The substance that may not exist

The field in brief01

Pedology treats soil not as dirt but as a natural body with anatomy. A vertical slice, called a profile, resolves into layers called horizons: dark organic topsoil, leached and enriched middle zones, weathered rock below. The smallest three-dimensional unit a pedologist will call a soil is a pedon. Reading how those horizons form and stack is the core craft of the field. [8]

Inside the topsoil lives the material at the center of this story: soil organic matter, the mass of dead and decomposing carbon left by roots, leaves, and microbes. It is not a minor pool. The world's soils hold more carbon than the atmosphere and all living vegetation combined, which is why what governs its lifetime is a question for climate science, not just agronomy. [1]

For most of the field's history, that dark matter was understood through a single laboratory ritual. In 1786 the chemist Franz Achard soaked peat in caustic potash, then added acid, and watched a dark amorphous solid fall out. [8] That precipitate became humic acid, and the recipe hardened into a taxonomy defined purely by what dissolves in what.

On this foundation rose the theory of humification: the claim that soil life and chemistry take small breakdown fragments and stitch them back up into large, complex, chemically distinct molecules found nowhere in living tissue. Their supposed toughness, their resistance to being eaten, was the textbook explanation for why soil carbon can persist for centuries. [2] The alternative on offer is the soil continuum model: organic matter as an unbroken gradient of ever-smaller decomposition products, whose long life comes not from molecular armor but from physical protection, clinging to mineral surfaces, locked inside clumps, simply out of a microbe's reach. [1]

The fight02

The shot that opened the modern war was fired in 2015, in the pages of a general-science journal, by two soil scientists with impeccable standing. Johannes Lehmann of Cornell University and Markus Kleber, then at Oregon State University, argued flatly that the evidence does not support the formation of large, persistent humic substances in soil. [1] Their case rested on a generation of newer instruments. When you point advanced spectroscopy, mass spectrometry, and direct imaging at undisturbed soil, they wrote, you do not find giant humic molecules; you find a smear of recognizable plant and microbial compounds caught at every stage of being taken apart. [10]

The deeper charge was about method. The extraction that births humic substances uses alkali near pH 13, a violence that no field soil ever meets, since soils live between roughly pH 3.5 and 8.5. [1] That harsh bath, the challengers argued, can weld small molecules into large ones on the spot, manufacturing the very objects it claims to reveal. Worst of all, the definition is circular: humic substances are simply whatever the procedure extracts, so their existence in nature can never be tested against the procedure itself. In a 2019 follow-up, Kleber and Lehmann tightened the language, calling the alkali-extracted fractions invalid proxies for what organic matter actually does in the ground. [2]

The defenders did not fold. The International Humic Substances Society, founded in the early 1980s and keeper of the standard reference materials the field extracts and compares against, mounted a direct rebuttal. Michael Hayes and Roger Swift, two elders of humic chemistry, published a paper whose very title, invoking the vindication of humic substances, framed the challengers as mistaken; they wrote that they disagreed fundamentally with the new view and set out to catalogue its errors and misinterpretations. [3] The society gathered its members' rejoinders into a collective set of viewpoints, and the soil chemist Daniel Olk assembled field case studies arguing the extracted fractions track real behavior in real landscapes. [4]

A third figure complicates any clean two-sided story. Alessandro Piccolo of the University of Naples Federico II had argued years earlier that humic substances are real, but not the giant covalent polymers of the textbook; they are supramolecular associations, loose assemblies of small molecules held together by weak forces. [6] That view is a knife that cuts both ways: it concedes the challengers' central chemical point, that the macromolecule was a fiction, while insisting on keeping the word. Meanwhile Philippe Baveye of AgroParisTech and Michelle Wander of the University of Illinois offered the most deflating observation of all, that the celebrated new view is roughly eighty years old, most of it already voiced by Selman Waksman in 1936, and that the field keeps rediscovering it only because its subdisciplines refuse to talk to one another. [7]

The fight is not a museum piece. As recently as 2026, Sen Dou and colleagues published a rejoinder in a soil-science journal under the banner of why humic substances still matter, insisting the concept earns its keep. [5] The stakes are why the argument will not die. Careers, curricula, and textbooks were built on humification. An entire commercial trade in humic and fulvic acid soil amendments, worth on the order of 1.4 billion dollars in 2024, is sold to farmers on the premise that these substances are real and beneficial. [9] And every model of how soil will hold or surrender carbon under a warming climate depends on the answer, because armored molecules and mineral-protected fragments behave very differently as the planet heats. [1]

What the fight reveals03

On the narrow factual question, the modern evidence has largely gone the challengers' way. Repeated attempts to observe distinct, large humic macromolecules inside undisturbed soil have not delivered them, and the wider fields of biogeochemistry and Earth-system modeling have broadly shifted toward the continuum-and-mineral-protection picture. [10] The burden of proof has flipped onto anyone claiming a distinct, naturally formed humic class; even Piccolo's supramolecular truce quietly abandons the giant-polymer version the textbooks taught. [6] What remains genuinely unresolved is whether the alkali-extracted fractions, though not a pristine natural kind, nonetheless capture organic matter that is functionally meaningful, and whether the commercial products work regardless of what humus turns out to be. [4] The deepest open question is the oldest one, dressed in new urgency: what actually governs how long carbon stays in the ground. The field spent two centuries answering with a substance it may have created itself, and the honest verdict is that it is still learning to see soil without the jar. [2]