A brief reflection on the use and application of these terms in Soil Science
Luiz Francisco da Silva Souza Filho1, Maria Eugenia Ortiz Escobar2
It has become increasingly common in scientific publications – including journal articles, conference abstracts, and posters – to use the term physicochemical to describe datasets that include particle-size distribution, soil and particle density, aggregation and aggregate stability, porosity, hydraulic conductivity, pH, exchangeable cations, potential acidity, base saturation, and related soil properties.
But is this use of the term physicochemical actually appropriate? Are all the attributes presented in these studies – often used as indicators of soil quality and soil health – truly physicochemical? Does the simple fact of measuring both physical and chemical attributes justify grouping them under a single designation as physicochemical attributes?
It is also worth noting that widely used expressions such as soil health carry conceptual – and, to some extent, metaphorical – meanings that are not always explicitly acknowledged. Although the term is now well established, particularly as an integrative concept encompassing the physical, chemical, and biological functioning of soils, it does not have a single universally accepted definition and may therefore be interpreted differently depending on the context. In this sense, much like the discussion proposed here regarding the terms physical, chemical, and physicochemical, the concept of soil health also benefits from careful and critical use whenever conceptual precision is sought in scientific communication.
With this in mind, we propose a reflection on the appropriate use of the terms physical, chemical, and physicochemical in Soil Science, particularly with respect to the attributes most commonly evaluated in soil studies. Rather than offering definitive answers, our intention is to encourage discussion within the scientific community and, hopefully, contribute to a broader consensus on the question posed in the title:
Physical Attributes + Chemical Attributes = Physicochemical Attributes?
For many readers, this question may seem trivial – or even self-evident. Yet the way these terms are currently used in scientific publications and conference presentations suggests otherwise.
As a starting point, it is worth considering the meaning of the terms physical, chemical, and physicochemical. This is by no means a straightforward task, given their broad application across different scientific disciplines and the fact that, in many natural phenomena, the boundaries between them are far from clear.
In a broad sense, the term physical refers to phenomena associated with the state, structure, and measurable properties of matter – such as mass, volume, energy, motion, and temperature – without changes in chemical composition, oxidation state, or the chemical form of the species involved. Water (H₂O) provides a familiar example: it may undergo changes in physical state, such as melting or boiling, while its chemical composition remains unchanged.
In contrast, the term chemical refers to processes that alter the composition of matter through chemical reactions, electron transfer, bond formation or cleavage, and the associated energy changes. Photosynthesis is a classic example, in which carbon dioxide (CO₂) and water (H₂O) are converted into glucose (C₆H₁₂O₆) and oxygen (O₂).
The term physicochemical, in turn, describes processes or properties in which physical and chemical aspects are intrinsically interconnected and cannot be fully understood in isolation. Adsorption, ionic diffusion, surface tension, and electrical conductivity are typical examples. Surface tension, for instance, can be observed when insects walk across the surface of water. In this case, hydrogen bonding between water molecules – a chemical interaction – generates cohesive forces that collectively give rise to a measurable physical property: the resistance of the liquid surface. This example clearly illustrates the intimate interplay between physical and chemical phenomena that characterizes physicochemical processes.
Within Soil Science, physical attributes refer to properties associated with the soil’s structure and physical condition, including mass–volume relationships, bulk density, texture, porosity, aggregation, permeability, water retention, and hydraulic conductivity, among others. These attributes are directly related to the spatial arrangement of soil particles and the movement of water and air through the soil profile. Their measurement does not alter the chemical composition of mineral constituents or soil organic matter, nor does it change the oxidation state or chemical form of the elements present.
Chemical attributes, in contrast, describe the interactions and transformations that occur between the solid phase and the soil solution, determining the ionic composition and chemical reactivity of the soil–solution system. These processes include ion exchange, dissolution and precipitation of mineral and organic compounds, metal–ligand complexation, and oxidation–reduction reactions involving elements such as iron, manganese, sulfur, and nitrogen. Ion exchange occurring on the reactive surfaces of soil particles underlies a wide range of interfacial phenomena that contribute to the chemical equilibrium of the soil system. Although these interactions take place at the solid–solution interface and inherently involve physicochemical processes, they are traditionally treated within Soil Science primarily from a chemical perspective. Together, they determine the chemical forms, concentrations, and availability of ions in the soil solution and influence how soils respond to both natural processes and anthropogenic interventions. Typical examples of chemical attributes include soil pH, potential acidity, and nutrient concentrations.
When applied to Soil Science, however, the term physicochemical should be reserved for attributes, properties, or processes in which physical and chemical phenomena are fundamentally inseparable – that is, when the behavior of the system depends simultaneously on the physical characteristics of the medium and on the chemical reactions occurring within it.
Although ion exchange also occurs at the solid–solution interface, it is generally regarded in Soil Science as a predominantly chemical process, since it is primarily associated with the equilibrium between ionic species distributed across different phases. This classification, however, does not diminish its physicochemical nature, as its occurrence necessarily involves interfacial interactions governed by both physical and chemical principles. By contrast, processes such as ion adsorption and desorption explicitly involve electrostatic forces, the structure of the electrical double layer, and the interaction energy between ions and solid surfaces. For this reason, they are classically recognized as physicochemical phenomena. The same applies to properties such as the electrical conductivity of the soil solution, the point of zero charge (PZC), and the zeta potential, where the distinction between physical and chemical phenomena becomes intrinsically blurred.
Some attributes, such as soil pH, may also raise questions regarding whether they should be classified as chemical or physicochemical. This uncertainty arises because pH measurement is based on electrochemical principles – which belong to the domain of physical chemistry – while the quantity being measured, namely the activity (or effective concentration) of hydrogen ions, is fundamentally chemical in nature. From the perspective of Soil Science, however, the primary purpose of measuring pH is to assess soil acidity and nutrient availability. Consequently, pH has traditionally been regarded as a chemical attribute. There are, nevertheless, specific situations – particularly in studies addressing soil–solution interactions and the behavior of surface charges, such as those involving the point of zero charge (PZC) and zeta potential – in which pH assumes a genuinely physicochemical role because it directly influences the equilibrium between charged surfaces and the surrounding soil solution.
It is important to emphasize, however, that these particular cases should not be generalized to encompass attributes that are clearly physical or clearly chemical. In most Soil Science studies, the attributes being evaluated fall predominantly into one of these two categories and are not directly associated with physicochemical phenomena. Maintaining clear and consistent terminology is therefore essential to avoid conceptual ambiguity and potential misinterpretations.
The purpose of this article is not to settle the discussion, nor to establish definitive boundaries between what should be regarded as physical, chemical, or physicochemical in Soil Science. Rather, it is intended as an invitation to reflection – one that remains open to discussion, refinement, and even the possibility that some of the ideas presented here may themselves be questioned or reinterpreted.
The indiscriminate use of scientific terminology can oversimplify concepts and ultimately lead to misunderstandings of the processes that govern soil behavior and functioning. Our primary objective is therefore to encourage dialogue and to contribute to a more consistent and widely shared understanding of how the terms physical, chemical, and physicochemical are used within Soil Science.
This discussion is particularly relevant for the education of future soil scientists and professionals, whose understanding of increasingly complex soil systems depends on clear conceptual foundations. As educators and researchers, we have a responsibility to promote precision in scientific language and to avoid the dissemination of imprecise terminology that may compromise both scientific understanding and professional practice.
Conceptual clarity and terminological rigor are not merely matters of writing style; they are fundamental components of scientific integrity and of our responsibility as members of the Soil Science community.
Ultimately, is this simply a matter of terminology, or does it reflect a deeper understanding of the processes that underpin Soil Science?
What do you think?
Physical Attributes + Chemical Attributes = Physicochemical Attributes?
Luiz Francisco da Silva Souza Filho1: Professor at the Federal University of Recôncavo da Bahia (UFRB), based at the Center of Agrarian, Environmental and Biological Sciences (CCAAB), Cruz das Almas, BA; Director of the Northeast Regional Center of the Brazilian Society of Soil Science (NRNE/SBCS), 2025-2027 term; Coordinator of Commission 4.3, History, Epistemology and Sociology of Soil Science, of the NRNE/SBCS. E-mail: lfsouzafilho@ufrb.edu.br.
Maria Eugenia Ortiz Escobar2: Professor at the Federal University of Ceará (UFC), based at the Department of Soil Sciences (DCS) of the Center of Agrarian Sciences (CCA), Fortaleza, CE; President of the Brazilian Society of Soil Science (SBCS), 2023-2025 and 2025-2027 terms. E-mail: mariaeugenia@ufc.br.
Statement on the use of generative AI and AI-assisted technologies in the writing process: This text was partially supported by the GPT-5 language model (OpenAI) in the stages of research, revision, and textual refinement. The use of artificial intelligence was assistive in nature, serving exclusively for editorial and linguistic support, including grammatical revision, improvement of clarity, cohesion, and textual style, as well as occasional suggestions on structuring. The impetus for the writing, as well as all ideas, interpretations, and conclusions presented, are the sole responsibility of the authors. The content was reviewed, edited, and approved by the authors, who assume full responsibility for all information and for the final version of the text.
