Reflections on the Use and Application of These Terms in Soil Science
Luiz Francisco da Silva Souza Filho1, Maria Eugenia Ortiz Escobar2
It has been observed, with some frequency, across various means of scientific communication such as articles, abstracts, and posters, the use of the term “physicochemical” to refer to data related to particle-size analysis, soil and particle density, aggregation and aggregate stability, porosity, hydraulic conductivity, pH, exchangeable cations, potential acidity, base saturation, among others.
But is the application of the term physicochemical in this sense correct? Are all the attributes presented in these works, often used as indicators of soil quality and health, truly physicochemical? Does the mere fact of determining physical attributes and chemical attributes allow us to group them under this designation?
It is also worth noting that terms widely used in the literature, such as “soil health,” likewise carry conceptual and, to some extent, metaphorical dimensions that are not always made explicit. Although its use is well established, especially as a concept that integrates the soil’s physical, chemical, and biological functions, it is an expression without a single, universal definition and may take on different interpretations depending on the context. Thus, similarly to what is discussed in this text regarding the terms physical, chemical, and physicochemical, the use of “soil health” also benefits from a critical and conscious approach, especially when greater conceptual rigor is sought in scientific communication.
With the aim of prompting a reflection on the appropriateness of the terms physical, chemical, and physicochemical in Soil Science, especially with respect to the attributes frequently analyzed, we have prepared this text to stimulate discussion within the scientific community. We hope, in this way, to contribute so that, in the near future, we may answer, even if not unanimously, the question posed in the title:
Physical Attributes + Chemical Attributes = Physicochemical Attributes?
For many, this question may seem trivial or even obvious, something so simple that it would not warrant debate. However, what is observed in different works and scientific events shows that the reality is not quite so.
As a starting point, it is necessary to seek the meaning of the terms physical, chemical, and physicochemical. This task, however, is far from simple, given the broad application of these designations across different branches of science and the way their boundaries frequently intertwine in certain natural phenomena.
In general terms, the term physical refers to phenomena related to the state, structure, and measurable properties of matter, such as mass, volume, energy, motion, and temperature, without changes occurring in the chemical composition of substances, in the oxidation state of elements, or in the chemical form of the species involved. A good example is the behavior of water (H₂O), which can change its physical state without altering its chemical composition, as occurs in the melting of ice (from solid to liquid) or in boiling (from liquid to gas).
The term chemical, in turn, involves transformations that modify this composition, comprising reactions, exchanges of electrons, the formation or breaking of bonds, and the energy variations associated with their constitution. Photosynthesis is a classic example: in simplified terms, carbon dioxide (CO₂) and water (H₂O) are transformed into glucose (C₆H₁₂O₆) and oxygen (O₂).
The term physicochemical, in turn, is used to describe processes or properties in which the physical and chemical aspects are inseparable and cannot be explained in isolation, as occurs in the phenomena of adsorption, ionic diffusion, surface tension, and electrical conductivity, among others. A classic example is surface tension, visible when insects walk over the “skin” that forms on the surface of a body of water. In this case, the hydrogen bonds between water molecules, chemical in nature, generate cohesive forces that, taken together, result in a measurable physical property: the resistance of the liquid’s surface. The phenomenon thus perfectly illustrates the interdependence between physical and chemical aspects, an essential characteristic of physicochemical processes.
Turning now to the application of these terms in Soil Science, physical attributes are understood as those related to the structure and physical conditions of the soil, such as the relationships between mass and volume, soil bulk density, texture, porosity, aggregation, permeability, water retention, and hydraulic conductivity, among others. These attributes are linked to the organization of soil particles and to the flow of water and air, among other aspects, and their assessment does not cause changes in the chemical composition of the mineral constituents or organic matter, nor does it alter the oxidation state or the chemical form of the elements present.
Chemical attributes concern the interactions and transformations that occur between the solid particles and the soil solution, determining the ionic composition and reactivity of the soil-solution system. These processes include ion exchange, dissolution and precipitation of mineral and organic compounds, complexation between metals and ligands, and oxidation-reduction reactions that alter the chemical state of elements such as iron, manganese, sulfur, and nitrogen. These ion exchanges, carried out on the reactive surfaces of soil particles, form the basis of various interfacial phenomena that contribute to the chemical equilibrium of the system. Although they occur at the solid-solution interface and involve processes of a physicochemical nature, they are, in most approaches within Soil Science, traditionally treated as predominantly chemical processes. Together, they define the form, concentration, and availability of ions in the soil solution, in addition to influencing its response to anthropogenic interventions and natural processes. This category includes characteristics such as pH, potential acidity, and nutrient contents.
The term physicochemical, when applied to Soil Science, should be reserved for attributes, properties, or processes in which the physical and chemical aspects act inseparably, that is, when the phenomenon depends simultaneously on the physical structure of the medium and on the chemical reactions occurring within it.
Although ion exchange also occurs at the solid-solution interface, it is generally treated in Soil Science, in many contexts, as a predominantly chemical process, as it is associated with the equilibrium between ionic species in different phases. This does not exclude, however, the fact that its occurrence involves interfacial processes of a physicochemical nature. On the other hand, processes such as the adsorption and desorption of ions explicitly involve electrostatic forces, the structure of the electrical double layer, and the interaction energy between ions and surfaces, and are therefore classically described as physicochemical phenomena. This category also includes properties such as the electrical conductivity of the soil solution, the point of zero charge (PZC), and the zeta potential, in which the boundaries between the physical and the chemical truly overlap.
Some attributes, such as soil pH, may give rise to discussion regarding their classification as a chemical or physicochemical attribute. This doubt stems from the electrochemical principles involved in its determination, which belong to the field of physical chemistry, and from the nature of what is measured, that is, the activity (or effective concentration) of hydrogen ions, an essentially chemical aspect. For Soil Science, however, the main interest in determining pH is of a chemical nature, aimed at assessing acidity and nutrient availability, which justifies its traditional classification as a chemical attribute. There are, however, specific situations, such as in studies of soil-solution interactions and the behavior of electrical charges, as in the point of zero charge (PZC) and the zeta potential, in which pH takes on physicochemical relevance, as it is associated with the equilibrium between surface charges and the soil solution.
It is important, however, to emphasize that these particular cases should not serve as a basis for generalizations, in the sense of including, under the same label, attributes that are clearly physical or chemical and that, as such, should be treated distinctly. In a large part of the works, the attributes assessed are predominantly physical or chemical and bear no direct relation to phenomena of a physicochemical nature, which is why terminological precision becomes essential to avoid conceptual confusion and mistaken interpretations.
This text does not intend to exhaust the discussion on the subject, nor to establish a definitive delimitation of what should be considered physical, chemical, or physicochemical in Soil Science. It is, rather, an initial reflection, open to debate and to collective improvement, including the acknowledgment that divergent interpretations, or even imprecisions in this very text, are possible. The indiscriminate use of terms can lead to the simplification of concepts and to mistaken interpretations of the processes that govern soil functioning. The main objective is to stimulate dialogue and to contribute to building a common understanding of the use and application of the terms physical, chemical, and physicochemical in Soil Science.
This discussion becomes even more relevant in the context of training new professionals, who depend on clear conceptual foundations to understand the complexity of natural systems. As professors and researchers, we have a duty to safeguard the precision of scientific language, avoiding the propagation of terminological errors that may compromise the understanding of phenomena and, consequently, the quality of future professional practice. Conceptual consistency and terminological rigor are not merely matters of style, but an essential part of the educational and scientific responsibility of those who teach, research, and contribute to the advancement of Soil Science.
After all, is this merely a question of nomenclature, or of understanding the processes that underpin Soil Science?
In this context, what is your opinion: 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.
