ӻӿӺ ஼ၟആ

J. Jpn. Soc. Soil Phys.
土壌の物理性No. 154, p.5 ∼ 10 (2023)

Pray, listen, and share ideas with others
to enhance your soil physics research

Robert HORTON

Soil science, including soil physics, is a relatively young discipline that seeks to deepen the fundamental understandingof soil properties and processes and to develop practical management skills required to use soils sustainably. Most research
efforts in the twentieth century centered on agricultural soils. Late in the twentieth century, the focus of soil science began
to shift from maximizing production (food, feed, and ﬁber) to sustainable production that sought to balance productivity
with environmental stewardship of the soil resource. It is clear that soils serve not only agricultural uses but much more. Asawareness increases on the importance of soils, the responsibility to care for soils as good stewards also increases. Because
soils are a prominent natural resource connected to food (world hunger), water (quality and quantity), climate (surface
energy balance and climate change), bioenergy, and transportation (road foundation), the opportunities and necessity forsoil physics research will increase. My thoughts about soils as a vital natural resource are found at the following link —https://youtu.be/CDJE45YPMu8.
Seeking a career in science research is a risky choice, because research involves pushing the knowledge envelope into
the unknown with no guarantees for success. To reduce risk and overcome anxiety, it is important to have multiple sourcesof motivation and inspiration to support a research career. Common sources of motivation and inspiration are direct
observations of the natural world, teachers and mentors, favorite journal authors, and attending conferences. These have all
helped me in my career. I also beneﬁtted from two additional sources that receive less attention in our science community— prayer, and having a trusted group of colleagues that use ‘brain-storming’ discussions (thinking and planning activities)
to develop new ideas. I will ﬁrst focus my comments on how faith in God and praying helped me as a scientist, and then I
will provide two examples on how discussions and collaborations with others helped to enhance my soil physics researchcontributions.
Importance of Faith and Prayer
Historically, early ‘thinkers’ or intellectuals who established our science disciplines were mostly people of faith. The
integration of their faith with reason led them to explore the beauty and wonder of creation in order to better know andserve God and to better serve humanity. Blaise Pascal, whom the S.I. units of pressure, pascals (Pa), are named after, wasa dedicated scientist and person of faith. Pascal argued that a wise person should seek to believe in God. If God does notexist, such a person will have only a ﬁnite loss (time or money), whereas if God does exist, they can receive inﬁnite gains(eternity in Heaven) and avoid inﬁnite losses (eternity in Hell). Pascal demonstrated that faith and reason complement
one another, because it is reasonable to have faith, but the faith must be reasonable.
Many scientists, whether they are persons of faith or not, do not regularly pray for inspiration. I was such a person
early in my career. I did not see the consistency of faith and reason. I went to church to pray, while at work I used myintellectual faculties. Aspects of my life were fragmented, with my work, family life, and spiritual life being somewhat
independent of each other. Work garnered most of my attention. Things changed early in my academic career, whentragedy hit my family and deeply affected my life. My 1-year old daughter died in an accident. This shook me to thecore. In my grief I turned to God, and over a period of time I began to experience the presence of God close to me. Witha growing relationship with God, I decided to strive for a healthy work-life balance by prioritizing faith, family, and work
Agronomy Department, Iowa State University, Ames, Iowa 50011 USA.

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(2023)(see https://doi.org/10.2134/csa2016-61-7-12). I continued to work hard, but I did not spend all of my waking time atwork.
Instead of partitioning my faith and science activities, I integrated them, striving to be the same whole person at home,
at church, and at work. Having God central to my life inspires, informs, and enriches all the aspects and relationships in
my life. It produces wonders for my work motivation by amplifying my desire to honor God and serve him by caring for
creation and caring for humanity. I regularly pray for guidance and inspiration in my research and in my interactions withothers. For example, to help develop the next generation of scientists, I ﬂipped my mentoring style to serve my students
rather than asking my students to serve me (see https://doi.org/10.1002/csan.20525).
I often begin my work day with a prayer composed by St. Thomas Aquinas, “Creator of all, source of all light andwisdom, you made everything. Let a ray of your light penetrate the darkness of my mind. Take from me all darkness of sin
or ignorance. Give me a keen understanding, a good memory, and the ability to grasp things correctly and completely.
Help me to be exact in my explanations and to express myself thoroughly and clearly. Show me how to begin my work,
direct my progress, and help me complete it. I ask this through Christ our Lord. Amen.”
Listening to and Sharing Ideas with Others
Another great source of motivation and inspiration for me has been group discussion. Whether planning, designing,
reviewing data, analyzing, interpreting, sharing ideas, thinking out loud, asking questions, pondering, or seeking guidance— group discussions are helpful. Ideas emerge from group discussions or ‘brain storming’ sessions. There is a table
outside of my Iowa State University ofﬁce where many group discussions have occurred. We just refer to it as ‘the table,’as in let’s meet at ‘the table.’ I have fond memories of many great discussions held at ‘the table,’ because many of theshared ideas enhanced my research collaborations. Everyone (faculty colleague and student alike) was free to express
their thoughts, ideas, concerns, and encouragements at ‘the table.’ Early career scientists, especially graduate students
and post-docs, taught me several things at these discussions. There are many success stories that resulted from fruitful
discussions at ‘the table,’ but I will limit myself to only share two stories — the automated tension inﬁltrometer and thethermo-TDR sensor. These are two examples depicting how good fruit can emerge from listening to and sharing ideas
with others.
Throughout my career, I have worked on various aspects of soil hydrology. Soil water inﬁltration, soil water storage, soil
water drainage, plant water relationships, and soil water evaporation. My research focus area is coupled heat, water, andchemical transfer in soils. I collaborated on a device, automated tension inﬁltrometer, to measure saturated and unsaturated
hydraulic conductivity in ﬁeld soils. It also determines soil pore size distribution and solute transport properties. Because
we did not have ways to measure energy and mass ﬂuxes in soil, I collaborated with others to develop ways to quantify
soil heat and mass ﬂuxes. A heat pulse sensor (reported by Gaylon Campbell, Keith Bristow, and Gerard Kluitenberg)
was combined with a TDR sensor (reported by Clarke Topp) to produce a thermo-TDR sensor. Analyzing thermo-TDR
measurements has enabled in situ determinations of sensible heart ﬂux, soil liquid water ﬂux, vapor ﬂux (evaporation),and transient ice formation and ice melting. These measurements are possible in rigid soils, and also in non-rigid soils
that experience dynamic porosity and bulk density. I think these instruments will assist future investigations of coupledheat and water processes at the land surface and in the subsurface. They will contribute to future energy balance and waterbalance discoveries.
The story behind developing the automated tension inﬁltrometerIn 1987 I was working at Iowa State University when a stranger knocked on my ofﬁce door. He introduced himself asMark Ankeny, a crop science graduate student. He told me that he was interested in plant roots, and he asked me if I had
any ideas on how to make measurements that would enable him to estimate how plant roots would distribute and grow ina particular soil. I told him that I did not know with certainty which soil measurements would provide the best estimatorsfor root development and growth. We discussed a series of measurements including cone penetration resistance, pore size
distribution derived from water retention curves, air permeability, and saturated and unsaturated hydraulic conductivity. I
recommended that he explore ﬂuid ﬂow measurements because ﬂuid ﬂow occurred in connected soil pores which might
relate well to how roots grow in connected pores. I had a homemade tension (disc) inﬁltrometer in my laboratory, andsince I was not using it at the time, I loaned it to Mark and asked him to make some saturated and unsaturated water ﬂow
measurements on soils to see if he could correlate them to root growth. He thanked me and left my ofﬁce. When he left, I
was not sure if I would ever see him or the tension inﬁltrometer again.
Several months later, Mark returned to my ofﬁce. He said that he had an idea on how to improve the design of thetension inﬁltrometer that I had loaned him. I listened to him as he described how adding pressure transducers to theメッセージ7

inﬁltrometer would enable automating the measurements. By connecting the transducers to a datalogger, saturated andunsaturated inﬁltration rates versus time could be measured directly. In Mark’s opinion, the inﬁltration data quality would
be greatly improved with the new design. After he explained his idea to me, I completely agreed with him. We hired
a local machine shop to build several newly designed automated tension inﬁltrometers. Ankeny et al. (1988) described
the automated tension inﬁltrometer design, which provided a large improvement in the quality of inﬁltration with timemeasurements. Ankeny et al. (1990) reported the effects of tillage and agricultural equipment trafﬁc on unsaturated
inﬁltration rates measured with the automated tension inﬁltrometer.
At ﬁrst, the automated tension inﬁltrometer simply improved the quality of inﬁltration measurements, and it was used
to quantify the effects of tillage and trafﬁc on soil water inﬁltration. However, it did not yet provide any new information on saturated and unsaturated hydraulic conductivity, nor did it address the original question on how to use soil
measurements to estimate potential root growth. I discussed these points with Mark, and I told him that I thought thehigh quality inﬁltration measurements could be used to quantify ‘pore size distribution’ and also to determine saturated
and unsaturated hydraulic conductivity. Mark felt quite challenged by my comments, in part because he did not have astrong background in soil physics or in mathematical analysis. Never-the-less he took my comments to heart and began
to think seriously about how to further interpret the tension inﬁltrometer measured inﬁltration with time curves. He had
an idea that measuring steady state inﬁltration ﬂuxes at a series of tensions could produce the saturated and unsaturated
hydraulic conductivity values for a particular ﬁeld. He came to my ofﬁce and sketched out his idea on a piece of paper.
Although his drawing was cartoon-like, I could follow his thought process, and I was convinced that his idea had merit. I
developed the mathematical steps that matched his conceptual idea, and we had a new method to determine saturated andunsaturated hydraulic conductivity from tension inﬁltration measurements (Ankeny et al., 1991). The automated tension
inﬁltrometer method became one of the most widely applied ways to determine ﬁeld hydraulic conductivity values. Later,a post-doc named Binayak Mohanty used tension inﬁltrometer measurements (Mohanty et al., 1996) to estimate soil poresize distributions, which brought us back full-circle to the original question Mark had asked on how to characterize soil
pores for plant root growth.
Jaynes et al. (1995) reported that a tension inﬁltrometer could be used to determine solute transport properties ofthe mobile-immobile model. They applied a suite of tracers to soil via a tension inﬁltrometer. Casey et al. (1998)
measured hydraulic conductivity and solute transport properties of ﬁeld soil. More recently, I hosted a Japanese scientist,
Junko Nishiwaki, who used tension inﬁltrometer measurements to characterize time changes in soil hydraulic conductivity
following tillage (Nishiwaki and Horton, 2020). Junko’s work clearly shows that soil bulk density increases with time
following tillage, and soil hydraulic properties change as bulk density changes. I think that the impact of soil bulk density
temporal changes on soil hydraulic and thermal properties will be increasingly studied in the future. We must reject theassumption that soil physical properties are constant in time. Developing a sensor that measures transient, in situ bulk
density values will provide new opportunities for comprehensive studies of coupled heat and mass transfer in non-rigid
soils. The thermo-TDR sensor is such a sensor.
The story behind developing the thermo-TDR sensor
In 1997 I was working in my ofﬁce when a man, Tusheng Ren, knocked on my ofﬁce door. I welcomed him into myofﬁce. He told me that he had recently completed his PhD program in Canada, and that he moved to Iowa to be withhis wife while she completed her graduate studies at Iowa State University. He said that he wanted to work with me
and asked if I could hire him as a post-doc. I told him that I had recently hired a post-doc, Kosuke Noborio, and that
I did not have funding for another post-doc position. Dr. Ren then asked me if he could work with me for free — nosalary. I did not want to take advantage of Dr. Ren, so rather than have him work for free, we agreed to ﬁrst meet anddiscuss possible research ideas. Meanwhile, I would acquire some funds to hire him, in case we decided on a suitable
collaborative research study. The research idea that emerged from our discussions was to try to improve a sensor that wasﬁrst described by Noborio et al. (1996). The sensor, which we later named the thermo-TDR sensor, was a combination ofa heat pulse sensor and a TDR sensor. It was able to measure soil thermal properties and soil electrical properties at thesame scale. The improved sensor design was reported by Ren et al. (1999), Ren et al. (2003), and Peng et al. (2019).
Initially, sensor measurements were used to characterize soil thermal properties, water content, and bulk electricalconductivity, but we hoped that additional soil properties could be derived from these measurements. Dr. Ren, Dr.
Noborio and I had many discussions at ‘the table.’ We held several ‘brainstorming’ sessions, where we allowed ourimaginations to explore potential uses of the new sensor. We discussed possible experiments to learn new things abouttransient soil properties and processes. Many of the ideas we considered at ‘the table’ discussions were actually studied
over the next two decades by a host of our subsequent graduate students and colleagues.

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Ochsner et al. (2001) described how thermo-TDR measurements could be used to simultaneously determine soil watercontent and soil bulk density. Soil water content was determined from the TDR measurement of apparent dielectric
constant, and bulk density was determined from the heat pulse measurement of volumetric heat capacity, which was afunction of water content and bulk density. Measuring thermal and electrical properties together enabled the determinationof water content and bulk density. Liu et al. (2013) described a way to self-calibrate sensors in situ. Liu et al. (2014) used
the method to measure in situ transient water content and bulk density. The ability to measure transient water content andbulk density opened the possibility to determine their effects on other soil physical properties, such as water retention,
hydraulic conductivity, and thermal properties. Initial studies were described by Zhang et al. (2018) on water retention,
Kool et al. (2019) on hydraulic conductivity, and Tong et al. (2020) on thermal properties. Much future work is neededon related topics.
Have you noticed how few methods exist to measure heat and mass ﬂuxes in soil? I am not aware of many practicalmethods to determine ﬂuxes in soil. It is somewhat surprising that soil physicists have not developed methods to directly measure soil water ﬂuxes. Most water ﬂux estimates require a known value of hydraulic conductivity. Because
hydraulic conductivity varies several orders of magnitude over the normal range of ambient soil water contents, estimatesof hydraulic conductivity are crude and noisy, and thus soil water ﬂux estimates are rough and more qualitative thanquantitative. Developing a reliable method to quantify soil water ﬂux without the need for hydraulic conductivity would
represent a much needed breakthrough for soil physics. In a progressive step by step manner, the thermo-TDR sensor
has been shown to be able to measure soil heat ﬂux, liquid water ﬂux, vapor ﬂux (evaporation), and latent heat ﬂux forfreezing and thawing ice contents. The ﬂux measurement approaches are new and should be greatly explored and appliedin future heat and mass transfer investigations. Here, I will brieﬂy describe how to determine ﬂuxes in soil.
Heat ﬂux plates are commonly used to measure soil heat ﬂuxes. However, heat ﬂux plates do not measure surface heat
ﬂuxes, because they must be buried in the soil. They cause problems by blocking liquid water movement and water vapor
movement, thus distorting the soil water distribution near the plates. Because water content effects soil thermal properties,
distortions in water content cause distortions in the temperature and heat ﬂux distributions. Ochsner et al. (2006) andPeng et al. (2017) reported that heat pulse measurements of soil thermal conductivity used in combination with ambient
temperature gradients could accurately measure soil heat ﬂux. The heat pulse sensors can be buried at shallow depths very
near to the soil surface, because they provide minimal interference to liquid and vapor water movement. Thus the heatpulse sensor has advantages over heat ﬂux plates by measuring heat ﬂux very near to the soil surface, which is importantfor surface energy balance studies. Additional future studies and applications of these measurements are warranted.
The soil physics community is in need of sensors to measure soil liquid water ﬂux. Wang et al. (2002) showed how heatpulse sensor measurements could be used to determine soil water inﬁltration ﬂux. Their mathematical analysis showedthat heat transfer could be used as a proxy for water ﬂux. A three-needle heat pulse sensor was placed in soil with an upper
needle positioned upstream from the center heater needle and a lower needle positioned downstream from the center heater
needle. Once a pulse of heat was applied to the center needle, heat transferred toward the upstream needle by conduction
alone, while conduction and convection carried heat towards the downstream needle. Thus, temperature increases at thedownstream needle were larger than those at the upstream needle. The logarithm of the ratio of downstream temperature to
upstream temperature was reported to be proportional to the soil water ﬂux density. At last, we had a method to determine
soil water ﬂux. Gao et al. (2006) veriﬁed the Wang et al. (2002) method under controlled laboratory conditions. Tian etal. (2018) presented a thermo-TDR based method to determine upward liquid ﬂuxes in response to soil water evaporation.
They veriﬁed the new method by combining their ﬁeld measured soil water ﬂuxes with water potential measurements todetermine unsaturated hydraulic conductivity values, which were similar to independently measured laboratory values.
Both of these soil water ﬂux methods are available for further veriﬁcation and application under ﬁeld conditions.
For quite some time it has been possible to estimate soil water evaporation rates from wet soil surfaces (Stage 1 conditions). However, it is much more challenging to estimate soil water evaporation rates from drying soil surfaces (Stage 2
and 3 conditions). Heitman et al. (2008) presented a sensible heat balance method based on heat pulse sensor measurements to estimate Stage 2 and 3 soil water evaporation rates. The Heitman et al. (2008) method determined evaporation
as a function of depth and time. Thus, they provided a way to determine water vapor ﬂuxes in soil. Wang et al. (2017)
veriﬁed the sensible heat balance method by comparing heat pulse sensor determined daily evaporation values with ﬁeld
weighing lysimeter soil water evaporation values. The daily evaporation values from both methods matched well. Thissoil water vapor ﬂux method is available for further applications under ﬁeld conditions.
The liquid water ﬂux and water vapor ﬂux methods perform well in unfrozen soil conditions. However, some soils
undergo periods of freezing and thawing. Therefore, it is important to develop a method to determine soil ice contentsメッセージ9during times of freezing and thawing. Kojima et al. (2013) performed a numerical study to evaluate the potential of asensible heat balance method to estimate soil ice contents during freezing and thawing conditions. Based on the numericalinvestigations, the sensible heat balance method performed well. Kojima et al. (2016) tested the sensible heat balancemethod to determine soil ice contents in a laboratory study. They used heat pulse sensor measurements to make thesensible heat balance calculations. Because heat of fusion was smaller than heat of vaporization, soil ice content wasmore difﬁcult to determine than soil water evaporation, and the sensible heat balance method was limited to temperatures
between −5 and 0 ◦ C. At colder temperature values, changes in volumetric heat capacity were used to estimate changes
in soil ice content. Tian et al. (2020) provided additional insights on using thermo-TDR sensors to determine soil ice
contents. Additional work is needed to further verify and develop methods to determine transient soil ice contents.
In closing, I hope that you, my soil physics colleagues, realize that we have much more soil physics to learn (properties,
processes, and applications) in the future than what we currently know. The expanding range of interest in and opportunities for soils makes this an exciting time period for soil science in general and soil physics in particular. I have described
two examples, tension inﬁltrometer and thermo-TDR sensor, of how I got started and followed through on unexpected, yetvery fruitful, research topics. Notice that in both examples my involvement began via discussions with young scientists.
Initial research ideas emerged from our discussions. After making some early advancements, additional discussions led to
new ideas for applications that we did not anticipate at the beginning. So, if you listen to others and discuss possibilities,
research ideas will emerge. Also, notice that there is much more research that can be done with tension inﬁltrometers andthermo-TDR sensors. They are instruments that enable measurements of soil properties and ﬂuxes, but it will take yourimagination to involve them in research studies to further explore coupled heat and mass transfer in soil.
In summary, for motivation and inspiration, I recommend that you pray, observe, listen, think, and engage in group
discussions.

Fig. 1 Group discussions at ‘the table’ (Iowa State University, left photo) and at Meiji University (right photo).

References

Ankeny, M.D., Ahmed, M., Kaspar, T.C. and Horton, R. (1991): A simple ﬁeld method for determining unsaturated hydraulicconductivity. Soil Sci. Soc. Am. J., 55: 467–470.
Ankeny, M.D., Kaspar, T.C. and Horton, R. (1988): Design for an automated tension inﬁltrometer. Soil Sci. Soc. Am. J., 52:
893–896.
Ankeny, M.D., Kaspar, T.C. and Horton, R. (1990): Characterization of tillage effects on unconﬁned inﬁltration measurements. Soil
Sci. Soc. Am. J., 54: 837–840.
Casey, F.X., Logsdon, S.L., Horton, R. and Jaynes, D.B. (1998): Measurement of ﬁeld soil hydraulic and solute transport parameters
as a function of water pressure head. Soil Sci. Soc. Am. J., 62: 1172–1178.
Gao, J., Ren, T. and Gong, Y. (2006): Correcting wall ﬂow effect improves the heat-pulse technique for determining water ﬂux insaturated soils. Soil Sci. Soc. Am. J., 70: 711–717.

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Heitman, J.L., Xiao, X., Horton, R. and Sauer, T.J. (2008): Sensible heat measurements indicating depth and magnitude of subsurface
soil water evaporation. Water Resour. Res., 44: W00D05, doi:10.1029/2008WR006961.
Jaynes, D.B., Logsdon, S.D. and Horton, R. (1995): Field method for measuring mobile/immobile water content and solute transfer
rate coefﬁcient. Soil Sci. Soc. Am. J., 59: 352–356.
Kojima, Y., Heitman, J.L., Flerchinger, G.N. and Horton, R. (2013): Numerical evaluation of a sensible heat balance method todetermine rates of soil freezing and thawing. Vadose Zone J., 12: vzj2012.0053.
Kojima,Y., Heitman, J.L., Flerchinger, G.N., Ren, T. and Horton, R. (2016): Sensible heat balance estimates of transient soil ice
contents. Vadose Zone J., 15: vzj2015.10.0134.
Kool, D., Tong, B., Tian, Z., Heitman, J.L., Sauer, T.J. and Horton, R. (2019): Soil water retention and hydraulic conductivity
dynamics following tillage. Soil Till. Res., 193:95-100, doi:10.1016/j.still.2019.05.020.
Liu, G., Wen, M., Chang, X., Ren, T. and Horton, R. (2013): A self-calibrated dual probe heat pulse sensor for in situ calibrating
the probe spacing. Soil Sci. Soc. Am. J., 77: 417–421.
Liu, X. Lu, S., Horton, R. and Ren, T. (2014): In situ monitoring of soil bulk density with a thermo-TDR sensor. Soil Sci. Soc. Am.
J., 78:400-407.
Mohanty, B.P., Horton, R. and Ankeny, M.D. (1996): Inﬁltration and macroporosity under a row crop agricultural ﬁeld in a glacial
till ﬁeld. Soil Sci., 161: 205–213.
Nishiwaki, J. and Horton, R. (2020): Temporal changes in soil hydraulic conductivity in saturated and unsaturated ﬁelds. Paddy
Water Environ., 18: 677–686, doi:10.1007/s10333-020-00810-x.
Noborio, K., McInnes, K.J. and Heilman, J.L. (1996): Measurements of soil water content, heat capacity, and thermal conductivity
with a single TDR probe. Soil Sci., 161: 22–28.
Ochsner, T.E., Horton, R. and Ren, T. (2001): Simultaneous water content, air-ﬁlled porosity, and bulk density measurements withthermal-time domain reﬂectometry. Soil Sci. Soc. Am. J., 65: 1618–1622.
Ochsner, T.E., Sauer, T.J. and Horton, R. (2006): Field tests of the soil heat ﬂux plate method and some alternatives. Agron. J., 98:
1005–1014.
Peng, W., Lu, Y., Xie, X., Ren, T. and Horton, R. (2019): An improved thermo-TDR technique for monitoring soil thermal properties,
water content, bulk density, and porosity. Vadose Zone J., 18: 190026, doi:10.2136/vzj2019.03.0026.
Peng, X., Wang, Y., Heitman, J., Ochsner, T., Horton, R. and Ren, T. (2017): Measurement of soil-surface heat ﬂux with a multineedle heat-pulse probe. Eur. J. Soil Sci., 68: 336–344, doi:10.1111/ejss.12421.
Ren, T., Noborio, K. and Horton, R. (1999): Measuring soil water content, electrical conductivity, and thermal properties with athermo-TDR probe. Soil Sci. Soc. Am. J., 63: 450–457.
Ren, T., Ochsner, T.E. and Horton, R. (2003): Development of thermo-time domain reﬂectometry for vadose zone measurements.
Vadose Zone J., 2: 544–551.
Tian, Z., Kool, D., Ren, T., Horton, R. and Heitman, J.L. (2018): Determining in-situ unsaturated soil hydraulic conductivity at aﬁne depth scale with heat pulse and water potential sensors. J. Hydrol., 564: 802–810, doi:10.1016/j.jhydrol.2018.07.052.
Tian, Z., Kojima, Y., Heitman, J.L., Horton, R. and Ren, T. (2020): Advances in thermo-time domain reﬂectometry technique:
Measuring ice content in partially frozen soils. Soil Sci. Soc. Am. J., 84: 1519–1526, doi:10.1002/saj2.20160.
Tong, B., Kool, D., Heitman, J.L., Sauer, T.J., Gao, Z. and Horton, R. (2020): Thermal property values of a central Iowa soil asfunctions of soil water content and bulk density or of soil air content. Eur. J. Soil Sci., 71: 169–178, doi:10.1111/ejss.12856.
Wang, Q., Ochsner, T.E. and Horton, R. (2002): Mathematical analysis of heat pulse signals for soil water ﬂux determination. Water
Resour. Res., 38: 10.1029/2001WR1089.
Wang, Y., Ochsner, T., Heitman, J., Horton, R., Xue, X. and Ren, T. (2017): Weighing lysimeter data conﬁrm the accuracy and precision of the heat-pulse technique for measuring daily soil evaporation. Soil Sci. Soc. Am. J., 81: 1074–1078,
doi:10.2136/sssaj2017.02.0049n.
Zhang, M., Lu, Y., Heitman, J., Horton, R. and Ren, T. (2018): Temporal changes of soil water retention behavior as affected bywetting and drying following tillage. Soil Sci. Soc. Am. J., 81: 1288–1295, doi:10.2136/sssaj2017.01.0038.