What Soil Is Really Made Of
Why microbial necromass may be the key to regenerative viticulture
It is easily assumed that soil represents some form of end point, a final destination if you will, for life that originates above ground. Plants, though rooted in the earth, senesce and shed leaves in autumn, and eventually die, returning their biomass to the soil. Over time these remains decay and are transformed into humus, allowing new plants to take root and continue the cycle. It is a tidy explanation, and for much of the last century it has been broadly accepted as the primary process through which fertile soils are formed.
Yet this picture, elegant though it may be, is rather incomplete.
The transformation of fallen leaves and dead roots into the dark organic matter we call soil is not simply the slow weathering of plant remains. Almost as soon as plant material enters the soil environment it encounters a vast and largely invisible community of bacteria, fungi, and other microorganisms that begin breaking it down. These organisms feed on plant tissues, dismantling complex compounds and drawing carbon and nutrients into their own bodies.
Because microbial life cycles are extraordinarily short, this living biomass is constantly turning over. Microbes grow, reproduce, and die in immense numbers, and the products of this continual activity become intimately mixed with the mineral particles and physical structure of the soil.
Only relatively recently has soil science begun to fully appreciate just how central this microbial activity is to the formation and persistence of soil organic matter. Far from being a simple resting place for decaying plant material, soil increasingly appears to function as a dynamic biological system whose internal cycles of growth, transformation, and decay shape the very substance that gives it fertility.
A Paradigm Shift
For most of the 20th century, the formation of humus was explained through what is often referred to as the classical humus model. According to this framework, which was roughly outlined above, soil organic matter originated primarily through the gradual decomposition of plant residues. Leaves, stems, and roots entering the soil were believed to be progressively broken down through physical weathering and microbial activity, leaving behind increasingly resistant compounds.
Over time these residues were thought to undergo a process known as humification, forming complex and chemically distinct substances collectively referred to as humus. Humus was imagined as a stable class of molecules that resisted further decomposition, slowly accumulating in soils and serving as the principal reservoir of soil fertility.
Around the turn of the millennium, however, advances in analytical chemistry, isotope tracing, molecular biology and computational modelling began to challenge this long-standing view. Researchers increasingly found that many compounds once assumed to be intrinsically resistant to decay, including lignin and other structurally complex plant molecules, could in fact be readily decomposed by soil microorganisms.
One of the most influential reassessments of this framework came with the publication of Schmidt et al. (2011), Persistence of soil organic matter as an ecosystem property. Drawing on evidence from isotope tracing, nuclear magnetic resonance spectroscopy, molecular biomarker analysis and soil fractionation studies, the authors argued that the classical concept of humus as a chemically unique and inherently stable substance could no longer be sustained.
Instead, their work suggested that the persistence of carbon in soil was not primarily determined by the chemical resistance of individual molecules. Most organic compounds, regardless of their origin, were found to be susceptible to microbial decomposition. What ultimately determined whether carbon remained in soil for years, decades, or centuries was the environmental context in which it existed. Factors such as the accessibility of organic matter to microbes, its physical protection within soil aggregates, and its association with reactive mineral surfaces were shown to play a far more decisive role in determining how long carbon persisted.
This marked a significant shift in how soil organic matter is understood. Rather than representing the final stage of plant decay, soil carbon is increasingly seen as the result of a continuous interaction between biological activity and the physical structure of the soil environment.
Within this framework, microbial communities occupy a central role. As microbes consume plant residues and root exudates, they convert these materials into their own biomass before releasing them again through metabolic processes and cell death. Portions of this microbial material can then become stabilised through interactions with minerals or by becoming physically trapped within soil aggregates.
Evidence for the scale of this process has grown rapidly in recent years. Using global datasets of amino-sugar biomarkers, Liang et al. (2019) showed that microbial necromass can constitute more than half of soil organic carbon. This is a fascinating discovery, which should reshape how we think about soil formation entirely.
Implications for Viticulture
The fact that microbes and their necromass are responsible for such a substantial portion of soil organic carbon has a couple of key implications for how we go about soil management. Primarily, it shifts the focus from simply adding organic matter to managing the biological processes that convert plant carbon into microbial residues and then stabilise those residues in soil. The vineyard floor therefore becomes less a space where organic matter such as compost is allowed to break down and accumulate, and should rather be thought of as a system where microbial life is fed and continuously cycled.
Importantly, this realisation lets us concentrate on additions of fungal and bacterial stimulants and food sources rather than direct inputs of carbon. The most immediate implication of course is highlighting the importance of a healthy and diverse cover crop, which, depending on the plant, can contribute as much as 20-40% of photosynthate to the soil in the form of root exudates, namely sugars, amino acids and organic acids.
The old model which ignores the importance of the microbial necromass would suggest that once the cover crop is terminated, the crops biomass gets broken down and contributes to long term storage. Rather, this new insight suggests that for viticulture, maintaining continuous plant cover between rows may be far more important for building soil carbon than periodically adding organic amendments. Cover crops keep the microbial community active throughout the year, fuelling microbial growth and the turnover that ultimately produces necromass.
The composition of this cover crop is also important, as in addition to carbon, microbial growth also requires nutrients, in particular nitrogen. When microbial communities are well supplied with balanced substrates, they convert a greater fraction of plant carbon into microbial biomass rather than respiring it as CO₂. In vineyards, cover crop mixtures that include legumes alongside grasses or cereals can help maintain this balance, providing both carbon-rich residues and biologically fixed nitrogen that supports microbial growth.
The growing recognition of fungal necromass as a particularly important component of stable soil carbon also carries implications for vineyard management. Fungi, especially mycorrhizal fungi, produce extensive hyphal networks that not only help vines acquire nutrients but also contribute to soil aggregation and carbon stabilisation. Practices that preserve fungal networks, such as reduced tillage, minimal soil disturbance, and maintaining perennial ground cover, can therefore help promote both soil structure and long-term carbon accumulation.
Another important factor is soil mineral interactions. Much of the stability of microbial necromass arises when microbial residues bind to clay particles or metal oxides in soil. This suggests that soil building is not only a matter of increasing biological activity but also of maintaining good soil structure and aggregation, which creates surfaces and protected spaces where microbial residues can persist. Excessive tillage, which breaks apart aggregates and exposes protected organic matter to oxygen and microbial attack, can therefore accelerate the loss of soil carbon even in systems where organic inputs are high.
Does It Mean Better Wine?
The ultimate question, at least from a mindset that prioritises wine quality, is whether accommodating this high microbial activity, and the regenerative soil building it implies, actually makes better wine. The answer to that is of course not entirely straightforward, in particular because the arguments linking microbial necromass-driven soil building to better wine is far from direct. They are however quite compelling when we follow the chain of mechanisms at play. The key argument is that soils rich in biologically derived organic matter tend to create more stable vine growth, better nutrient balance, improved water regulation and, critically, healthier soil-root interactions. These are factors that, when a vine is thriving and the rest of the factors influencing terroir and climate play along, can greatly influence grape composition and the availability of critical flavour and aroma precursors for the yeast that eventually turn the juice into wine. In the same way that the inorganic rocks that make up part of the soil do not impart specific flavours to the wine, microbes only affect grape composition indirectly by influencing nutrient cycling and availability. Microbial residues contain significant amounts of nitrogen, phosphorus, and sulphur. Because necromass is stabilised in soils rather than immediately mineralised, it acts as a slow-release nutrient reservoir which leads to a more gradual and balanced supply of nutrients to vines, rather than the sudden pulses that can accompany synthetic fertilisation.
Furthermore, microbial necromass is one of the primary components of mineral-associated organic matter, which helps form stable soil aggregates. These aggregates improve porosity and water retention while also maintaining drainage and oxygen availability around roots. In vineyards, this creates a soil environment that buffers vines against both drought and excessive moisture. When water supply is more stable, vines tend to experience moderate, controlled stress rather than severe stress events, which is widely associated with more consistent ripening and improved phenolic development.
The presence of abundant fungal networks, closely tied to the formation of fungal necromass, also plays an important role. Mycorrhizal fungi extend the effective root system of the vine, increasing the surface area through which nutrients and water can be acquired. They can improve access to relatively immobile nutrients such as phosphorus and certain micronutrients. In addition to improving nutrient uptake, these fungal networks contribute to soil aggregation and structure, reinforcing the physical stability of the soil environment around the vine roots.
There are of course far more, and highly interconnected processes at work here, but what is important to note is that contrary to the idea that the vine needs to struggle through poor soil in order for its roots to grow deep and far, a much more resilient system can be encouraged that leverages the many incredible benefits of symbiotic associations with mycorrhizal fungi and other microbes.
Simply put, soils rich in microbial-derived organic matter tend to support more diverse microbial communities. These communities can suppress soil-borne pathogens through competition and predation, creating a more balanced soil ecosystem. Healthier vines that experience fewer stress shocks or disease pressures tend to ripen fruit more evenly and consistently, which is critical for producing balanced wines.
Taken together, it appears evident that soils rich in microbial necromass help create stable, well-structured, biologically active vineyard soils. These soils support vines that grow with moderate vigour, access nutrients and water in a balanced way, and experience fewer physiological extremes during the growing season. It is reasonable to expect the resulting fruit to ripen more evenly and develop the compositional balance that winemakers often associate with high-quality wine.
Final Thoughts
From a practical standpoint, much of this is in line with current regenerative practices, but thinking of soil building as being a primarily necromass driven process should open the door for some ways to optimise regenerative processes.
Those inputs currently focused on adding biology and microbes directly for instance, while causing no harm, might be replaced or supplemented with efforts to feed the microbes that are already present in the soil. Taking bacteria as an example, the fact that their populations increase exponentially, doubling roughly every 20 minutes provided conditions allow for it, implies that every soil system out there is effectively operating at bacterial capacity. If more food was available, there would be an almost immediate increase in the bacterial population to take advantage of it. The microbial food that can be introduced to that system therefore is the limiting factor for regenerative efficiency, and potentially, the key healthier vines and better wine.
As always, I look forward to your thoughts on this topic, and if you have any first hand experiences with soil building in a viticultural context it would be amazing to hear from you.
Another thoughtful, well written article on a subject near and dear to me and my livelihood.
A couple of thoughts/observations. Every soil/site is different and depending on its origin and content (both organic and inorganic) requires a different approach to creating the healthiest soil for the crop being farmed. Location/Location/Location! In my case farming soil created 65 milliion years ago by the formation of the coastal mountains in Northern California (Sonoma) by tectonic plates coming together.
My soil is deep, but very low CEC and OM, made worse by haviing been farmed over 135 years. We are in a climate where rain rarely falls from mid-April to late October/early November. One of the current beliefs is that cultivation is bad. But in my soil without minimal and shallow cultivation I could not "dry farm" my vineyard successfully (and by success I mean achieving eonomically sustainable yields). Even if I did irrigate (drip) the roots would not have access to soil available nutrients and would have to "fertigate" or put nutrients under drip. By cultivating late/light I maintain soil moisture and subsequently access to nutrients in the second foot of soil which contains the majority of the OM and available nutrients. If I did "no-till" the soil would dry out down to around 2' (60cm). Roots will not grow in soil without available water and therefore cannot absorb any nutrients. Additionally it is said that cultivation wipes out micro flora/fauna. That is correct but once rains return in the late fall these guys come back with a vengeance. Especially given that I do an annual compost addition right after harvest (12-15 tons/ha). Originaly I thought the main benefit ot the compost was to feed the newly planted cover crop (bell beans, snow peas, vetch, barley and oats), but now appreciate how it stimulates all the stuff you can't see (though the mychorrizae is visible to the naked eye). You can successfully do "no-till" where you have summer rains or soils with OM/sufficient nutrients below the 60cm depth in my experience.
I will probably not be the first or last commentator to say "yeah, that's kind of what I figured". I have zero understanding of soil microbiology but it seemed like some of the answers to what influences obvious flavour differences in different terroirs had to lie to some degree in this invisible and barely understood "last frontier" of bacteria, fungi and other members of this invisible but influential menagerie in the soil. For me, this vague hunch was hastened by the work of Suzanne Simard in her book The Mother Tree. Using radioactive isotopes she was able to determined that certain types of trees were not competitive but actually tribal and even cooperated with other types of trees. In just one example, she showed that a conifer would through the mycorrhizal network transmit carbon to young seedlings in need and even "lend" it to deciduous trees. I thought, well if she has proved that this is possible what is the implication for vineyards? What and how are their networks transmitting? Maltman and others were lecturing on the impossibility of minerals influencing wine's flavours, but we all knew we could taste it, right? So perhaps there was something about the presence of minerals, something in between that was influenced by them that produced what our palates were clearly perceiving? It seems like the answers are starting to arrive. Thanks again George. It's so great where you boldly go.