by Roberto
Fig. 1. Das Edaphon book cover. Source
Soil, the material on the surface of the ground on which plants grow, is itself teeming with life. And there's a term that unifies all that life as an interconnected whole: edaphon. Though it is seldom found in the scientific literature, I will argue that it should be more widely taught and used.
Edaphon, from the ancient Greek ἔδαφος (édaphos, "ground"), was coined by the pioneer soil microbiologist Raoul Heinrich Francé and used in the title of his 1913 book "Das Edaphon: Untersuchungen zur Oekologie der bodenbewohnenden Mikroorganismen" ("The Edaphon: Investigations into the Ecology of Soil-Dwelling Microorganisms" ) (Fig. 1). How's that for early, visionary microbial ecology? For Francé, soil contained an interconnected and interdependent network of microscopic life. Perhaps hoping for a land equivalent to the aquatic term plankton (in use since the late 1800s), he came up with edaphon.
For whatever reasons, the term did not catch on with the scientific community. A PubMed search today retrieved ten publications that use the term, in contrast to a search for edaphic (adjective meaning "of soil") which yielded nearly two thousand publications. Granted, PubMed may not be the best database to search for soil publications, but still... So, what happened to Francé's idea? Interestingly, his most notable legacy is the influence that the edaphon concept has had on the development of present-day organic farming and gardening practices (e.g., see here).
Serendipitously, research into edaphon taught me much that I didn't know about, of all people, Selman Waksman. I learned about his many contributions long before his Nobel Prize-winning discovery of streptomycin (and before he failed to recognize the co-discoverer, his student Albert Schatz). Waksman was 64 in 1952, the year he received the Nobel Prize. But several decades before that, he was already a widely recognized authority in soil microbiology. In 1927, when he was 39, he published the gargantuan tome (over 900 pages with over 2500 references) "Principles of Soil Microbiology," which went on the serve as a key reference in the field for many years. In that text, Waksman writes: "In the complex medium of the soil and under the influence of various physical, chemical and environmental conditions, soil microorganisms carry on their activities, not as individual forms or even as groups, but as a soil population." And he refers to this population as edaphon. The concept of edaphon thus forces us to see all of the living beings present in soil as an interdependent population.
Fig. 2. The edaphon's food web. Source
What did Waksman, as well as other soil microbial ecologists of the time, mean by soil microorganisms? I find the answer utterly fascinating; the first paragraph from the preface of another book by Waksman, this one from 1931, co-authored with Robert Starkey, and beautifully titled "The Soil and The Microbe" says it all:
The soil is not a mass of dead debris, resulting simply from the physical and chemical weathering of rocks and of plant and animal remains through atmospheric agencies, but it is teeming with life. Every small particle of soil contains numerous types of living organisms belonging both to the plant and animal kingdoms, yet so small that they cannot be recognized with the naked eye. These organisms are, therefore, called microbes, microorganisms or microscopic organisms. These microbes comprise numerous types of bacteria, fungi, algae, protozoa, nematodes and other invertebrates which vary considerably in their structure, size, mode of living and relationship to soil processes.
Nematodes and other invertebrates as microbes! I love it! You see, early investigators saw no sharp boundaries as life forms went from microns to millimeters. Rather, they viewed microscopic life as a continuum of sizes. This idea of a continuum of sizes, without a focus on molecular classification, is retained in today's organic farming practices. The edaphon is usually categorized by size into micro-fauna (2 to 200 µm), meso-fauna (200 to 2,000 µm) and macro-fauna (over 2,000 µm). (I do find it curious, this practice of stating dimension ranges using "2s" rather than "1s." I wonder, why?)
I think present-day soil microbial ecology could benefit greatly from remembering that soil functionality depends on the interactions among all the organisms present, i.e., the edaphon (Fig. 2). Nowadays, most of the soil microbial ecology studies that I read or hear about are dominated by "omics" approaches focused on bacteria and archaea (and sometimes, fungi). But only seldom do these studies include nematodes, protists, rotifers, tardigrades, mites, springtails, millipedes, or even worms and snails (these latter two would have to be the mega-fauna of soil!). In my mind, there's no doubt that a more widespread embrace of the concept of the edaphon will lead to a more holistic understanding of the earth beneath our feet. And hopefully more respect for it!
Acknowledgment: In full disclosure, until recently I had never heard the term edaphon. Last week, while participating in a Microbiome Symposium organized by Pieter van 't Hof at Universidad San Francisco de Quito, Ecuador, I met John van der Laag, who introduced me to the term. Muchas gracias John!
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