Humus
The part of soil organic matter most transformed by soil organisms, the part that darkens the earth.
Explanation
Leaves, roots, droppings: whatever dies on the ground is eaten, digested and transformed by soil life. The most transformed part is what we call humus. The FAO describes it as the part of organic matter that has been used and transformed by many different soil organisms: it colours the soil darker, increases soil aggregation and aggregate stability, helps the soil attract and retain nutrients, and buffers fluctuations in soil acidity (Bot & Benites, FAO, 2005). Recent research, however, revises the image of a humus that is “stable” by nature.
Refined definition
In the classic view, humification produces “humic substances” (fulvic acids, humic acids, humin), defined by what dissolves or not during a chemical extraction (Bot & Benites, FAO, 2005). Lehmann & Kleber (2015), in Nature, argue that the available evidence does not support the formation of such large-molecular-size, persistent substances in soils: soil organic matter would rather be a continuum of progressively decomposing organic compounds. What lasts owes it mostly to its environment: molecular structure alone does not control the stability of organic matter, where environmental and biological controls predominate (Schmidt et al., 2011).
Key points
Where humus comes from
From plant, animal and microbial residues, broken down step by step by soil organisms: earthworms, arthropods, fungi and bacteria (see Living soil). The FAO specifies that humus itself slowly decomposes, releasing nitrogen, phosphorus and other nutrients (Bot & Benites, 2005).
“Stable humus”: a phrase to handle with care
The phrase is common. The FAO explains the slow decomposition of humus by its intimate interactions with soil minerals and by its chemical complexity (Bot & Benites, 2005); more recent work challenges the second argument (Schmidt et al., 2011; Lehmann & Kleber, 2015). What lasts is protected, not inert.
In the garden
Seen as a continuum that keeps decomposing, soil organic matter is not acquired once and for all: it is maintained by regular inputs, soil cover, living roots, compost, mulch. This is the practice described in the Mulch and Living soil definitions.
Watch out for
- • The word has a second meaning among foresters and soil scientists: the “humus form”, that is, the set of organic and organo-mineral surface layers of a soil, classified into broad types such as mull, moder and mor (Jabiol et al., L'humus sous toutes ses formes, 2007).
Examples
- • A dark, crumbly soil under an old mulch
- • The litter of a woodland floor transforming, layer after layer
- • A mature compost, already well transformed, that keeps evolving in the soil
Sources
- Bot, A. & Benites, J. (2005). The Importance of Soil Organic Matter: Key to Drought-Resistant Soil and Sustained Food Production. FAO Soils Bulletin 80. Rome : FAO.
- Lehmann, J. & Kleber, M. (2015). The contentious nature of soil organic matter. Nature, 528, 60-68.
- Schmidt, M.W.I. et al. (2011). Persistence of soil organic matter as an ecosystem property. Nature, 478, 49-56.