What is humus?

In brief

The dark, stable fraction of organic matter in soil, the part that holds water and nutrients and gives a living soil its colour, smell and structure.

Learning path: From soil to forest

Station 1 of 7

Humus is the dark, stable part of the organic matter in soil. Leaves, roots, dung and dead soil creatures arrive at the surface or in the ground, soil life takes them apart, and a fraction of what passes through that process ends up in forms that resist further breakdown. That fraction is humus. It is not a substance you can buy in a bag, and it is not the same as the compost you might add. It is a condition the soil reaches.

What it does is easier to state than what it is. Humus holds water, several times its own weight of it, which is why soils rich in it survive dry spells that leave poor soils cracked. It carries a negative surface charge, so it binds calcium, magnesium, potassium and trace elements and keeps them available to roots instead of letting rain wash them down beyond reach. And it acts as glue, binding sand, silt and clay into crumbs, which is what creates the pore space that lets a soil breathe and drain at the same time.

There is a genuine argument in soil science about the word itself, and it is worth knowing about. The classical view described humus as a set of distinct large molecules, humic and fulvic acids, that could be extracted and named. A prominent paper in Nature in 2015 argued that those molecules are largely artefacts of the harsh chemistry used to extract them, and that what actually exists in soil is a continuum of organic fragments at every stage of decay, held in place by minerals and by soil structure rather than by their own chemical stubbornness. The practical consequences are real: if stability comes from where organic matter sits rather than from what it is, then protecting soil structure matters as much as adding material.

We keep the word because it names something every gardener and farmer can recognise by hand and by eye. We just try not to pretend it is simpler than it is.

How Terra Humana understands humus

For Terra Humana, humus is the single clearest measure of whether renaturalisation is working. Trees can be planted and counted in an afternoon. A soil that has begun to hold water and life cannot be faked, and it cannot be rushed.

The Mediterranean makes this harder than the textbooks assume. Heat and dry summers speed up the breakdown of organic matter, so a large share of what reaches the ground is respired away rather than stabilised. Add the loss of tree cover across the island's interior over centuries of grazing, fire and cultivation, and what remains on many slopes is mineral soil with very little organic matter left in it. Building it back is not a season's work.

That leads to a plain order of priority. Protecting the organic matter already in the ground beats adding new material: every summer a soil spends bare and exposed costs more than a mulch layer can return. Shade, permanent cover and undisturbed ground do more for humus than any amendment.

We also decline to turn this into a climate claim. We describe humus as what it is, a matter of soil fertility, water and life, and we attach no further promises to the soil we build.

Sa Medada

What this looks like at Sa Medada

There is no laboratory analysis of the soil at Sa Medada yet, and no pH value has been measured so far. So we start with what is right without a measurement: shading the soil in summer instead of leaving it bare, and returning the cuttings of the support plants as mulch. The analysis will provide the baseline against which any change in humus can be measured.