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J. Jpn. Soc. Soil Phys.
土壌の物理性No. 150, p.7 ∼ 10 (2022)

The evolving world of soil physics
Rien van Genuchten

This short essay is about the discipline of soil physics, how I became part of it, and where I think we are heading.
First, let us look at the term “Soil Physics”, which has been deﬁned in different ways. One standard (but circular)deﬁnition refers to soil physics as the application of physical principles to soils, while others suggest that soil physics
should refer to the state and transport of matter and energy in soils (e.g., Hillel, 1971). Whatever the exact deﬁnition,different topics have been associated with soil physics, starting with the early view that soil physics is an inherent part of
the agricultural curriculum with a focus on both laboratory and especially practical ﬁeld-scale applications. This would
include soil textural analysis, soil density and permeability measurements, water retention, and horizontal or vertical waterﬂow (including capillary rise). This early focus certainly is understandable since soil management, crop production andirrigation were the initial themes when agricultural operations started several thousand years ago. Excellent overviews ofearly soil physics from the 18th century and later are by Philip (1974) and Swartzendruber (1977). While the initial focus
was on agricultural issues, starting in the 1960s and 70s the focus broadened increasingly to more environmental issues
involving protection of our soil and water resources and global climate. This included then also more emphasis on thephysics part of “soil physics”, leading to more fundamental descriptions of soil as a living porous medium in which water,
heat and solutes operate, with or without vegetation (e.g., Sposito, 1986; Raats, 2001; Raats and van Genuchten, 2006).
Several textbooks at that time very much inﬂuenced my work, the most important being books by Kirkham and Powers(1972), Nielsen et al. (1972), and Taylor and Ashcroft (1972).
My own involvement with soil physics started in the late 1960s when I attended Wageningen University in the Netherlands to study irrigation and drainage, leading to a MS research report on drip irrigation (van Genuchten, 1971). I remember being very proud about that report and still refuse to delete it from my publication list, even though I do not have a copy
anymore. The report documented some drip (trickle) irrigation experiments carried our using a ﬁne- and a coarse-textured
soil, along with analyses of the developing moisture distributions using analytical solutions of the Richards equation as
derived by Philip. Not sure what to do next, I went to Las Cruces, New Mexico, where Peter Wierenga convinced me todo a PhD with him in soil physics. There I learned about chloride, pesticide, boron and pesticide transport processes inlaboratory soil columns, and how physical (e.g., aggregation) and chemical (e.g., kinetic sorption) nonequilibrium processes affect observed breakthrough curves. The PhD work resulted in several papers published in SSSAJ, the leading
soil physics outlet at the time (e.g., van Genuchten and Wierenga, 1976; van Genuchten et al., 1977).
After graduating in 1975, I spent 3 years as a postdoc with George Pinder at Princeton University looking at ﬁnite
element solutions of the Richards equation using Hermitian (smooth cubic) ﬁnite element basis functions. There I initiallystruggled with the numerical solutions when using Brooks and Corey (1964) type formulations of the unsaturated soil
hydraulic functions, which were not ﬁrst-order continuous at the air entry value. This caused me to look at alternative
formulations, leading to a Princeton research report (van Genuchten, 1978). I thought that that report was the end of it. But
I had shared a card-deck copy of the optimization program (SOHYP) I used for this work with several, including Jacob
Dane which was impressed and urged me to publish a paper about it. I initially hesitated, but ﬁnally submitted a paper
to SSSAJ in 1980 when I had accepted a position at the U.S. Salinity Laboratory in Riverside, California. The paper wasreturned by SSSAJ for major revisions: they told me the material was marginal, and that even the title was not original.
I nearly gave up, but in the end still revised things and resubmitted. The resulting paper (van Genuchten, 1980) becamevery popular later on, and very much changed my career; I became the presumed expert about multiphase soil hydraulicproperties, Pc-S curves, residual water contents, pore-size distribution models, and so-on (which I never felt relative to theDepartment of Earth Sciences, Utrecht University, Utrecht, Netherlands.
Department of Nuclear Engineering, Federal University of Rio de Janeiro, Brazil.

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pesticide transport work I did for my PhD). In any case, the hydraulic functions are now used in many unsaturated ﬂow
studies, in environmental and global climate research, in studies about fuel cells, fabrics, bio-membranes, ink inﬁltration
processes, and other studies, gathering some 30,000 citations or more at present. My goodness!!
I am reﬂecting on all this in that one never knows (at least I did not) how well one’s research may ﬁt into the broader
picture. In that sense soil physics has changed tremendously the last 30 or 40 years. Change has come in several aspects,
especially the need to address environmental challenges, soil and groundwater pollution, the lack of fresh water, soil
salinization, climate change, and many other issues. Contaminants include a range of agricultural and industrial pollutants, pollutants generated by mining and milling operations (including radioactive materials), viruses and bacteria, anda suite of emerging chemicals (pharmaceuticals, disinfection byproducts, nanomaterials, microplastics, PFAS, personalcare products, microplastics, and more). This has caused soil physicists to focus on a much broader array of problems than
the narrow ﬁeld of “soil physics”. The soil physics ﬁeld certainly has become much more multidisciplinary. Moreover,research includes now spatial scales covering molecular scales, pore scales, core scales, plot scales, ﬁeld scales, regional
scales and even global scales. I have certainly seen this in my own activities. In that sense I am not sure anymore if Ishould call myself a soil physicist, a soil scientist, a soil hydrologist, an agricultural or civil engineer, or maybe just ahydrologist.
Publications by soil physicists now also appear in a much broader set of outlets, changing from mostly soil sciencejournals (e.g., SSSAJ, Soil Science) to journals organized by other societies (AGU, GSA, EGU, ASCE, ACS, …), andjournals operated independently by such publication companies as Elsevier and Wiley, including to me far too many new
open-access journals outside the publication mainstream. The number of papers now being published is truly astonishing,
and to me nearly depressive. So many papers are now being published each year that you must be a full-time reader of
thousands of papers in hundreds of journals, or you have to limit yourself to only a few and concentrate on doing yourwork and writing it up. It seems that one cannot be both (reader and writer). In that sense everything is accelerating,the bad versus the good: the environmental problems this planet faces, versus the number of scientists that work on theproblems, their creativity and their enormous output. Still, one thing that worries me is the stress that all this causes onespecially young scientist in the academic world. When I started my career, one good peer-reviewed publication per year
was thought to be sufﬁcient in many places. Now researchers are expected to have several (three, four, ﬁve, ...) papers
to be published in high-impact journals, with the publications needing to have many citations. Beginning academicians
are at the same time often pressured to submit research proposals and have them approved. This even though the successrate at present is often less than 25 %, while the proposal sometimes also needs to include enormous overhead costs.
Additionally, beginning scientists often are expected to serve as editor or associate editor of one or more journals, serve
on a range of internal and/or external committees, and show activities across disciplinary, cultural and socio-economic
boundaries (with which I actually agree), all this often at a time in their life when many start a new family. Clearly thepressure to perform must be overwhelming.
Because our planet is faced with so many environmental problems, which area is best to address in one’s research? I
regularly get questions about this: what topics should I study to have impact; where to begin? My answer is simple: Do
the things you enjoy most, albeit of course within the structural boundaries of your department of employer. And interact
with your friends and colleagues that pursue the same topics and possible solutions. In this sense I was extremely fortunate
in that I could work nearly always on the conceptual-mathematical part of things, with lots of help from others. I enjoyedthings so much that there was no difference for me in working and studying a particular topic at work, or playing with theproblems at home on my free time. There were no boundaries in this. For example, I could work on analytical solutions,alone or with my friends and colleagues (e.g., Leij and van Genuchten, 2002; Toride et al., 1999; van Genuchten andWagenet, 1989; van Genuchten et al., 2012, 2013), or do the same for numerical solutions addressing variably-saturated
ﬂow and contaminant or heat transport problems (van Genuchten et al., 1977; Šimůnek et al., 2008, 2009, 2016), The
latter included documentation of the codes into user-friendly software, notably the RETC, STANMOD and HYDRUS
codes (https://www.pc-progress.com/en/Default.aspx).
Looking back now on these publications, it may seem to have all been relatively straightforward; but they were not!
Last month I scanned through some of my old notes on deriving the analytical solutions. I would not have the patience
anymore to do this again. For example, some of the solutions contained unusual functions, such as complementary error(erfc) functions with complex arguments (van Genuchten, 1985) or modiﬁed Bessel (K0 , I0 , K1 , etc.) and Kelvin (Ber, Bei,Ker, Kei, etc) functions (van Genuchten et al., 1984). I remember needing a lot of time and patience to mathematicallyverify and program those functions correctly. This since I noticed that several standard textbooks at the time, such as
the popular text by Abramowitz and Stegun (1970) I often used, sometimes contained small errors or misprints. Lots

メッセージ9of patience and persistence were indeed needed to derive the solutions and associated computer programs. But then,patience and resilience are generally needed with any research project, whether mathematically as I always enjoyed, orexperimentally, or otherwise.
To conclude, I want to thank the many giants in the ﬁeld with who I could work and interact over the years. My ultimate
advice to beginning scientists and engineers: Do the research you enjoy most within the conﬁnes of your assignment asa student or scientist in your department, or as a professional at work, collaborate as much as possible with others, andfreely share your work if possible. But if you do not enjoy your work, give it up and change to another path in life.

References
Abramowitz, M., and Stegun, I.A. (eds.) (1970): Handbook of mathematical functions. Dover, New York.
Brooks, R.H., and Corey, A.T. (1964): Hydraulic properties of porous media. Hydrology paper 3. Colorado State Univ., Fort Collins,CO.
Hillel, D. (1971): Soil and water: Physical principles and processes. Academic Press, Inc , New York and London.
Kirkham, D. and Powers, W.L. (1972): Advanced soil physics. John Wiley and Sons, Inc., New York.
Leij, F.J., and van Genuchten, M.Th. (2002): Solute transport. In: A.W. Warrick (ed.), Soil Physics Companion, pp. 189–248. CRCPress, Boca Raton, FL.
Nielsen, D.R., Jackson, R.D., Cary, J.W. and Evans, D.D. (eds.) (1972): Soil water. Am. Soc. Agron., Madison, Wisconsin.
Philip, J.R. (1974): Fifty years progress in soil physics. Geoderma, 12: 265–280.
Raats, P.A.C. (2001): Developments in soil-water physics since the mid 1960s, Geoderma, 100: 355–387.
Raats, P.A.C. and van Genuchten, M.Th. (2006): Milestones in soil physics, Soil Sci., 171S1: S21–S28.
Šimůnek, J., van Genuchten, M.Th., and Šejna, M. (2008): Development and applications of the HYDRUS and STANMOD software
packages and related codes. Vadose Zone J., 7, doi:10.2136/vzj2007.0077.
Šimůnek, J., Jacques, D., Twarakavi, N.K.C. and van Genuchten, M.Th. (2009): Selected HYDRUS modules for modeling subsurface ﬂow and contaminant transport as inﬂuenced by biological processes at various scales, Biologia, 64(3): 465–469,
doi:10.2478/s11756-009-0106-7.
Šimůnek, J., van Genuchten, M.Th., and Šejna, M. (2016): Recent developments and applications of the HYDRUS computer
software packages. Vadose Zone J., 15, doi:10.2136/vzj2016.04.0033.
Sposito, G. (1986): The “physics” in soil water physics. Water Resour. Res., 22: 83–88.
Swartzendruber, D. (1977): Soil physics — Reﬂections and perspectives. In: M.D. Thorne et al., ASA Special Publ.,
doi:10.2134/asaspecpub30.c6.
Taylor, S.A. and Ashcroft, G.L. (1972): Physical edaphology. W.H. Freeman and Co., San Francisco.
Toride, N., Leij, F.J. and van Genuchten, M.Th. (1999): The CXTFIT code for estimating transport parameters from laboratory orﬁeld tracer experiments. Version 2.1. Research Report No. 137, U.S. Salinity Laboratory, USDA, ARS, Riverside, CA. 121 p.
van Genuchten, M.Th. (1971): Trickle irrigation. Research Report, Department of Irrigation, Agricultural University, Wageningen,Netherlands (in Dutch). 58 p.
van Genuchten, M.Th. (1978): Calculating the unsaturated hydraulic conductivity with a new, closed form analytical model. Research Report No. 78-WR-8, Water Resources Program, Dept. of Civil Engineering, Princeton Univ., Princeton, NJ, 63 p.
van Genuchten, M.Th. (1980): A closed form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc.
Am. J., 44(5): 892–898.
van Genuchten, M.Th. (1985): Convective dispersive transport of solutes involved in sequential ﬁrst order decay reactions. Computers & Geosciences, 11(2): 129–147.
van Genuchten, M.Th. and Wierenga, P.J. (1976): Mass transfer studies in sorbing porous media: I. Analytical solutions. Soil Sci.
Soc. Am. J., 40(4): 473–480.
van Genuchten, M.Th. and Wagenet, R.J. (1989): Two site/two region models for pesticide transport and degradation: Theoreticaldevelopment and analytical solutions. Soil Sci. Soc. Am. J., 53(5): 1303–1310.
van Genuchten, M.Th., Wierenga, P.J. and O’Connor, G.A. (1977): Mass transfer studies in sorbing porous media: III. Experimental
evaluation with 2,4,5 T. Soil Sci. Soc. Am. J., 41(2): 278–285.
van Genuchten, M.Th., Pinder, G.F. and Frind, E.O. (1977): Simulation of two dimensional contaminant transport with isoparametricHermitian ﬁnite elements. Water Resour. Res., 13(2): 451–458.
van Genuchten, M.Th., Tang, D.H. and Guennelon, R. (1984): Some exact solutions for solute transport through soils containing
large cylindrical macropores. Water Resour. Res., 20(3): 335–346.

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van Genuchten, M.Th., Šimůnek, J., Leij, F.J., Toride, N. and Šejna, M. (2012): STANMOD: Model use, calibration and validation.
Trans., ASABE, 55(4): 1353–1366.
van Genuchten, M.Th., Leij, F.J., Skaggs, T.H., Toride, N., Bradford, S.A. and Pontedeiro, E.M. (2013): Exact analytical solutions
for contaminant transport in rivers. 2. Transient storage and decay chain solutions. J. Hydrol. Hydromech., 61, 2013, 3, 250–259,
doi:10.2478/johh-2013-0032.

(Most of our papers can be downloaded from the personal pages on the HYDRUS website, http://www.pc-progress.com/
en/Default.aspx?rien-van-genuchten)