Does Humus Exist?

"Humus" is a word used in many gardening articles and books - even recent textbooks - about soil. You may find bags of "humus" for sale at home improvement stores. But Drs. Johannes Lehmann and Markus Kleber argue convincingly that humus doesn't exist in their 2015 Nature article "The Contentious Nature of Soil Organic Matter."

Due to lab procedures that are nothing like real-world conditions, we incorrectly believed that some organic matter (which is primarily carbon) stuck around in soils by being transformed into large, stable molecules traditionally called humus. The truth, exposed by Lehmann, Kleber, and their colleagues, is that - rather than some organic matter becoming persistent and chemically unique humus - all organic matter is instead constantly being broken down into smaller and smaller molecules. The reason some organic matter seems to persist in soils is not that it is especially resistant to decomposition, but that small fragments of organic matter end-up in places where they are sheltered from further decomposition. By binding to mineral particles and by being incorporated into soil aggregates, organic matter fragments are protected from becoming immobilized in the bodies of soil organisms or from getting lost from the soil as carbon dioxide.

One takeaway message is that our soil organic matter is never completely safe from loss (as carbon dioxide). The organisms (bacteria, fungi, protozoa, nematodes, etc.) that create soil health through their biological processes need to be fed by constant inputs of organic matter. We should primarily achieve this by always having living plants on the soil - typically by using cover crops or perennials. But applying imported organic matter in the form of composts or manure can also be very beneficial.