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III. Determination of Soil Salinity from Soil Paste or Bulk Soil Electrical Conductivity

# III. Determination of Soil Salinity from Soil Paste or Bulk Soil Electrical Conductivity

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MEASURING AND MAPPING SOIL SALINITY

213

air

.-.

.-.

A

Figure 3. Schematic representation and model of electrical conductivity in soil. (A) The

three paths that current can take in unsaturated soil. (B) Simplified soil model consisting of

the three conductance elements (a-c) in parallel. (After Rhoades eta/., 1989a).

series-coupled and continuous pathway volumetric contents of soil water;

0, is the total volumetric content of soil water; 0, is the total volumetric

content of soil particles; and EC, is the average specific electrical conductivity of the soil particles. The soil water in the continuous pathway, dWc

(=0, - Ow), is envisioned as the “mobile” water phase. It can be different

in electrolyte composition (i.e., EC,) than that in the “immobile” water

phase (i.e., EC,), which is associated with the fine and intraped pore water

(i.e., the immobile water, 0,). At equilibrium, EC, and EC, would be

the same, but during transient-state periods, such as immediately after

irrigation or rainfall, they would likely be different. This model assumes

that EC, is independent of 0, and EC,, which appears to be the case for

most practical purposes (Shainberg et al., 1981; Bottraud and Rhoades,

For conditions of EC, greqter than about 2 dS/m and for soils with

typical values of EC, (I 1.5 dS/m), the product 0,EC, is so much larger

than the product B,EC, that the latter can be neglected. Equation ( 5 ) then

simplifies to

[

1

EC, -I-(0, - O,)EC,

0,

For such cases, the relation between EC, and EC, in Eq. (6) is linear for

EC,

=

(”

+

-

214

any value of O,(Ow - 0,) beyond some threshold level and the y intercept

depends on EC,, O,, and 0,. Because the ratio [(O, O,)z/O,]

is typically

close to the value 1, the intercept of Eq. (6) is approximately equal to EC,.

The earlier EC, model of Rhoades et al. (1976) is analogous to this limiting

case version of Eq. (9,as discussed elsewhere (Rhoades et al., 1989a).

At low levels of EC,, the relation between EC, and EC, is curvilinear,

as described in Eq. (5). The first term of the equation determines the shape

of the EC, - EC, curve. Over the remainder of the EC, range, EC, and

EC, are linear, with Ow - 0, representing the slope, as described above.

So, although Eq. ( 5 ) describes the full relation between EC, and EC,, Eq.

(6) can be used for conditions of EC, 2 2-4 dS/m (which corresponds

approximately to EC, 1 1- 2 dS/m).

A typical set of data illustrating the appropriateness of the abovedescribed model and of generalizationsusing the model is shown in Fig. 4

for Waukena loam soil. The solid line is that described by Eq. ( 5 ) and the

+

-

.c

0

h

c

4-

L - - - _ - - - _ - - - _ _ - - _ _ - _ :

9,-

Waukena loam

12

16

20

Electrical Conductivity of Soil Water, ECw,dS/m

Figure 4. The electrical conductivity of Waukena loam soil as a function of the electrical

conductivity and volumetric content of soil water. The measured data points (0)are shown

and the solid line is the “fit” of these combined data by Eq. (5). (After Rhoades et al., 1989a.)

MEASURING AND MAPPING SOIL SALINITY

3

a

I .o

1

I

-

0.6

-

0.4

-

I

- 0.0209

EC,= 0.023%C

0.8

I

215

-

-

0.2

I

1

I

I

Clay Content, %

Figure 5. Correlations between EC, and clay percentage for a number of soils from the

San Joaquin Valley of California. (After Rhoades et af.,1989a.)

circles represent experimental data. EC, represents the EC of the equilibrating water or the water expressed from the soil by pressure filtration.

The soil had been extensively leached with waters of different salinities

(EC,), therefore EC, was essentially equal to EC, and to EC, under the

conditions of this experiment. The data and model relations also show that

the ability to determine accurately EC, (or EC,) from EC, decreases as 0,

decreases. This is so because the required accuracy of measurement of EC,

becomes limiting as the EC, =f(EC,) relation flattens at low values of 0,.

At very low values of 0, (ZO.l), it is not possible to determine EC, (or

EC,) from EC, at all (see Rhoades et al., 1976).

To use Eq. ( 5 ) or (6) to assess soil salinity (EC, or EC,) from EC,, the

values of EC, , Ow, and 0, must be known. EC, and 0, can be estimated

using Figs. 5 and 6, respectively. The means of obtaining these relations are

described elsewhere (Rhoades et al., 1989a). The value 0, can be measured

in the field using time domain reflectometry (TDR) methods or it can be

adequately estimated, for many practical purposes, by “feel.” The TDR

method is described later. The value 0, can be estimated from bulk density

(pB) as 0, = &/2.65, where 2.65 is a reasonable estimate of the average

particle density of most mineral soils.

Equation (5) may be solved for EC,, with the assumption that EC, =

EC,, by arranging it in the form of a quadratic equation and solving for its

216

Volumetric Content of Soil Water, 8,

Figure 6. The volumetric content of soil water in a series path as a function of the total

water content for various soils. (After Rhoades el a/., 1989a.)

positive root:

EC,

=

-b+dFXG

+

where a = (O,)(Ow- Om), b = (0, Om)z(EC,)

(O,EC,), and c = O,EC,EC,.

If EC, is desired, it can be obtained from

(ECwOw)= (EC,O,

(7)

2a

+ EC,O,)

+ (Ow - O,)(O,EC,)

= EC,(SP/lOO)p,

-

(8)

where SP is the gravimetric water content of the saturation paste expressed

as a percentage and p~ is the bulk density of the soil (Rhoades et al.,

1989a).

The relations described above give the opportunity to distinguish between the “mobile” and “immobile” contents of soil salinity. Because the

soil water that plants draw on to meet transpiration requirements is primarily 0, and because the salinity effect is primarily one of reducing water

availability to the plant, the salt in the mobile water phase, EC,, ideally

should be used as a basis for relating crop responses to soil salinity. In as

much as EC, is the dominant contributor to EC, in saline soils, it may be

MEASURING AND MAPPING SOIL SALINITY

217

determined from Eq. (6) for a given value of EC, using an intercept term

estimated from the relations of Figs. 5 and 6. This procedure is applicable

2 dS/m. For conditions of lower salinity, the intercept term

where EC,

of Eq. [ 5 ] must be used. The appropriate y intercept value, and thus EC,,

may be determined using successive approximation techniques.

Because of the linear relationship [Eq. ( 6 ) ] that exists at a fixed water

content (such as field capacity) between EC, and EC, at significant values

of EC, (or EC,), soil salinity (either EC, or EC,) may be empirically

related to EC, by an expression of the following type:

EC, (or EC,) = m(EC, - EC:)

(9)

where EC: is [(O, 8,)2/8s](EC,), or -EC,, and is predicted as previously

described (i.e., using Figs. 5 and 6), and rn is the slope of the EC, (or

EC,) =f(EC,) relation at field capacity. A typical linear relationship

between EC, and EC, of this type is shown in Fig. 7. Analogous relations

+

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Electrical Conductivity of Soil, ECa (dS/m)

Figure 7. The relation between bulk soil electrical conductivity and the electrical conductivity of the saturation paste extract (soil salinity) for Dateland soil at field capacity

218

have been developed between an EC, and EC, (Rhoades, 1980). With such

calibrations, one can predict EC, (or EC,) from EC, for field soils of

various types, provided they are at or near field capacity water content at

the time of EC, measurement. Simple calibration procedures have been

developed in order to obtain the calibration relations of Eq. (9) appropriate

to field soils with their natural structures, pore size distributions,and water

1971;Rhoades and van Schilfgaarde, 1976; Rhoades et al., 1977). Numerous satisfactory field calibrations (as in Fig. 7) have been obtained for

many soils around the world and they have been found to be very similar

for soils of similar textures (Rhoades and Ingvalson, 1971;Halvorson and

Nadler, 1981; Bohn et al., 1982). It has been shown that the calibrations

[slopes and intercepts of Eq. (9)] can be predicted from soil properties such

as texture (or percentage clay content) and saturation percentage (or field

information on methods for calibrating EC, =f(EC,), see Rhoades (1976)

The effect that varying the soil water content (i.e., departure from Calibration water content) has on salinity determined from Eq. (9) depends on

whether or not salt loss from the soil occurs with the change of water

content (Rhoades et al., 1981). Immediately following an irrigation, salt

loss occurs as the water drains to field capacity; hence, EC, is very sensitive

to changes in 0, during such times. Afler the rapid drainage ceases and the

soil is at field capacity, further major losses of soil water in cropped soils

occur mainly through evapotranspiration. Almost all of the salt in the

water taken up by the plant root system is excluded from entering by the

root membranes; the salt is left behind in the remaining water. Likewise,

no salt is lost through evaporation. Hence, the salt concentration (or

electrical conductivity) of the remaining soil water is increased proportionately as 0, is reduced by evapotranspiration. Because of this inverse proportional relationship between EC, and Ow, the product EC,0, found

within a given soil volume at field capacity will not change appreciably as

0, is reduced below field capacity; thus, the product EC,0, may be considered (ignoring salt precipitation) as approximately constant at water

contents of field capacity or less. However, changes in 0, do affect EC,

through its influence on the partitioning of 0, and Om, as shown in Fig. 6.

As 0, decreases below field capacity due to evapotranspiration, EC, will

show an approximately linear decrease according to the relationship

where (Y

A EC, = aA0,K

(10)

is a factor related to the relation between 0,, and ,

8 and K =

MEASURING AND MAPPING SOIL SALINITY

219

EC, 0, = a constant. For typical soils the error in EC, caused by A 0, is not

large, with reasonable deviation in 0, from field capacity water content.

Experimental data supporting these conclusions about the relatively insignificant effects of A 0, on AEC, and AEC, during and following an irrigation are given by Rhoades et al. (198 1). The appropriateness of using 0, as

a reference for water content in establishing EC, =f(EC,) calibrations is

supported by the results of Bottraud and Rhoades (1985).

It is apparent from the above discussionsthat EC, is primarily a measure

of the total dissolved salt in a soil on a volumetric basis. Hence, given the

value of (EC, 0,) at field capacity, an estimate of EC, at any lower water

content can be calculated from Eq. (8). Hence, the limits of EC, (or

osmotic potential) over an irrigation cycle can be ascertained (estimated)

from EC, (and hence from EC,). For many such practical applicationsthis

procedure can be used in place of Eqs. (5) and (6) to estimate EC, from

EC,.

An equation analogous to Eq. (5) established for bulk soil electrical

conductivity exists for saturated soil pastes, as follows:

where EC, is as defined previously, EC, is the electrical conductivity of the

saturated paste, 0, and 0, are the volume fractions of total water and solids

in the paste, respectively, ,8 is the volume fraction of water in the paste

that is coupled with the solid phase to provide a series-coupled electrical

pathway through the paste, EC, is the average specific electrical conductivity of the solid particles, and the difference e, - e, is e,, 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 0,).

Assuming the average particle density ( p , ) of mineral soils to be 2.65 g/cm3

and the density of saturation soil paste extracts (p,) to be 1.00, 6, and 0,

for saturated pastes can be directly determined from SP as follows:

and

The saturation percentage of many mineral soils can be adequately estimated in the field for purposes of salinity appraisal from the weight of a

paste-filled cup of known volume (Rhoades et al., 1989b). Figure 8 may be

used for this purpose; for details of the relations inherent in this figure, see

Wilcox (195 1).

220

CUP

GRAMS PASTE

Figure 8. Theoretical relation between saturation percentage (SP) and weight (in grams)

of 50 cm3of saturated paste, assuming a particle density of 2.65 g/cm3.(After Rhoades et al.,

1989b.)

B. DETERMINING

SOILSALINITY

FROM SATURATED

PASTE

ELECTRICAL

CONDUCTMTY

EC, can be determined from measurements of EC, and SP [using Eqs.

(1 1) - ( 13), if values ofp,, Ow, and EC, are known. These parameters can be

(1989b,c). For typical arid land soils of the southwestern United States, ps

may be assumed to be 2.65 g/cm3; EC, may be estimated from SP as

EC, = 0.019SP - 0.434, and the difference 0, - Om may be estimated

from SP as Ow -,8 = 0.0237(SP).0-M57

The measurement of EC, and SP

MEASURING AND MAPPING SOIL SALINITY

22 1

can be easily 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 EC, can be made much more quickly than that

of EC, .

In this method a saturated soil paste is made as described previously and

is then placed in a conductivity cup of known volume. From the weight of

the paste, SP is determined; from the conductance, EC, is determined.

Then EC, is obtained from Fig. 9 given EC, and SP, using the curve

corresponding to the SP value, or else it is calculated using the following

equation:

EC,

=

-b+-

2a

+

+

- O,EC,,

where a = O,(Ow - OA, b = (0, 0,,,J2EC, (Ow - O,)O,EC,

and c = - O,EC,EC,. The values of EC,, O,, Ow, and 0, are estimated

from SP using Eqs. ( 12) and ( 13) and the relations given above.

Sensitivity analyses and tests have shown that the estimates used in this

method are generally adequate for salinity appraisal purposes of typical

mineral arid-land soils of the southwestern United States (Rhoades et al.,

1989~).For organic soils or soils of very different mineralogy or magnetic

properties, these estimates may be inappropriate. For such soils, appropriate values for p,, EC,, and 0, will need to be determined using techniques

analogous to those of Rhoades et al. (1989b). The accuracy requirements

of these estimates may be evaluated using the relations given by Rhoades et

al. (1 989c).

It should be noted that EC,O, is not equivalent to ECJ, because different amounts of soil are involved in the two measurements. The relation

between these two products is

Ecw~w/Pb= EC,~,/p,

(15)

Data to support this are given by Rhoades (1981) and Rhoades et al.

(1990b). The ratio OJp, is equivalent to SP/lOO (see Rhoades et al.,

1989a,b).

C. DETERMINING

SOILSALINITY

FROM BULKSOILELECTRICAL

CONDUCTIVITY

Soil salinity can be determined from bulk soil electrical conductivity by

essentially one of three ways. Here we discuss these alternative methods of

salinity appraisal, but first the various instrumental means of measuring

EC, will be briefly reviewed.

222

10

80

\

I

I

I

v)

U

90 I

' 100 I

I

I

6

aJ

0

w

I

6

I

1 4

I

I

I

I

2

c

I

I

I

I

w

1 2

I

*0

X

C

0

c

I

I

I

I

I

I-

2a

c

Q

v)

v-

0

>I

zw

,-

c

0

a

U

C

0

0

Q

0

I-

L

c

0

Q,

iii

0

2

4

6 8

10 12 14 16 18 20

Electrical Conductivity of Saturation

Paste, EC,,,

dS/m

Figure 9. Relations between electrical conductivity of saturated soil paste (EC,), electrical conductivity of a saturation extract (EC,),and saturation percentage(SP),for representative and-land soils. (After Rhoades et al., 1989b.)

MEASURING AND MAPPING SOIL SALINITY

22 3

1. Sensors for Measuring Bulk Soil Electrical Conductivity

Three types of soil conductivity sensors presently exist that are capable

of measuring bulk soil electrical conductivity. Two are field-proven, portable sensors that are now commercially available: (1) a four-electrode sensor

and (2) an electromagnetic induction sensor. A third sensor, based on time

domain reflectometry technology, has shown good promise and utility in

certain experimental applications. Each method has its own advantages

and limitations.

a. Four-Electrode Units

Bulk soil electrical conductivity can be measured using four electrodes

inserted into the soil, a combination electric current source/resistance

meter, and connecting wire. Such a surface array of electrodes and a

generator/meter unit are shown in Fig. 10. The current source-meter unit

may be either a hand-cranked or a battery-powered type. Units designed

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

salinity appraisal, should measure from 0.1 to 1000 ZZ. A commercially

Figure 10. A “fixed-amy” four-electrode apparatus and commercial generator/meter.