Psychroinetric Measurement of Soil Water Potential: Temperature and Bulk Density EffectsJ

G. S. CAMPBELL AND WALTER H. GARDNER-

ABSTRACTChanges in soil water potential with temperature were deter-

mined using thermocouple psychrometers on samples of foursoils. Results indicate that change in soil water potential withtemperature becomes greater as soils become drier. The finertextured soils showed a greater change in water potential withtemperature than did coarser soils at similar water potentials.

Water potential change with bulk density also was deter-mined for samples of three soils. Little change was notedexcept on a clay subsoil sample.

These results indicate that in most cases negligible differenceswill arise from measuring water potential at laboratory temper-ature rather than at the temperature existing in the field andfrom use of disturbed field samples with altered bulk densityrather than undisturbed field soil. However, at water potentialsbelow about —20 bars, temperature effects may become sig-nificant, especially in fine textured soils, and measurements onclay subsoils may be significantly affected by changes in bulkdensity.

Additional Key Words for Indexing: soil water potential,thermocouple psychrometer, soil temperature, soil bulk density.

EBORATORY measurements of soil water potential usingthermocouple psychrometers on field samples usually

are conducted at temperatures and bulk densities differentfrom those existing in the field at the sampling site. Infor-mation on the temperature and bulk density dependence of

CAMPBELL & GARDNER: PSYCHROMETRIC MEASUREMENT OF SOIL WATER POTENTIAL

the water potential of several soil samples at various watercontents is presented here. This information will give someidea of how well extrapolations of laboratory data will fita given field situation and will facilitate use of such dataunder conditions where soil temperature is an importantfactor in analyses.

Water potential measurements were made using thermo-couple psychrometers of the Spanner (1951) type. Thesewere constructed as outlined by Campbell et al. (1968).A psychrometer measures the relative vapor pressure(pip0) of the water vapor in the system. This is related tothe water potential by

,/, = (RT/MJ In (p/Po) , [1]

where p and p0 are the actual and saturated vapor pres-sures, Mw is the molecular weight of water, T is the abso-lute temperature, and R is the universal gas constant. Thewater potential is the sum of the matric (\f/m) and osmotic(<Ao) potentials, so a change in psychrometer reading mayrepresent a change in either or both of these components. Italso is possible that changes in one of these components willbe masked by changes in the opposite direction of the othercomponent. A change in psychrometer reading also canoccur as a result of changes in psychrometer sensitivitywith temperature (Rawlins, 1966). Thus psychrometercalibrations are necessary for each temperature at whichsoil water potential is measured if high accuracy is required.

METHOD OF MEASUREMENT

Psychrometers were calibrated over standard KC1 solutionswhere the logarithm of the vapor pressure ratio, In p/purequired by equation [1] is obtained as a function of themolality of the solution from the activity

= W8 + RT ln P/Po [2]

where ^i and /ij0 are the chemical potentials of the solventwater and the water at the reference state. The chemical po-tential of the solute, KC1, is

= ̂ + RT In a2

= p? + 2 RT In (ym)

[3]

[4]

For KC1 the ionic activity, a2, may be replaced by (ym)2,where 7 is the mean ionic activity coefficient and m the meanionic molality of the solution. At equilibrium the sum of theproducts of the number of moles of each constituent presentand the differential chemical potential for each must be zero,thus

+ "2 [5]

Differentiating equations [2] and [4] and substituting inequation [5] yields

din1000/MH2o

(m din m + m din y) [6]

where n2 is replaced by m, the molality of KC1 in the solution,

and /i] is 1,000/molecular weight of water. The logarithm ofthe KC1 activity coefficient is obtained in terms of molalityusing the Debye-Huckel equation as given by Harned andCook (1937).

In v =2.303/x(4,/n — A'm2)1

1 + AV2(d0m - A'm2)1''2

+ 2.303 B(d0m - A'm2)

- In (1 + 0.036m) , [7]

where d0 is the density of water and /t, A, A', and B are tem-perature-dependent parameters for which they give values at0, 10, 20, 25, and 40C. They show reasonable agreementbetween values of the activity coefficients obtained by meansof this equation and experimental values found by severalinvestigators. Obtaining din y from equation [7] for use inequation [6], integrating the resulting equation, and substi-tuting in equation [1] using integration limits of m andm = 0 gives

#0 = -16.62864 T

-N K2/4'

VT 2VT 2VT

+ (-V2N - V2A: - VI In M

— K.+

+

+ 2.303*

5P

-3M4

A: 5P2 VI >«"

M5-

l n ( l + 0.036m) \m

0.036 Jo [8]

where

L = (d0m — A'mM = 1 + \/2ALN = A'/(d0

2A3)K = (P = (

and where four terms of a converging infinite series in (d0m —A'mz) have been used to provide desired precision. Values ofthe parameters in the Debye-Huckel equation are given byHarned and Cook (1937). Values of water potential *0 forvarious values of molality are shown in Table 1. Water poten-tials of KC1 solutions have been calculated by Oster (U. S.Salinity Laboratory, Riverside, Calif., private communication)using osmotic coefficients, thermal data, and appropriateequations given in Lewis and Randall (1961) for tempera-tures between 0 and 50C. Values for NaCl have similarly beencomputed by Lang (1967). Oster's values differ from thevalues shown in Table 1 by no more than 16 J kg-1, with mostvalues being within 5 J kg-1. The method of Stokes (1945)was used by Taylor, Evans, and Kemper (1961) to provide

10 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971

Table 1—Water potentials of KC1 solutions at temperaturesbetween 0 and 40C, computed from data of

Harned and Cook (1937)Water Potential, Joules/kg,

Molal-. ity

0.050.10.150.20.250.30.350.40.450.50. 550.60.650.70.750.80.850.90.951.0

OCx 102

- 2.14- 4.21- 6.25- 8.27-10.29-12.29-14. 29-16. 28-18. 26-20. 25-22. 22-24. 20-26. 17-28. 14-30. 11-32. 08-34. 05-36. 01-37. 97-39. 93

sotxlO2

- 2.18- 4.29- 6.37- 8.43-10.49-12. 53-14. 57-16.61-18.64-20. 67-22.70-24. 73-26.75-28.78-30. 80-32. 83-34. 85-36. 87-38.89-40. 92

10Cx 102

- 2.22- 4.36- 6.48- 8.59-10.68-12.77-14.85-16. 93-19.01-21.08-23. 16-25. 23-27.31-29.38-31.46-33. 53-35. 61-37. 69-39.77-41. 85

15CfxlO2

- 2.25- 4.44- 6.60- 8.74-10.87-13.00-15. 12-17. 24-19.36-21. 48-23. 60-25.72-27. 84-29. 96-32.08-34. 21-36.33-38. 46-40. 59-42.72

20CxlO2

- 2.29- 4.52- 6.71- 8.90-11.07-13. 24-15. 40-17. 57-19.73-21. 90-24. 06-26. 23-28. 40-30. 57-32.74-34. 92-37. 10-39. 28-41. 47-43. 66

or bars25C

x 102

- 2.33- 4.59- 6.83- 9.05-11. 26-13. 47-15. 68-17. 88-20. 09-22.30-24. 51-26. 72-28. 94-31. 16-33.38-35.61-37. 84-40. 07-42. 31-44. 55

x 102*30Ct

XlO2

- 2.37- 4.67- 6.94- 9.20-11.45-13.70-15. 94-18. 19-20. 43-22.68-24. 93-27.19-29. 45-31.71-33. 98-36. 25-38. 52-40. 80-43. 09-45. 38

35CtxlO2

- 2.41- 4.74- 7.05- 9.35-11.64-13.92-16. 20-18.49-20. 77-23. 06-25. 36-27.65-29. 95-32.26-34. 57-36. 88-39. 20-41. 53-43. 86-46.20

40CXlO2

- 2.45- 4.82- 7.16- 9.49-11.82-14. 14-16. 46-18.79-21.11-23. 44-25.78-28. 11-30. 46-32. 80-35. 16-37. 52-39. 88-42. 25-44. 63-47. 02

Values given for water potential In Joules/kg are roughly the equivalent of those giv-en In dyne-cm/cm3/106 or bars when the multiplying factor 102 is ignored.

t Interpolated values.

activity values at 5, 25, and 40C. Except for what appears tobe a typographical error at 0.7 molal for 40C (should havebeen p/p0 = 0.977561), their activities, when converted towater potentials, are all within 6 J kg-1 of the correspondingvalues given in Table 1 for temperatures of 25 and 40C. How-ever, at 5C their values are appreciably low, leading to thesuspicion that an error was made in computations at this tem-perature. Our computations at the same molalities, using themethod of Stokes (1945), at 5C gives values within 22 Jkg-1, with all but one value falling within 6 J kg"1, of thoseshown in Table 1. Although some uncertainty exists as to theexact water potential values, those shown in Table 1 are con-sidered to be as accurate as the precision of the psychrometerwarrants.

TEMPERATURE DEPENDENCE

The temperature dependence of the soil water potentialwas obtained using a psychrometer in a sample changer(Campbell et al., 1966). Calibration errors were mini-mized by including the sample and a standard solution inthe sample changer at the same time. Three constant tem-perature baths controlled at 10, 25, and 40C were used.The control was such that short and long term fluctua-tions always were less than 0.005C, and usually less than0.001C. At the start of each run, three soil samples andthree standard solutions were placed in the sample changercylinder. To prevent condensation on the sample holdercover after assembly and placement in the constant tem-

1 '2

10

10C

o

perature bath at temperatures below ambient, the cylinderand samples were first cooled on ice, the changer quicklyassembled, and the assembly placed in the 10C bath. Afterattainment of equilibrium, readings were taken on all sam-ples and standard solutions. The samples were not left inovernight, thus reducing the possibility for vapor exchangebetween sample compartments and errors due to changesin samples or standard solutions. Typical calibration curvesfor a psychrometer against water potentials of standardsolutions are given in Fig. 1 for the three temperatures.

Data were taken on surface samples of four soils:Palouse silt loam and Ritzville fine sandy loam from east-ern Washington, and Millville silt loam and an unclassified,saline alkali clay loam from northern Utah. The Utah soilswere high in calcium carbonate, while the Washington soilswere low. The comparisons desired involved texture andsalinity. Figure 2 shows the temperature dependence ofwater potential for the four soils with each point represent-ing a single observation. These figures suggest that in wetor coarse soils water potential changes little with tempera-ture. Changes are more pronounced in dry or fine-texturedsoil. The saline alkali soil showed little change of waterpotential with temperature even at low water potentials.

The objective of this study was to determine how welllaboratory measurements of water potential would matchfield measurements; thus, readings were made as soon as

10

Temperature - *C

25

-10

-20 •

a. 1 06 _j- — ~~~ —— : .071.09_4___ —— _- — •—-"""

.057 — ———

1

_.

. 080^ —-ClI _ Palouse s i l t loam•-—

——— R i t z v i l l e f ine sandy loam

Temperature -aC10 25 ^0

-10

-20

-20 -10 0

Fig. 1—Typical psychrometer calibration at three temperatures.

i .113

.083

JJS7.

- — — — Unclassified clay loam———— M i l l v i l l e s i l t loam

Fig. 2—Temperature dependence of water potential in foursoils. The parameter is water content (g/g).

CAMPBELL & GARDNER: PSYCHROMETRIC MEASUREMENT OF SOIL WATER POTENTIAL 11

vapor equilibrium was reached to avoid changes due toslow chemical reactions. The direction of change shown isin agreement with the data of Taylor (1958), Taylor andStewart (1960), and Wilkinson and Klute (1962) whichwere obtained using tensiometers or a pressure plate appa-ratus. Previous vapor pressure measurements on soil haveshown either no change of water potential with temperature(Klute and Richards, 1962) or a change in the oppositedirection to that shown here (Kijne and Taylor, 1964).Since the wet loop psychrometer (Richards and Ogata,1958) was used for both of these studies, the possibilityfor error due to the addition of water to the system existed(Zeilinger et al., 1967). This was particularly true in thestudy by Kijne and Taylor (1964) because the same soilsample was used with repeated additions of water to thepsychrometer. The method used in this study eliminatedthis possibility for error in addition to establishing calibra-tion points under the exact same experimental conditionswhich existed for the samples each time the samples wererun. These factors along with the essential agreement ofthese data with tensiometer and pressure plate data ob-tained by others indicates that the direction of changeshown here is correct, at least for the samples having asmall ^0 contribution.

BULK DENSITY DEPENDENCEDependence of water potential on bulk density was

determined using the apparatus shown in Fig. 3. The soilsample was compressed against a porous, stainless steeldisc. Bulk density was measured by determining the posi-tion of the piston with a micrometer, calculating the volumeoccupied by the soil plug, and dividing this into the massof soil in the cylinder. At equilibrium the water potentialin the soil is equal to the water potential in the disc, so thatpsychrometer measurements made in a cavity under thedisc indicated the soil water potential. For these measure-ments the system was maintained at atmospheric pressureand at a temperature of 25C.

Samples of two surface soils and one subsoil were used.The surface samples were Millville silt loam and Palousesilt loam. The subsoil was a swelling clay from an unclassi-fied soil in eastern Washington. Water potential measure-ments as bulk density was varied are shown in Fig. 4.There is apparently little change of water potential withbulk density except in the swelling clay subsoil. Also,changes are less pronounced in drier soil. These resultsare in essential agreement with results of Box and Taylor(1962) who used tensiometers for the water potentialmeasurement.

:;';V '.-•'.- '•'••'•:• ' ' • '' .• . .'.~̂

I

-j£^-Ha — 0— J

- —— Soi 1

Fig. 3—Apparatus for determining bulk density dependence ofwater potential.

Bulk densi ty - g/cm

1.5 Z.O

-10

-20

.183

.082

.081

— — — Palouse s i l t loam— — — M i l l v i l l e s i l t loam

,,0.5

Bulk densi ty - g/cm

1.0 1.5

-20

-kO

.432

Swe l l i ng clay subsoil

-371

Fig. 4—Bulk density dependence of water potential for threesoils. Parameter is water content (g/g).

12 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971