How should soil and the field that studies it be defined? Experts disagree on what should be called soil, reflecting the tension between long-standing traditions and the field’s changing frontiers.
“Soil science has enormous, untapped potential,” Nebraska’s Aaron Lee M. Daigh said. “If we define our subject too narrowly, we artificially limit our science.”
“I propose to redefine soil,” Daigh said. “In a nutshell, the changes would tremendously expand what counts as soil — it starts much younger, goes much deeper and exists much farther out in the universe, literally, than scientists have previously thought or given credit for.”
Over the past decade, newly proposed definitions have offered valuable viewpoints. But they still draw on past traditions and often leave aspects of what qualifies as soil open to interpretation. Each viewpoint — historical, societal and fundamental — contributes to refining the definition of soil.
What if the frozen surface of a distant comet, icy moon or dwarf planet qualified as soil? And what if icy worlds like these turn out to be the most common type of soil in the universe?
Daigh is an associate professor of Vadose Zone Science in the Departments of Agronomy and Horticulture and Biological Systems Engineering, and the 2026 president of the Soil Science Society of America — the 90th person to hold the position. His new article raises these questions and introduces three new approaches for redefining soil and the field that studies it.
“Defining Soil: Proposals of Zero-states, Active Layer, and Págousols (From Ice Parent Materials) to Fundamentally Expand Soil Science,” was first published March 14, 2026, in the Soil Science Society of America Journal.
Daigh proposes three new ideas. First, recognize soil’s starting point in a consistent manner, which he calls “zero states.” At that moment, all the forces that will shape it — climate, living things, landforms, underlying material and time — are just beginning to act. This first proposal acknowledges that in many areas, soils are barely developed and may not show the classic layering that will eventually evolve. For example, fresh volcanic ash deposited on a hillside or sediment newly laid down by a flooding river represents a real-world zero-state. The clock of soil formation has just begun, even before any plants or organisms are present.
“I argue that soil formation should be recognized even when one or more of these factors, such as organisms and even time, are absent,” Daigh said. “A sterile sediment immediately exposed to changing forces at its surface can qualify as soil from its very first moment of existence — it does not need to wait until those forces have built up distinct layers within it.”
Second, avoid assigning soil a fixed depth, since any single cutoff is difficult to defend scientifically. Instead, define soil’s lower boundary by the depth to which those changing forces at its surface cause mass and energy to shift and move over time.
“Rather than setting a one-size-fits-all depth limit, such as six feet, the bottom of the root zone or looking for visual cues, I propose defining soil’s lower boundary by the extent of the active flux layer — the depths where surface-driven exchanges of water, energy, gases and dissolved chemicals actively occur. Below this zone, fluxes stabilize and soil-forming processes effectively cease. This depth marks the boundary between soil and the deeper materials that are unaffected as forces shift and change at its surface.”
This science-based approach replaces judgment-based visual indicators and blanket depth cutoffs with measurable, real-world processes. Overall, Daigh’s proposed approach means that most soils go much deeper than previously acknowledged.
Third, include icy soils on Earth and other celestial bodies, which Daigh coins as “págousols” — a new term for this novel category of soil. These págousols are made directly from grains of ice, such as snowpacks, and may occur on any size of celestial body, provided they don’t violate the prior two proposals.
The word comes from the Greek págos, ice mass, and the Latin sol, soil. Págousols are soils made from ices — frozen chemical compounds such as water ice, nitrogen ice, methane ice and carbon monoxide ice.
Existing terms like cryosols and gelisols exist, but those describe rocky soils altered by cold and frost. They do not capture soils that come from ice itself as the starting material. So, Daigh coined the new term to reflect the basic difference.
“Icy moons like Europa, Enceladus, Titan and Triton, along with dwarf planets like Pluto and objects in the Kuiper Belt and Oort Cloud, all become candidates for soil science study under this definition,” Daigh said. “Seasonal and permanent snowpacks here on Earth would also qualify.”
The solar system alone may contain tens of billions to trillions of icy celestial bodies, leading to Daigh’s prediction that págousols are likely the most abundant type of soil in the universe.
Daigh is not proposing that practical definitions of soil used in agriculture, land management and policy be replaced. Rather, he supports the centuries-long tradition of maintaining two complementary definitions. The first is a broader scientific definition that keeps pace with new discoveries. The second is a practical, easy-to-use one for government agencies and policymakers.
Daigh first presented these ideas at CANVAS 2025, the annual meeting of the Soil Science Society of America, held jointly with the American Society of Agronomy and Crop Science Society of America, Nov. 9–12 in Salt Lake City, Utah. A recording of the presentation, titled “Ice Worlds: Fundamental Questions of Ice Vadose Zones and Astropedology” is publicly available.
He contends that these changes make the definition of soil more precise and open new research areas. This work matters because clear definitions help scientists better study Earth and other worlds. They support practical efforts like environmental stewardship and food production, as well as broader frontiers such as the study of life beyond Earth and space exploration.
Daigh, a vadose zone hydrologist and soil physicist, has research spanning water quality, remediation, fundamental and applied soil science and engineering. He is also a member of the U.S. National Committee for Soil Sciences at the National Academies of Sciences, Engineering, and Medicine.
