Soil Science Society of America Journal 64:1285-1290 (2000)
© 2000 Soil Science Society of America
DIVISION S-1-SOIL PHYSICS
Soil Thermal Conductivity
Effects of Density, Moisture, Salt Concentration, and Organic Matter
Nidal H. Abu-Hamdeha and
Randall C. Reederb
a Agric. Engineering and Technol. Dep., Jordan Univ. of Science and Technology, P.O. Box 3030, Irbid, Jordan
b The Ohio State Univ., Agric. Engineering Bldg., RM 228C, 590 Woody Hayes Dr., Columbus, OH 43210-1057 USA
nidal{at}just.edu.jo
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ABSTRACT
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The thermal conductivity of soil under a given set of conditions is most important as it relates to a soil's microclimate. The early growth and development of a crop may be determined to a large extent by microclimate. The effect of bulk density, moisture content, salt concentration, and organic matter on the thermal conductivity of some sieved and repacked Jordanian soils was investigated through laboratory studies. These laboratory experiments used the single probe method to determine thermal conductivity. The soils used were classified as sand, sandy loam, loam, and clay loam. The two salts used were NaCl and CaCl2, while addition of peat moss was used to increase the organic matter content. For the soils studied, thermal conductivity increased with increasing soil density and moisture content. Thermal conductivity ranged from 0.58 to 1.94 for sand, from 0.19 to 1.12 for sandy loam, from 0.29 to 0.76 for loam, and from 0.36 to 0.69 W/m K for clay loam at densities from 1.23 to 1.59 g cm-3 and water contents from 1.4 to 21.2%. The results also show that an increase in the amount of added salts at given moisture content (volumetric solution contents 
ranged from 0.03–0.12 m3 m-3 for the sand and from 0.09–0.30 m3 m-3 for the clay loam) decreased thermal conductivity. Increasing the percentage of soil organic matter decreased thermal conductivity. Finally, it was found that the sand had higher values of thermal conductivity than the clay loam for the same salt type and concentrations.
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INTRODUCTION
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SOIL THERMAL PROPERTIES are required in many areas of engineering, agronomy, and soil science, and in recent years considerable effort has gone into developing techniques to determine these properties. Seed germination, seedling emergence, and subsequent stand establishment are influenced by the microclimate. Thermal properties of soils play an important role in influencing microclimate (Ghuman and Lal, 1985).
Modeling water and energy movement in soils requires knowledge of heat, salt, and water interaction. Heat flow in soil may be determined from knowledge of the thermal conductivity and temperature gradient. Fundamental information on how salts and water affect thermal conductivity of soil is also needed in modeling water and energy movement in systems containing salt-affected soils (Noborio and McInnes, 1993)
The thermal conductivity of a soil depends on several factors. These factors can be arranged into two broad groups: those which are inherent to the soil itself, and those which can be managed or controlled, at least to a certain extent, by human management. Those factors or properties that are inherent to the soil itself include the texture and mineralogical composition of the soil (Wierenga et al., 1969). Factors influencing a soil's thermal conductivity that can be managed externally include water content and soil management (Yadav and Saxena, 1973). Water content plays a major role in a soil's thermal conductivity. Water content is also the most difficult to manage. The way a soil is managed will play an important part in determining its thermal conductivity. Any practice or process which tends to cause soil compaction will increase bulk density and decrease porosity of a soil. This in turn will have a significant effect on thermal conductivity.
The effect of water content on soil thermal conductivity has received more attention than the effects of other physical characteristics (Kunii and Smith, 1960; Al Nakshabandi and Kohnke, 1965; Fritton et al., 1974; Parikh et al., 1979; Riha et al., 1980). The effects of salts on thermal conductivity of soil have also received little attention, and studies to date have left much uncertain. Noborio and McInnes (1993) found that the apparent thermal conductivity of soils decreased with increased CaCl2, MgCl2, NaCl, or Na2SO4 salt concentration in solution from 0.1 mol kg-1 to solubility limits. On the other hand, Van Rooyen and Winterkorn (1959) found no noticeable effect of salt on the thermal conductivity of quartz sand at high solution contents with concentrations of CaCl2 up to 0.18 mol kg-1, or with NaCl up to 0.34 mol kg-1. Globus and Rozenshtok (1989) concluded that the thermal conductivity of quartz sand moistened with 0.25 mol kg-1 solution of the base KOH was lower than that of quartz sand moistened with water.
Thermal properties can be determined indirectly by measuring the rise or fall of temperature in response to heat input to a line source at the point of interest (Jackson and Taylor, 1965). De Vries (1952, 1963) developed models that allow estimation of thermal conductivity and volumetric heat capacity of soils from the volume fractions of their constituents and the shape of the soil particles. The dual-probe heat-pulse technique (Campbell et al., 1991; Bristow et al., 1993; Kluitenberg et al., 1993; Bristow et al., 1994a) has also been used to make measurements of soil thermal properties. It consists of two parallel needle probes separated by a distance (r). One probe contains a heater and the other a temperature sensor. With the dual-probe device inserted in the soil, a heat pulse is applied to the heater and the temperature at the sensor probe is recorded as a function of time. All three soil thermal properties, including thermal conductivity, can be determined from these data.
For Jordanian soils, however, information on thermal properties has been lacking. These data are needed for constructing models to predict the thermal regime of soils. Such information assumes greater importance with increasing attention being paid to developing the agricultural industry in Jordan. Since the early growth and development of a crop may be determined to a large extent by microclimate, the practical significance of knowing the thermal conductivity of a soil under a given set of conditions is most important. Our first objective was to determine the effects of soil density and water content on the apparent thermal conductivity of four soils with different textures: a sand, a sandy loam, a loam, and a clay loam. Because of the general lack of information about the effects of salt and organic matter on thermal conductivity, our second objective was to determine the effects of two common salts and organic matter on the apparent thermal conductivity of clay loam soil.
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Theory
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The single probe methodology is based on a solution of the heat conduction equation for a line heat source in a homogenous and isotropic medium at a uniform initial temperature. Because of the linear heat source and cylindrical geometry of these heat dissipation sensors, sensor temperature (T) during heating is related to time (t) according to the theoretical solution for a line heat source (De Vries and Peck, 1952; Campbell et al., 1991; Bristow et al., 1994b; Reece, 1996)
 | (1) |
Where T0 is the initial temperature (°C), q' is the energy input per unit length of heater per unit time (W m-1), 
is the thermal conductivity of the material surrounding the line source (W m-1 °C-1), t0 is a time correction used to account for the finite dimensions of the heat source and the contact resistance between the heat source and the medium outside the source, and d is a constant.
Nonlinear least-squares regression is used to solve for . An alternative approach is to assume t0 << t so that ln(t + t0) 
ln(t). With this assumption, linear regression can be used to calculate from heating data with Eq. [1] and ln(t) as the independent variable. Furthermore, if the relation between T and ln(t) is linear, then can be simply estimated from the change in sensor temperature between two times, t1 and t2, by
 | (2) |
The corresponding equation for sensor temperature during cooling after th s of heating is (Reece, 1996)
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Equation [2] and [3] can be approximated by substituting I2 R for q' as
 | (4) |
where is the thermal conductivity (W/m K), I is the current in the line source (A), R is the specific resistance of the wire (W/m), and S is the slope of the straight-line portion of the temperature rise or fall vs. the logarithm of time (°C). We assumed that energy transported in soil by radiation and convection of heat was negligible (De Vries and Peck, 1958; Jury et al., 1991). The experiments were carried out at room temperature.
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Materials and methods
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Thermal conductivity of soils was measured using the single probe methodology. In this method, a line heat source, i.e., a thin straight wire through which a constant electric current is passed generating constant heat, is installed and a thermocouple is glued to it. The soil is then packed around the wire to the desired density. When the sample and wire are at uniform and constant temperature, constant power is supplied to the heater element and the temperature rise of the heating wire is measured by a thermocouple and recorded with respect to time during a 200-s heating interval. Actual current through the heater element was calculated with Ohm's law by measuring the voltage drop across a 10-W reference resistor in series with the heater wire. Heating power input to a sensor was calculated by multiplying the resistance per unit length of heating wire (300 W m-1) by the square of the applied current. Heating power inputs of 11 to 12 W m-1 was used in this study. Thermal conductivity of the sample may be calculated from the temperature–time record and power input according to Eq. [4]. In our experiments rectangular steel boxes of dimensions 17 cm length, 14 cm width, and 20 cm height were constructed in which the soil was packed (Fig. 1)
. A hole was drilled in the center of each end through which an electrical wire was inserted. This wire ran through the center of the box lengthwise and was fastened at both ends. The wire leads from the box were connected to a power supply that would heat the wire running through the box. A thermocouple was inserted through a small hole in a removable lid of the box and glued to the electrical wire. The thermocouple leads were attached to a data logger that was employed to record wire temperature at the thermocouple terminal at specified time intervals.
Measurements of thermal conductivity were made on four types of soils: sandy soil (95% sand, 4% silt, and 1% clay), sandy loam soil (75% sand, 9% silt, and 16% clay), loam soil (40% sand, 36% silt, and 24% clay), and clay loam soil (21% sand, 39% silt, and 40% clay). Soils were air-dried and screened through a 0.2-cm sieve. For the study of variation of thermal conductivity with bulk density and moisture content, the soil sample was brought to a desired water content by adding a known amount of distilled water and then mixing thoroughly. Then the soil of known weight at the given moisture content was packed around the wire to different known volumes that were marked on the box to bring the soil sample to the desired bulk density. Various levels of bulk density and moisture content were used. After that, the electrical wire was connected to the power supply unit. Temperature was measured and recorded every 5 s for the first minute and then every 10 s until the end of the heating process (200 s in our study). Then the power supply unit was disconnected and cooling process started immediately. The thermocouple continued to record the temperature after the battery was disconnected. The temperature was recorded every 5 s for the first 30 s and then every 10 s till the end of the cooling process. The temperature was plotted vs. the logarithm of time and slopes of the straight-line portions of these curves were determined and used in the calculations of thermal conductivity. Figure 2
shows an example of these plots. On average, these curves became straight lines 20 s after heating began at the wire. The same procedure was repeated for different soil bulk densities at various levels of moisture content for each soil type.
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Fig. 2 Wire temperature as a function of lnt during heating for sandy loam at moisture content of 6.1% and different soil densities
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For the study of variation of thermal conductivity with salt concentrations and percentage of organic matter in the soil, only the sandy soil and the clay loam soil were used. Soil samples were air-dried and mixed with amounts of either CaCl2 or NaCl solution to produce desired solution contents. These salts were chosen because their ions are found in many salt-affected soils (Noborio and McInnes, 1993). The mixture was screened through a 0.2-cm sieve and then mixed in plastic bags. The samples were allowed to equilibrate overnight at room temperature and were then placed in the rectangular steel boxes. Volumetric solution contents ranged from 0.03 to 0.12 m3 m-3 for the sand and from 0.09 to 0.30 m3 m-3 for the clay loam. The thermocouple temperature was recorded as a function of time following the procedure described above and slopes of temperature vs. logarithm of time were obtained. Concentrations used were 0.01, 0.03, 0.05, 0.07, and 0.09 kg kg-1 for NaCl and 0.010, 0.015, 0.020, 0.025, and 0.030 kg kg-1 for CaCl2. The experiment was repeated (the box was emptied and repacked with soil) four times for each treatment. Distilled water was used in all experiments. Finally, six samples of clay loam soil were air-dried and screened through a 0.2-cm sieve. These soil samples were amended with peat moss, which was used as organic matter. The amendment was done on a weight basis such that the weight of peat moss was 5%, 10%, 15%, 20%, 25%, and 30% the total weight of the six samples, respectively. Then the mixture of known weight was packed to a known volume to bring the soil sample to the desired 1.13 g cm-3 bulk density. Single probe thermal conductivity measurements were made using the same procedure described above. The experiment was repeated two times for each peat moss percentage and average values were reported.
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Results and discussion
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A paired t-test was used to test the null hypothesis that obtained from heating data was not different from the obtained from cooling data. The P value was 0.29, indicating that both the heating method and cooling method yielded identical thermal conductivity values. The average of the heating and cooling estimates of are reported in this study.
Thermal conductivity of five sieved and repacked Jordanian soils as a function of bulk density, water content, salt concentration in solution, or organic matter is shown in Fig. 3 through 9
. Figures 3 through 6 show thermal conductivity of the different soils as a function of bulk density and water content. The sandy soil had higher thermal conductivity values than the other soils at all bulk densities. Thermal conductivity increased with increasing bulk density for all soils as a result of particle-contact enhancement as porosity was decreased. For the clay loam and loam soils, thermal conductivity did not continue to increase rapidly with increasing bulk density at various water contents (Fig. 4 and 6). There was a rapid increase in the thermal conductivity of the two soils with the first increment in bulk density; however, further increase in bulk density caused only a slight increase in thermal conductivity. Such a phenomenon was absent in the sandy and the sandy loam soils.
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Fig. 3 Thermal conductivity as a function of soil density for sand at three different moisture contents (1.4, 2.5, 3.3%)
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Fig. 4 Thermal conductivity as a function of soil density for clay loam at three different moisture contents ( 9.3, 14.2, 18.3%)
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Fig. 5 Thermal conductivity as a function of soil density for sandy loam at four moisture contents (6.1, 9.4, 12.1, and 16.0%)
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Fig. 6 Thermal conductivity as a function of soil density for loam at three moisture contents (7.7, 17.5, and 21.2%)
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Fig. 7 Soil thermal conductivity of sand as a function of concentrations of both NaCl and CaCl2 solutions
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Fig. 8 Soil thermal conductivity of clay loam as a function of concentrations of both NaCl and CaCl2 solutions
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It appears that increase in bulk density of sandy and sandy loam soils beyond 1.32 g cm-3 did improve contact between the relatively larger sand particles vs. between the silt and clay. At a given bulk density, thermal conductivity increased with increasing soil water content. It is observed that beyond a certain bulk density, higher values of moisture content increased thermal conductivity less rapidly in the case of clay loam and loam soils, and more rapidly in the case of sandy and sandy loam soils. Increasing water content perhaps completed water films around sand particles, which are relatively larger than silt and clay, and thus increased the contact area between sand particles that causes the thermal conductivity to increase rapidly.
The maximum thermal conductivity (1.94 W/m K) was observed in sandy soil. Clay loam soil had a lower thermal conductivity than sandy soil at all water contents and bulk densities studied. Thermal conductivity values reported here lie well within the range 0.3 to 2.25 W/m K for sandy soil, as given by Van Wijk (1963), and within the range 0.15 to 0.79 W/m K for loam soil, as given by Ghuman and Lal (1985). The values obtained for thermal conductivity are higher than the 0.59 W/m K obtained by Ghuman and Lal (1985) for sandy loam soil at a 10% moisture content. The differences in mineralogy and sand, silt, and clay fractions in their sandy loam soil may account for this variation. In the present study, higher values of thermal conductivity were obtained for the sandy soil than for the clay loam soil. The decrease of effective thermal conductivity with a decrease in grain size may be explained by the fact that as the grain size decreases, more particles are necessary for the same porosity, which means more thermal resistance between particles (Tavman, 1996). This suggests that clay loam soils with low thermal conductivities would exhibit larger surface temperature changes, compared with sandy or sandy loam under equal heat flux densities. If this were true, it could influence the successful raising of temperature-sensitive crops on the clayey soils in Jordan.
The effect of salt concentration in solution on thermal conductivity of the sandy and the clay loam soils are depicted in Fig. 7 and 8. Thermal conductivities of the soils decreased with increasing salt concentration in solution. These results are similar to those of Noborio and McInnes (1993). Sodium chloride caused less reduction in thermal conductivity than did CaCl2. On the basis of their experimental observations, Noborio and McInnes (1993) reported that NaCl caused less of a reduction, but not significantly, in thermal conductivity than did other salts. For our study, based on the individual data points obtained from heating and cooling data at different repetitions, it is observed that the NaCl solution caused a significantly greater reduction in the soil thermal conductivity than did the CaCl2 solution. This might be due to the fact that most of the sodium compounds have relatively higher solubility than calcium compounds. Shainberg and Otoh (1968) speculated that this greater reduction might attributed to the reduction in tactoids and the corresponding increase in platelets with an increased fraction of Na ions on the exchange sites. Values of thermal conductivity at a given solution content for the sandy soil were higher than those for the clay loam soil at all salt concentrations in solution. Noborio and McInnes (1993) stated that for soils with a significant amount of clay, flocculation and aggregation might be strongly influenced by the interactions of clay particles with salt ions. The less ordered the structure of clay (i.e., more flocculated), the lower the thermal conductivity.
Thermal conductivity of the clay loam soil decreased as the percentage (on weight basis) of organic matter increased in the soil samples. These results are depicted in Fig. 9, which shows that thermal conductivity was 0.17 W/m K at 30% organic matter content. Because of the lack of studies on the effect of organic matter on thermal conductivity of soils, we were unable to compare our results with others from previous studies. The reason for the lack of research in this area might be due to the fact that in a soil's natural state, the organic matter content is relatively fixed as it is in relative equilibrium with the climate and amount and type of biomass produced, as well as with the level of biological activity occurring in the soil. On the other hand, it can be argued that the amount of organic matter in a soil can be altered by cultural practices; e.g., conservation tillage.
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Conclusions
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Our results show that thermal conductivity varies with soil texture, water content, salt concentration, and organic matter content. For all soils studied, an increase in bulk density at a given moisture content increased thermal conductivity, and increasing moisture content at a given bulk density increased thermal conductivity. Loam and clay loam soils exhibit slight increases in thermal conductivity beyond a certain bulk density threshold. Clayey soil generally had lower thermal conductivity than did sandy soil. Thermal conductivity of sandy and clay loam soils decreased with increasing salt contents at a given water content with higher for the sandy soil at the same salt concentration. Our results support the suggestion of Noborio and McInnes (1993) that solution–clay interactions significantly affect the thermal conductivity by altering the microstructure of clayey soils. Thermal conductivity for clay loam soil decreased with increasing organic matter (peat moss) content.
Received for publication July 15, 1999.
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ABSTRACT
INTRODUCTION
