MARDI Res. Bul l . , (1985) 13, 3: (309-321)

THE THERMAL CHARACTERISTICS OF SOME HIGHLY WEATHEREDSOLS OF PENINSULAR MALAYSIA

H. GHULAM MOHAMMED*

Keywords: Conductivity, Diffusivity, Texture, Stability, Moisture, Porosity.

RINGKASAN

Sifat-sifat terma tanah yang utama ditakrif dan dibincangkan.

Ragi tanah-tanah terlulubawa di Semenanjung Malaysia, kandungan pasir mempengaruhi dayapengaliran dan daya peresapan terma. Kandungan pasir yang tinggi menyebabkan peningkatan sifat-

sifat terma tersebut. Saiz pasir juga memainkan peranan yang penting dan ini ternyata apabilaperbandingan dibuat di antara tanah-tanah dari siri Bungor dan Rengam. Peranan air dalam perubahan

sifat-sifat terma tanah juga bergantung kepada kandungan pasir. Kandungan pasir yang tinggi

menyebabkan peningkatan daya pengaliran dan daya peresapan terma yang lebih tinggi, terutama

apabila kandungan air volumetrik > 0.16.

Pemindahan terma berkurangan bagi tanah-tanah yang mengalami kepecahan agregat akibat

terlalu basah. Selain daripada kestabilan agregat, saiz agregat dan keronggaan tanah juga mem-pengaruhi sifat-sifat terma tanah.

INTRODUCTION

The thermal properties of the soilinfluence the soil temperature regime andcharacterize the soil's capacity to absorb,release and transmit heat energy. Some ofthe more important properties are heatcapacity, thermal conductivity, thermaldiffusivity and damping depth.

The temperature of a soil dependsdirectly on its heat capacity since theamount of temperature change in responseto the absorption or release of heat isinfluenced by the heat capacity. Fluctua-tions in soil temperature, which areimportant factors in plant growth, arestrongly influenced by these thermalproperties.

These fluctuations are greatest in thesurface layer of the soil which receives solarradiation during the day and loses heat tothe atmosphere at night. The heat receivedat the surface is propagated to the lowerlayers by heat conduction in the form ofwaves, but with a reduction in amplitudeand a progressive increase in time-lag.

However, heat capacity and thermalconductivity vary greatly between soilsbecause different soils are made up ofdifferent proportions of sand, silt, clay andorganic matter, and are of differentstructure. In Peninsular Malaysia there arelarge variations in soil texture and structure.Moreover, being situated in the equatorialzone, with air temperatures of around 30'Cduring the day, soil temperatures whichare in excess of the optimum for the growthof many crops do occur in this country(MaENE, MeesscHaLr, Ltv andMorrnnnuoDrN MANAN. 1979 ToooRov.1980).

Although the principles of heat con-duction are universally well-known and soilthermal characteristics can usually beinferred from soil physical properties, up tillnow no quantitative information on thermalproperties is available with respect toMalaysian soils. This paper reports anddiscusses the thermal characteristics ofseveral soils of different textural andstructural properties in an attempt to con-tribute to the pool of basic soil data, and toprovide examples of how temperature

*Central Research Laboratories Division, MARDI, Serdang, Selangor, Malaysia.

309

changes in soils can be varied by differencesin their physical properties.

DEFINITIONS

The explanation of symbols is given inAppendix 1. Specific heat is the amount ofheat in joules required to raise the tempera-ture of 1 kg of soil 1 K, and heat capacity, Cis the product of specific heat and the massof the soil. The volumetric heat capacity, C,in J/m3/K of a soil constituent is equal to theproduct of its specific heat and its density,p . The volumetric heat capacity of thewhole soil is the sum of the volumetric heatcapacities of all the constituents of the soil(JeNsE and Bonrl, 1965; MoNrerrH, 1973).Thus,

C , s o i l : L C , , x , . . . . . . ( 1 )

where i is a soil constituent and X is thevolume fraction of a soil constituent.

The specific heat and the volumetricheat capacity may also be referred to asgravimetric specific heat and volumetricspecific heat respectively.

The thermal conductivit y, ),in J lml slKis the amount of heat that flows through aunit area in a unit time under unit tempera-ture gradient. The thermal diffusivity, c in- t

m2ls is equal to f and represents the

temperature change ihut tuk", place in anyportion of the soil as heat flows into it froman adjacent layer (JeNse and Bonel, 1965;Tevr-oR and Jecxsol, 1965).

The damping depth, D is the depth atwhich the amplitude of the wave of theperiodic temperature variations is reducedto e-r or 0.37 times the amplitude at thesurface (VeN WIrx and De Vntes, 1966;MoNtEttH, 1973). The relation between Dand the other thermal variables is

1 ^ _p : ( 2 t r l C " A ) ) , r . . . . . . . . . . . . . . . . . . ( 2 )

where (d is the angular frequency in persecond.

For the diurnal temperature variation,0) : 7.27 x 10-5/s and for the annualvariation, w : 7.99 x 10-7 per second. Dgives an idea of the depth to which the soilwill be warmed up.

MATERIALS AND METHODS

Soils

Soil samples of.12 mm in diameter ofBungor, Kuantan, Munchong and Rengamseries were used. These soils have largedifferences in texture (Table 1) and arerepresentative of the sedentary soils ofPeninsular Malaysia. Other relevant detailsof the soils are also given in Table 1.

In order to investigate the role ofaggregate size in the variations of thermalproperties of soils, measurements were alsomade on aggregates of diameter 3-5 mmand 5-8 mm and on mixtures ofdifferent-sized aggregates of Munchongseries soil.

Theory of Method

In order to minimize the effects of heattransfer by water movement when tempera-ture gradients are introduced to the soil, atransient heat flow technique was used tomeasure thermal conductivity.

Thermal conductivity was determinedfrom temperature variations in theproximity of a heat source which was aneedle in which a heating filament and athermocouple were built. The referencejunction of the thermocouple was placed ina column of sand which was maintained at aconstant temperature in a water bath. Theneedle, or probe, was placed in a column ofsoil and heated. As the heat spread outradially from the probe, the probe tempera-ture was recorded continuously in relationto time.

The temperature varies according tothe heat input, the thermal contactresistance between the probe and the soil

310

n

()

O

V)

(J

a

O

c

(t

.jOJ

,

\o

o o 6 i

t ) - ,

E

N

Ei

N

IN

a -

\a

.E

. E

!

N' a

! E

N

IN

EN

t r a )

f € 9' A : d

i - Y 7v ' " ;

= 9, ; 9

!r+

o ( , r Et v , o c . ca i + a i . d

r

rr. ;

c9

n o \ c6 i r i - c .* \ C r *

T

qN

N

z ! ;i 1 i € Eo ! 9 o q' : c - * o ' F. = j . r . t . =F * t - r t -

; i n v P i ; o = ; oj L , - - = ! H !

9 3 1 - Y F €

, i 6 6 c i , ; , 5 6 , ;

oo- =

; E Z E

c c Y

and the thermal properties of the probe andthe soil. The heat flux is proportional to thetemperature and the thermal conductivity,

r \U : A l 6 ' . + r 0 r 1 . . . . . . . . . . . . . ( 3 )S r c . [ 6 / , 6 , )

Equation 1 is the one-dimensionalform of Fourier's law of heat conduction(Dn Vnres, 1966). With the exact initial andboundary conditions, and at times /1 and t2,Equation 3 becomes

T r - 7 , = O h t r l t r . . . . . ' . ' . ( 4 )4n)"

Where Z1 and T2 are temperatures at/1 and /2 respectively, and r is the distancefrom the centre of the needle.

Calibration of the Thermocouple

The scheme for the calibration of thethermocouple is shown in Figure 1. The coldjunction of the thermocouple and a thermo-meter were placed in a cylinder filled withwater at room temperature. The needle,containing the hot junction, was placed inthermostatic water bath. The temperatureof the bath was increased in steps of 1'C inthe range from 14"C (287 K) to 25"C (298K). At every temperature increase thepotential difference between the cold andhot junctions was measured with a recorder.

The calibration curve is shown inFigure 2. The slope of the graph of potentialdifference versus temperature gives theincrease in potential per unit temperature(pVlK). ln Figure 2, the slope is 0.017microvolt per degree Kelvin.

Measurement of Thermal Conductivity

The soil was moistened to the desiredmoisture content by adding water dropwisefrom a burette. While water was beingadded the soil was stirred vigorously with aspatula to ensure thorough mixing. Themoistened soil was packed into a 250-mlmeasuring cylinder. To ensure a uniformbulk densitv. the soil was added in small

t l t- ) l r

Thermometer

Water bath

0.20

0 . 16

0 . 1 2

0.08

0.04

0.00281 289 291 293 295 297 298

Temperature (K)

Figure 2. Calibration curve forthe thermocouple.

portions, each addition being followed bystamping of the cylinder on a piece of thickcloth placed on a wooden table. Absoluteuniformity of packing was not achieved.

Rubber stopper

Thermometer

Water

Cold junct ion

Rubber band

Graduatedcyl inder

However, errors were minimized by varyingthe number of stampings to suit the prevail-ing conditions for the various samples.

The cylinder was placed in a thermo-static water bath maintained at a tempera-ture of 25' Celsius. The cylinder containingthe soil sample, and a cylinder containingsand, in which the cold junction was placed,were allowed to equilibrate in the bath forone hour. The apparatus is shown in Figure 3.

When a measurement was made, theneedle was placed in the soil sample and thecurrent switched on for 100 seconds. Thepotential difference between the hot andcold junctions was measured continuouslywith a recorder.

The slope of the graph of potentialdifference versus log t (time in seconds) isrepresented by br, whilst the potential

Figure 1. Apparatus for the calibration of a thermocouple.

, q

a

312

Thermocouple(cold junct ion)

difference per unit temperature difference isrepresented by b2. The thermal conductivitywas calculated using Equation 7 (shownbelow) which can be derived from Equation 4.

Equation 4 can be rewritten as

Water bath

Soi l sample

Thermocouple (hot junct ion)

Heat ing wire

Needle

The volumetric heat capacity, Cv :n2 C, X; where X, is the volume fraction

i : 1 .of a soil component. As the major soilcomponents are mineral matter, organicmatter and water,

c, : Q w x^ + 2 512 x " + 4 186 X.) 1d J/m3/K .. .. . . . . (8)

where X^ is the volume fraction ofmineral matter, X" is the volumefraction of organic matter and X. isthe volume fraction of waier.

RESULTS AND DISCUSSION

A Comparison of the Thermal Properties ofMineral Soils, Pure Sand, Clay and anOrganic Soil

Since soils are mixtures of severaldifferent materials, the thermal propertiesof different soils vary considerably, depend-ing on the proportions of the constituent

Figure 3. Apparatus for the measurement of thermal conductivity

Tr-Tr=ry

But b1 - PRlog t2ltt

and b2 : P RTz-Tt

Therefore log ttltt -Tr-Tt

There fore , l . : 037 PRbz lb l . . . . . . . . . . . . . . . (7 )

Calculation of Volumetric Heat Capacity

The values of bulk density, moisturecontent (w/w) and organic matter content(w/w) were used to calculate the volumefractions of water, mineral matter andorganic matter.

(6)b2

b l

J I J

Table 2. Thermal properties of some soils and soil constituents

MaterialSand content (7o)

Coarse sand Fine sand

Totalporosrty

Volumetricmoisturecontent

Thermal Volumetr ic Damping

conductivity heat capacity depth(J/m/s/K) (106J/m3/K) (m 10-'?)

Sand*

Clay*

Peat*

Kuantan soil<2 mm

Munchongsoil <2 mm

Rengam soil<2 mm

Bungor soil<2 mm

1 t A

A 1

10.5

36.6

22.9

7 .7

J J . J

0.4000.4m0.4000.4000.8000.8000.5660.ffi40.5850.6200.6200.6300.5890.6080.6000.5370.6260.592

0.0000.2000.0000.2000.0000.4000.0140.1850.2700.0150.0640.0880.0040.1760.2130.0180.16'70.222

0.2931.7580.251,1 .1720.0590.2930 .1 100.2430.7300.1000. r580 .1800 . 1 1 10.5840.9780 .1410.3870.866

1 . 2 5 62.093

1.2562.093

0.5022 . 1 7 7

0.9371 .5901.988

0.8431 . M 71.129

0.8541.5361 . 7 2 1

1 . 0 1 31.4521.765

8.015.2

7 . 412.4

5 . 66 . 1

5 . 96 .5

1 0 . 1

5 . 7o . 4

6.66 .0

10.212.5

6.28 .6

1 1 . 6*Average values reported by VnN Wt.lx and Dr Vntrs (1966)

Table 3. Thermal properties and density of some soil constituentsr

SubstanceThermal conductivity

(J/m/s/K)Specific heat(103 J/m3/K)

Density(kg/m3)

Quartz

Clay minerals

Organic matter

Water

Ice

Air

8.7900.2930.2510.5732.1 ' t70.025

2 0092 0092 5124 1861 884

1 .26

2 6602 6501 3001 000

9201 .25

*Adapted from Dr Vnrss (1966).

materials. This is i l lustrated in Table 2,where the data obtained from soils ofPeninsular Malaysia are compared with datareported by VeN WI.lx and De VnlEs(1966). Their values show that the thermalconductivity of sand is slightly higher thanthat of clay. The thermal conductivity of anorganic soil, as represented by peat, is muchlower than those of mineral materials. Thevolumetric heat capacity of sand does notdiffer from that of clay, but that for organicmatter is lower. Since the heat capacities ofquartz and clay minerals are the same(Table 3), at equal porosity and moisturecontent, their volumetric heat capacities areequal. But the heat capacity of organic

matter is higher than those of minerals and,therefore, at equal porosity and moisturecontent, one would expect the volumetricheat capacity of peat to be higher than thatof a mineral soil. In the example given, peatin fact has a lower volumetric heat capacity.The reason for this is that its porosity ismuch higher than those of the mineral soils,giving a lower percentage of solid material.

The value for damping depth dependson both the thermal conductivity and the heatcapacity, and changes according to these twoparameters, as can be seen in the examples inTable 2. Soils with higher sand contents havehigher values of damping depth.

3r4

Apart from some slight variations,

corresponding to differences in porosity andmoisture content. the values of thermalproperties obtained for the soils under studyare comparable to the average valuesdiscussed above. From the results obtainedfor the four soils; Rengam, Bungor,Munchong and Kuantan, the effect of sand

content is quite apparent. Bearing in mindthe slight differences in porosity, the

thermal conductivity is observed to behigher for soils with higher sand contents(for example, Bungor and Rengam), especiallyat high moisture contents. However,differences in damping depth are very small.

Since the organic matter content is lessthan 5Vo in all the mineral soils, its effect onthe thermal properties is negligible.

Though the results of this studyprovide us with a better appreciation of therelationships between soil physical charac-teristics and thermal properties, it must beemphasized that the values of thermalproperties obtained from laboratorymeasurements do not necessarily representthe values under field conditions. This isbecause laboratory samples have been dis-turbed and have a different structure fromthe field soil.

The Effect of Texture on Thermal Properties

The effect of texture on thermal pro-perties can be attributed to differences inmineralogy and particle size. As can be seenin Table 3, the thermal conductivity ofquartz is 30 times greater than that of clayminerals. Therefore, at equal porosity andmoisture content, soils containing higherproportions of sand have higher values ofthermal properties (Table 2).

Apart from the mineralogical make-up, this effect can also be explained by thefact that sand particles are much larger than

clay particles, and thus conduct heat more

readily. Heat conduction occurs at the mole-

cular level and is enhanced by intimatecontact of molecules. Even if clay particles

are aggregated to form secondary particles

as large as sand, heat conduction would sti l lbe greater in the sand because the sandparticle is more dense. An aggregate of clayparticles would be more porous and wouldhave a lower density than a sand particle,and thus would conduct heat less readily.

The Role of Moisture in Heat Transfer inSoils

Water and solid material are muchbetter heat conductors than air. Thus highmoisture content and high density enhancesoi l thermalconduct iv i ty (VeN WIx and Dr,Vnrns, 1966; Russgrt, 1973; Lnl, 1980).However, the influence of moisture variesaccording to soil characteristics.

Figure 4 shows the relationshipbetween the volumetric moisture contentand the thermal conductivity of three soilsof Peninsular Malaysia. The three soilsdiffer in their sand contents, that forRengam being 44.37o, Bungor 56.2% andKuantan 11.2Va (Table 2). Apart fromdifferences in total sand content. thecontent of coarse sand (fraction 200 -

2 000pm) of Rengam is greater than that ofBungor (Table 2).

Figure 4 also shows that, at volumetricmoisture contents below 0.16, the differ-ences in thermal conductivity among thethree soils are small although those withhigher sand contents tend to have slightlyhisher values. At these low moisture

0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6Volumetr ic moisture content

Figure 4. Change in the thermal conductivitywith volumetric moisture content for Bungor,

Kuantqn and Rensam series soils.

v l X< l 6

> 1 . 2

E o .a

a I l 4

?E

F

3 1 5

contents, the amount of water is insufficientto exist as films on the soil surface.

At volumetric moisture contentsgreater than 0.16, the thermal conductivityincreases more sharply. The differencesbetween soils also become more apparent.At these higher moisture contents, thewater films act as bridges between soilparticles, providing a continuous solid-liquid path for heat conduction.

With further increases in moisturecontent, the thermal contact between soilparticles continues to improve, until up to acertain moisture content where the thermalconductivity reaches a maximum. Moisturein excess of this critical point does notprovide any further improvement to thethermal contact.

The soil with high clay content,Kuantan, reaches the maximum thermalconductivity at a higher moisture contentthan the other two soils. The volumetricmoisture contents corresponding to themaximum thermal conductivity are 0.30,0.33 and 0.48 for Rengam, Bungor andKuantan respectively. For the two soils withrelatively high sand contents, Bungor andRengam, the moisture content is greater forthe soil with more sand (Bungor) andapparently contradicts the earlier observa-tion that a soil with lower sand contentachieves the maximum thermal conductivityat a higher moisture content. However,although the total sand content of Bungor ishigher than that of Rengam, its coarse sandcontent is much lower; 83% of the sandfraction of Rengam is coarse compared with41.% of that of Bungor. Thus Rengam has asmaller total surface area than Bungor,whilst Kuantan, with its high clay contenthas the largest surface area. Since themaximum thermal contact is achieved whenall the soil particles are joined to each otherby water f i lms on the soil surface, soils withlarger surface areas will require largeramounts of water. Thus, the values ofvolumetric moisture content mentionedabove (0.30, 0.33, 0.48) increase in the

order of increasing surface area (Rengam,Bungor, Kuantan).

The variation in thermal diffusivitywith volumetric moisture content is shownin Figure 5. Soils containing more sand(Bungor, Rengam) have higher thermaldiffusivity at equal moisture contents. Thisis because sand has a higher thermal con-ductivity than clay while their heatcapacities are equal.

0 0 . 1 ( \ . 2 0 . 3 0 . 4 0 . 5 0 . 6

Volumetric moisture content

Figure 5. Change in the thermal diffusivitywith volumetric moisture content for Bungor,

Kuantan and Rensam series soils.

The difference in diffusivity betweenthe soils is very large at high moisturecontents. For example, at a volumetricmoisture content of 0.3, the thermal diffu-sivity of Rengam is almost twice that ofKuantan. This shows that a difference intexture such as that between Rengam andKuantan can contribute significantly todifferences in the rate of heat transfer.

The volumetric heat capacity increaseslinearly with volumetric moisture content(Tables 4-6). The increase is l inear becausethe only component that increases is water,while the mineral matter and organic mattercontents are constant and the bulk densitv isalmost constant.

The Effect of Aggregate Size on ThermalConductivity

Many of the highly weathered soils ofPeninsular Malaysia are composed of stableaggregates of various sizes. Aggregate size

b

- U h.=

jt-

t 0 .2F

F

3r6

Table 4. Volumetric heat capacity and related parameters at various moisture contents forBungor series soil

Moisturecontent(% wlw)

Totalporosity

(vo)

Volumetricorganic

matter content

Volumetricmineral

matter content

Volumetricmoisturecontenl

Volumetricheat capacity(103J/m3/K)

1 . 6

4 .5

4 .6

7 .4

10.3

12.6

1 5 . 5

17.5

17.6

21.8) 7 4

26.2

28.5

28.9

2'7.7

29.4

34.1

) 3 . I

55.8

54.4

55.5

56.s58.6

62.3

62.6

u.959.259.656.254.050.252.150.6A 1 <

0.0260.0240.0260.0250.0240.0230.0210.0210.0200.0230.0220.0250.0260.0280.0270.0270.029

0.4340.4160.4340.4250.4060.3970.3590.3490.3310.3870.3780.4160.4350.4720.4530.4630.491

0.0180.0510.0530.0850 .1 100.1360 .1510.1670.1580.2220.2410.2970.3380.3720.3430.3710.442

I 013

I 1 1 1

1 158

I l t 3

| 337| 4271 4071 4521 3761 7651 8222 1382 3522 5752 4112 5502 908

Table 5. Volumetric heat capacity and related parameters at various moisture contents forKuantan series soil

Moisturecontent(Vo wlw)

Totalporosrty

(vo)

Volumetricorganic

matter content

Volumetricmineral

matter contenl

Volumetricmolsturecontent

Volumetricheat capacity(103J/m3/K)

1 . 3A 1

6.6

1 0 . 1

12.8

15.7

18.0

25.1

26.9

28.7

32.5

38.3

41.6

45.7

56.6

57.0

57.4

57.7

56.6

57.0

ffi.4

58 .5

58.5

) I . I

54.35 1 . 354.054.0

0.0280.0280.0280.0280.0280.0280.0260.0280.0280.0280.0300.0320.0300.030

0.4020.4020.4020.3920.4020.4020.3740.3920.3920,3920.4300.4580.4300.430

0.0140.0550.0720.1080 .1410.1600.1850.2700.2890.3080.3830.4810.4900.s38

937

I r08

1 180

1 3 1 1

1 468

1 549

1 590

1 988

2 068

2 149

2 544

3 012

2 989

3 191

distribution is an important factor in thethermal regime of soils because it partlydetermines the pore size distribution. Thesize and shape of pores determine thenature and number of contact pointsbetween soil aggregates.

The effects of aggregate size on thethermal conductivity of Munchong seriessoil are shown in Figure 6 and Table 7.Figure 6 shows the variations in thermalconductivity with volumetric moisturecontent for columns of aggregates of

317

Table 6. Volumetric heat capacity and related parameters at various moisture contents forRensam series soil

Moisturecontent(Vo wlw)

Totalporosity

( % )

Volumetricorganrc

matter content

Volumetricmineral

matter content

Volumetricmolsturecontent

Volumetricheat capacity(103J/m3/K)

0.43 .87.09.8

11.41 1 . 515.717.720.823.225.126.028.5

58.9

58.9

ffi.4

6 1 . 5

59.2

60.0

9 .2

60.8

60.0

58.9

56.2

54.0

52.5

0.0300.0300.0290.0280.0300.0290.0260.0280.0290.0300.0320.0330.03s

0.3800.3800.3710.3530.3800.3710.3340.3620,3710.3800.4080.4270.445

0.0040.0400.0710.0950.1200.1170.1450.1760.2160.2420.2820.3060.349

8541 0041 lt71 173I 339I 3081 3421 5361 721I 8542 0792 2212 441

Table 7. The effect of aggregate size (at equal soil porosity) on

thermal conductivity (Munchong series soil samples)

Sampleno.

Aggregatesize (mm)

Volumetricmoisture contenl

Porosity(%)

Thermal conductivity(J/n/s/K)

s-8 (90 )-2 -3 (10s-8 (702-3 (30s-8 (s0t - ? r / sn

s-8 (302-3 (70

>t -2 (450-1 (ss

>1-2 (27.6)0-1 (72.4)

> 1 - 2 ( 1 3 . 8 )0- 1 (86.2)

0.018

0.018

0.016

0.017

0.022

0.021

0.026

69.4

68.6

67.7

67.2

62.4

61.4

59.4

0.0668

0.0661

0.0547

0.0613

0.1122

0.1051

0.0987

*Values in parentheses indicate composition (7o) in soil sample

E

';!

E

F

0.200 . 1 60 . 1 20.080.04

< 2 mm (Total porosity = 627.)

3 - 5 m m ( T o t a l p o r o s i t y : 6 3 f t 1

5-8 mm (Tota l poros i ty = 697a)

different sizes havins different totalporosity.

Table 7 shows the differences inthermal conductivity between variousmixtures of different-sized aggregates, whenboth moisture content and total porosity arekept constant.

It is observed that the soil columncontaining aggregates I 2 mm has higherthermal conductivity than those containing

0.00 0 .02 0 .04 0 .06 0 .08 0 .10 0 .1 I

Volumetric moisture content

Figure 6. The effect of aggregate size onthermal conductivity at various volumetricmoisture contents for Munchong series soil.

3 1 8

larger aggregates (Figure 6). The conduc-tivity decreases as the size of aggregatesincreases. This trend is due to differences inthe densities of the soil columns. Largeaggregates create wide pore spaces and havelow density. At low density, thermal contactis lower because aggregates tend to beseparated by air space.

However, when porosity and moisturecontent are almost constant, the presence ofa greater number of large aggregates tendsto increase the conductivity. However, thedifferences in conductivity shown in Table 7are very small, and no definite conclusioncan be drawn.

The Influence of Soil Structure on ThermalProperties

From the results obtained in thisstudy, the influence of soil structure appearsto be related to porosity, aggregate size andaggregate stability.

Low porosity results in higher thermalconductivity because of better thermalcontact. A large aggregate is expected to bea better heat conductor than a group ofsmaller aggregates if their moisture contentsand porosity are the same. A soil aggregatecan be regarded as a solid body whichconducts heat better than liquids and gasesbecause the individual particles which makeup the aggregate are in intimate contact.

At high moisture contents, soil aggre-gates are easily disrupted by physical distur-bance. When this happens, the conductivitydecreases because the continuity of heatflow in a solid body (soil aggregate) isbroken. This phenomenon is il lustrated bythe points in Figure 5 at the high range ofmoisture content. For Rengam, at volumetric

moisture contents greater than 0.3, thethermal conductivity recorded is lower thanthe maximum. For Bungor and Kuantan,the drop occurs at volumetric moisturecontents greater than 0.33 and 0.48 respec-tively. At these high moisture contents, theaggregates become less stable and break up,and become less effective as conductors ofheat.

CONCLUSIONS

Differences in soil texture andstructure affect the thermal properties of thesoils under study, emphasizing the role ofparticle size in enhancing heat conduction.

Water greatly increases thermal con-ductivity when present in sufficient quan-tities to exist as films on soil surfaces.Thermal contact between particles isimproved by water 'bridges'.

However, the increase in thermal con-ductivity due to the presence of water isgreater for soils with high sand content thanfor clayey soils, whereas the increase in heatcapacity is the same for all mineral soils.Thus, thermal diffusivity, which charac-terizes changes in soil temperature, increasesmuch more for sandy soils than for clayeysoils. Greater fluctuations in soil temperatureare expected in soils such as Rengam andBungor, with high amounts of coarse soilparticles, than in clayey soils such as Kuantan.

ACKNOWLEDGEMENTS

This study was conducted while theauthor was at State University, Ghent,Belgium. He would like to thank Prof. M.De Boodt and the staff of the laboratory forSoil Physics, Soil Conditioning and Horti-cultural Soil Science of the Universitv.

ABSTRACT

The important thermal properties of soils are defined and discussed.

In the highly weathered soils of Peninsular Malaysia, the effect of sand content on thermalconductivity and thermal diffusivity is clearly shown. Higher sand content increases the value of thesevariables. The effect of the grain size of the sand fraction is also clearly brought out in a comparison

319

between soils of the Bungor and Rengam series. The role of water in increasing the thermal conductivity

and the thermal diffusivity of soils is greatly modified by the sand content. A higher sand content results

in a greater increase in conductivity and diffusivity, especially in the range.of volumetric water contents

> 0 .16 .

Heat transfer is greatly reduced in soils whose aggregates were broken down due to excessive

wetting. Besides aggregate stability, other elements of soil structure such as aggregate size and porosity

were also shown to influence the thermal properties of soils'

REFERENCES

Dr VnIrs, D.A. (1966). Thermal properties of soils. InPhysics of Plant Environment (ed. Vnx Wux,R.W.), pp. 210-35. Amsterdam : North-Holland Publishing Co.

JeNse, A. R.P. and Bonrl, G. (1965). Measurementof thermal conductivity in.rit! in mixed materialsa.g. soils. Neth. J. agric. Sci. 13,57-61.

Lel, R. (1980). Physical and mechanical characteristicsof Alfisols and Ultisols with particular referenceto soils in the tropics. ln Characterization ofSoib in Relation to their Classification andManagement for Crop Producion (ed. TlrENG,B.K.G.), pp. 188-201. Oxford: ClarendonPress.

MerNr, L.M., MeEsscHALK, G.G., Ltu, K.H. andMoxnrnnuoplx MeN,tx, M. (1979). Ann. Rep.

for SoiI Physics Proiect. Fac. Agric., UniversitiPertanian Malaysia, Serdang.

Accepted for publication on 10th September, 1985.

MoNTEITH, J.L., (1973). Principles of EnvironmentalPhysics. l-ondon : Edward Arnold.

Russeu, E.W. (1973). Soil Conditions and PlantGrowth 10th ed. London : l.ongman.

Tlvlon, S.A. and Jncrsox, R.D. (1%5). Heat capacityand specific heat. In Methods of Soil Analysk.Part I. Physical and Mineralogical Properties,Including Sraasrrcs of Measurement and Satnpling'(ed. Bncx, C.A., Evexs, D.D., Wsrrr, J.L.,ExsvIxcr.n, L.E,., Cr-enr, F.E. and Dneuen,R.C.), pp. 345-8. Madison, Wisconsin: Amer.Soc. Agron. Inc.

Tooonov, A.V. (1980). Soil temperature in Malaysia.MARDI Rep. No.69.

Vax Wrrx, R.W. and DE VRIEs, D.A. (1966). Periodictemperature variations in a homogeneous soil. InPhysirs of Plaw Envirownent (ed. Ver Wur,R.W.). 6t*1"t6am: North-Holland Publishing Co.

320

Appendix 1. Symbols and units of variables used

Symbol Variable Unit

CC,Ipa

rTI

oIRb1

b2

Heat capacityVolumetric heat capacityThermal conductivityDensityThermal diffusivityDistance of heat flow from the centre of the thermocoupleTemperatureTimeTotal heat input per unit timeElectric currentElectrical resistanceSlope of the curve of the potential difference between the hot andcold junctions versus log t (response lag curve)Potential difference between the hot and cold junctions per unittemperature difference (thermocouple output)Damping depthVolumetric organic matter contentVolumetric moisture contentVolumetric mineral matter content

@ Angular frequency

JIK

J/m3/K

J/m/s/K

kglm3

m2ls

m

KS

J/sAohm

V/s

V/Kmm3/m3m3/m3m3/m31/s

Dxox-x^

-) l I