-paste electrical conductivities: Sensitivityanalysis of models. Soil Sci. Soc. Am. J. (In Press).

14. Wilcox, L. V. 1951. A method for calculating thesaturation percentage from the weight of a known volumeof saturated soil paste. Soil Sci. 72: 233-237.

15. Rhoades, J. D., N. A. Manteghi, P. J. Shouse and W. J.Alves. 1989c. Soil electrical conductivity and soilsalinity: New formulations and calibrations. Soil Sci.Soc. Am. J. 53: 433-439.

16. Rhoades, J. D., P. J. Shouse, W. J. Alves, N. A. Manteghiand S. M. Lesch. 1989d. Determining soil salinity fromsoil electrical conductivity using different models andestimates. Soil Sci. Soc. Am. J. (In Press).

17. Rhoades, J. D. and A. D. Halvorson. 1977. Electricalconductivity methods for detecting and delineating salineseeps and measuring salinity in Northern Great Plainssoils. ARS W-42.

18. Rhoades, J. D. and J. van Schilfgaarde. 1976. Anelectrical conductivity probe for determining soilsalinity. Soil Sci. Soc. Am. J. 40: 647-651.

19. Austin, R. S. and J. D. Rhoades. 1979. A compact, low-costcircuit for reading four-electrode salinity sensors.Soil Sci. Soc. Am. J. 43: 808-810.

20. Halvorson, A. D. and J. D. Rhoades. 1976. Field mappingsoil conductivity to delineate dryland saline seeps withfour-electrode technique. Soil Sci. Soc. Amer. J. 40:571-575.

21. Rhoades, J. D. and D. L. Corwin. 1981. Determining soilelectrical conductivity - depth relations using asinductive electromagnetic soil conductivity meter. SoilSci. Soc. Am. J. 45: 255-260.

22. Corwin, D. L. and J. D. Rhoades. 1982. An improvedtechnique for determining soil electrical conductivitydepth relations from above ground electromagneticmeasurements. Soil Sci. Soc. Am. J. 46: 517-520.

1926 RHOADES

23. Corwin, D. L. and J. D. Rhoades. 1984. Measurement ofinverted electrical conductivity profiles usingelectromagnetic induction. Soil Sci. Soc. Am. J. 48:288-291.

24. Rhoades, J. D., S. M. Lesch, P. J. Shouse and W. J. Alves.1989e. New calibrations for determining soil electricalconductivity - Depth relations from electromagneticmeasurements. Soil Sci. Soc. Am. J. 53: 74-79.

25. Rhoades, J. D. 1979. Inexpensive four-electrode probe formonitoring soil salinity. Soil Sci. Soc. Am. J. 43:817-818.

26. Bernstein, L. and L. E. Francois. 1973. Leachingrequirement studies: Sensitivity of alfalfa to salinityof irrigation and drainage waters. Soil Sci. Soc. Amer.Proc. 37: 931-943.

27. Rhoades, J. D. and S. D. Merrill. 1976. Assessing thesuitability of water for irrigation: Theoretical andempirical approaches. FAO Soils Bulletin. 31: 69-109.

28. Rhoades, J. D. 1982. Reclamation and management ofsalt-affected soils after drainage, pp. 123-197. In:Proceedings of the First Annual Western ProvincialConference Rationalization of Water and Soil Res. andManagement. Lethbridge, Alberta, Canada.

29. Rhoades, J. D. 1989. Effects of salts on soils andplants, pp. 39-48. In: Proceedings of SpecialityConference Sponsored by Irrigation and Drainage DivisionWater Resources Planning and Management Division,American Society Civil Engineers. University ofDelaware, Newark, DE.

30. Rhoades, J. D. and S. Miyamoto. 1989. Testing soils forsalinity and sodicity. Soil Testing and Plant Analysis.(In Press).