‘Humus’ to the Romans meant the soil (earth, ground) as a whole. Carl Linnaeus, the inventor of binomial nomenclature, used this word in his system to classify soils as he did plants and animals: Humus daedalea, Humus ruralis, and Humus damascena were his terms for garden soil, rural soil and clay soil respectively.
The Swedish chemist Johan Gottschalk Wallerius used the word ‘humus’ in his 1761 work Agriculturae fundamenta chemica to refer specifically to the decomposed organic matter within soil, and it is this concept which remains with us to this day.
Though the concept of ‘humus’ as we know it today dates over two and a half centuries ago, surprisingly little is actually known about it except that it is a brown-to-black substance of no definite form, with jelly-like consistency, and which is very stable and resistant to further decomposition. The structure of a ‘humus molecule’ can’t be described, unlike the complex lignin class of molecules, which can.
In fact, in 2015 Johannes Lehmann and Markus Kleber published a ‘perspective’ paper in Nature claiming that humus doesn’t even exist! (Also available here without a paywall.) (Note that this is a perspective paper, ie an opinion piece, and not original scientific research by the authors.)
As I understand it, not being part of it myself, the soil science world collectively sat up and said “What?!” And, as scientists are wont to do, calmly discussed the matter.
Lehmann and Kleber’s claim focused on the observation that the extractable ‘humic substances’ — complex compounds known as fulvic acids and humic acids — could only be extracted from soils with extremely alkaline solutions of pH 13. And even this extraction left behind an unextractable fraction collectively called ‘humins’, of indeterminate nature. They argued that this extraction technique was outdated, incomplete, and invalid, as this crazy-high (my words) pH solution was extracting soil components that were not meant to be included, such as the living microbial biomass and non-decomposed organic matter such as leaf fragments.
Lehmann and Kleber instead proposed a ’soil continuum model (SCM)’, in which there was not a soil entity called ‘humus’, but rather a continuum of organic molecules of all sizes at various rates of ‘churn’ in decomposition.
The authors did have valid points. These extraction solutions were, after all, devised by soil chemists over 200 years ago, who couldn’t possibly have known the role of biology, microbiology and biochemistry in soil chemistry — those fields came much later.
It was perfectly reasonable of these chemists to regard soil as nothing more than some lifeless medium for them to pick apart through chemical analysis. Yet such a high pH extraction technique with all those hydroxide ions (OH-) in solution practically guaranteed (via chemical reactions) the creation, if not alteration, of at least some of the very compounds being extracted.
Yet the ’soil continuum model’ was hardly a new and radical concept. While others had also covered the subject some years earlier (eg Piccolo 2002), the soil microbiologist and Nobel Laureate Selman Waksman was the first back in 1936 with his impressive 508 page book called Humus: Origin, Chemical Composition, and Importance in Nature. He wrote (emphasis mine):
“Chemically, humus consists of certain constituents of the original plant material resistant to further decomposition; of substances undergoing decomposition; of complexes resulting from decomposition, either by processes of hydrolysis or by oxidation and reduction; and of various compounds synthesized by microorganisms. Humus is a natural body; it is a composite entity, just as are plant, animal, and microbial substances; it is even much more complex chemically, since all of these materials contribute to its formation. Humus possesses certain specific physical, chemical, and biological properties which make it distinct from other natural organic bodies. Humus, in itself or by interaction with certain inorganic constituents of the soil, forms a complex colloidal system, the different constituents of which are held together by surface forces; this system is adaptable to changing conditions of reaction, moisture, and action of electrolytes. The numerous activities of the soil microorganisms take place in this system to a large extent.”
You can extract ‘humus’ (and organic matter) from soil — ’humus’, whatever it is, does exist and perhaps a definition everyone agrees on will come with time.
Thus to me, coming from this with a multidisciplinary approach, it makes perfect sense to consider humus to be a mixture of many different substances, of different origins, and in many diffferent states of decomposition. All reacting with each other via any number of different biological, chemical and physical actions, and each of these determined by a specific soil’s type and environment.
A hot, dry desert sand, for example, will have the high temperatures conducive to biological and chemical reactions, but will lack the organic matter and moisture that can drive those reactions to their full potential. A moist, temperate rainforest soil is likely to be ‘just right’ in all regards, and an open grassland is likely to be somewhere in between — but having said that, the hard to decompose lignin of woody trees will still break down more slowly than herbaceous grasses on an open plain.
With so many different inputs and variables it really is no surprise that ‘humus’ from one soil to the next is likely to vary widely in colour, composition, complexity, and stability.
On the whole, humus is regarded as a very stable component resistant to further degradation, with a lifespan of years to hundreds of years. This apparent stability may well be hiding an underlying ‘churn’ as larger and more complicated molecular structures are broken down into ever simpler ones, while simpler ones are simulataneously combined into ever more complex ones.
It is intuitive that larger molecules are broken down into smaller ones, but the reverse, not so much. Yet if a tree can make the very slow to decompose lignin molecules, there is no reason why microbes can’t also produce hard-to-break chemically complex structures as byproducts from their own enzymatic activities.
Humus adds fertility to a soil via the properties its stable and chemically complex nature brings. It improves soil structure by increasing porosity and water retention — when people dig compost into a soil so as to improve it, they are really providing the means by which soil microbes can both mineralise that organic matter to feed plants, as well as to slowly produce the humus for long-term soil health.
(If you have a really poor soil, by all means dig plenty of rich compost in to kick-start this process, but once added once, it’s best to add future organic matter to the surface as a top dressing, for slow and more natural incorporation, rather than to dig it in again. Digging disturbs the precious soil structure and microbial communities that had been forming all that time.)
Humus, like clay, is a colloid with very large negatively-charged surface areas. This increases a soil’s cation exchange capacity, holding excess plant nutrients ready for slow-release as needed by plants instead of being leached away. Humus also directly feeds the microbes able to break down the complex chemical bonds within it.
Humus acts as a buffering agent in soils, keeping a soil within a constant pH range neither too alkaline nor acidic. This lack of fluctuation creates the conditions for maximum nutrient availability and minimal disruption to supply.
An added benefit of humus is its dark colour, which helps soils to warm earlier at the end of winter. This has many benefits including seed germination, root stimulation, and faster microbial activity.
Faster microbial activity leads to faster mineralisation of organic matter and faster development of still more humus, which leads to still more improved soil structure and all-round soil fertility and health.