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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
/ 0.25
).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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30
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-0.20r-0.25
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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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=2.65-0.10
--0.I5—0.20^-0.25V0.30V0.35'0.40
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
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ivi
o3
• a
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•5
ric
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OO
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3 0
25
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^0.10
*-0.l5-0.20--O.25.^0.30
.35
.40
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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%Clay = 40 p =2.65SP=65 0S =0.49 /3B = I.29
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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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UJ
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30
25
20
15
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T-00.10
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SP=83 0S = O.44 />B«U7
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0.200.250.300.350.40
0 8
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).