Humus, the dark soil layer whose backbone can outlast centuries
Under the leaf litter lies a dark, spongy layer that soil scientists still cannot fully define. Humus is decomposed plant and animal matter, yet it is far from dead, teeming with bacteria and fungi. Much of it has sat in the ground for more than a century, shielded inside tiny soil crumbs or clinging to clay.
Humus forms when organisms feeding on fallen litter break it down, a process called humification in which earthworms play a starring role alongside fungi, microbes and other invertebrates. Anything still visibly rotting counts as detritus; only the fully transformed material is humus, though no sharp line separates the two. Its raw ingredients include leaves, dead animals and microbes, the droppings of soil creatures, and even charcoal left by old fires. The result is brown to black, rich in carbon and nitrogen, and usually holds about twelve parts carbon to one of nitrogen.
What the ground looks like depends on the local worms. In acidic, well-drained soil under conifers or heath, big earthworms cannot survive, decomposition crawls along, and a thick mat of litter sits on the humus with a crisp boundary above the mineral soil. That is mor. On chalky soils beneath deciduous woods and lowland grass, earthworms churn humus and mineral soil together into mull, and litter vanishes quickly. Moder falls in between.
Chemically, humus is a tangle of giant molecules built partly from lignin and other polyphenols in wood, bark and leaves, with proteins and amino sugars hooked on. Radiocarbon dating shows the polyphenol backbone can be very old while the attachments are young, because microbes strip protein off far more easily than they crack the core. Much humus persists for more than 100 years when it is hidden inside soil aggregates or bound to clays; unprotected material mostly breaks down within a decade. Attempts in the 1940s and 1960s to separate it chemically failed because the extracting agents themselves altered the sample.
Stable humus supplies few nutrients directly but holds soil together, raises its capacity to retain water, and carries negatively charged sites that grip plant nutrients. Finely powdered charcoal worked into topsoil, perhaps by the grinding of a tropical earthworm, may explain the remarkable fertility of the Amazonian dark earths known as terra preta.
Source: Humus
