Is Wine Made in the Vineyard?

How soil biology, vineyard management, and microbial succession intersect to influence wine quality

A while ago I wrote an article an article which explored the role of native yeasts in shaping our perception of terroir. Specifically, it looked at how the yeast strains found on grapes, though consisting largely of the same species, can vary tremendously in their composition depending on where in the world they are found. This microbial biogeography influences how native fermentations progress, and ultimately impacts the flavour profile of the finished wine. While the alcohol tolerant Saccharomyces cerevisiae eventually comes to dominate the must, the effect of supporting yeast strains is notable and will have an effect even when the wine is inoculated with commercial strains.

Yeast however represent only a relatively small portion of the microbial community that exists both in and on the vine, as well as in the soils they grow in. In fact, with the exception of the highly useful yeasts, the role of microbes has largely been ignored, unless it is a matter of spoilage bacteria which puts our wines at risk. This is in part a consequence of our ability to analyse these microbial populations. With improvements in RNA sequencing techniques however, it is now possible to analyse the relative abundance of bacteria and fungi in grape juice and must. It is an area of study that potentially has consequences for vineyard management, particularly because evidence suggests that there may be a far closer link between the microbiota in the soil, over which we exert substantial influence, and the population of bacteria and fungi that eventually make their way into the grape must.

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For biodynamic practitioners this might intuitively fit with experience, but it is a potential connection that is almost entirely neglected in modern viticultural practices. I should add of course that biodynamic guidelines, though recommending the inoculation of soils with highly microbially active preparations such as fermented cow manure, do not make any claims as to these preparations ability to influence the microbial composition of grape must. In part that is because many of the fundamental guidelines stem from a time before we had any way of measuring this impact. We also have no method of quantifying the cosmic energy imparted to water swirled in a certain manner either, but that’s rant for another day.

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The Unseen Architects of Flavour

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An Old Adage Revisited

We’ve all heard the cliché that wine is made in the vineyard. It is a sentiment that every now and then meets with resistance from certain parts of the wine press. Recently, an instalment of the VinePair podcast argued passionately that this trite statement is not only misleading, but actively diminishes the skill and craftsmanship of the winemaker. While the idea that wine is grown is certainly a romantic notion, there is no doubt that it only becomes the refined product it is as a direct result of the interventions made in the cellar. Yet the nature and the quality of the raw materials a winemaker has to work with is a direct consequence of what goes on in the vineyard, and great quality wine cannot be made unless the fruit is up to a certain standard.

What is beginning to emerge, however, is that this separation between vineyard and fermentation is far less clean than we tend to assume. A number of studies now point toward a continuous microbial gradient running from soil to vine to grape, and ultimately into the ferment itself. The communities present in grape must are not assembled at random at the moment of crushing, nor are they dictated solely by cellar conditions. They appear, at least in part, to be inherited. The consequences of this are quite interesting.

Photo by Vindemia Winery on Unsplash

Work published in Proceedings of the National Academy of Sciences demonstrated that microbial communities associated with grapes exhibit clear regional signatures, to the extent that fermentations could be distinguished based on their site of origin. Both bacterial and fungal populations showed consistent, site-specific patterns, and importantly, these patterns were not erased during fermentation. While Saccharomyces cerevisiae inevitably came to dominate, the early and mid-fermentation stages retained a fingerprint of the initial community, influencing metabolite production in ways that remained measurable in the finished wine. It is a finding that alone challenges the long-standing assumption that the winery is the primary site of control over fermentation outcomes. Instead, it suggests that part of that control is exercised upstream, in the vineyard, through processes that are far less visible.

Not only that, it supports the notion that a description of terroir cannot be complete on the basis the physical growing conditions for a particular vine alone. While drainage, aspect, latitude and the many other conditions shaping the environment are critical, the microbial community clearly cannot be ignored as a key variable influencing the regional identity of a wine.

The most well studied group of plant-associated microbes are endophytes, which live in the soil surrounding the roots and in the roots themselves. While colonising plant tissue, endophytes are generally known to cause no harm to the host plant and can in many instances alleviate plant abiotic stress, promote plant growth and antagonise pathogens that might pose a danger to the host plant. Microbes living in the soil and colonising the root system of a plant can also migrate further through the plant to colonise aerial tissues, either internally or externally (in which case they’ll be considered epiphytic microbes). The exact mechanism of this migration through the host plant is however quite poorly understood and more research is needed. Some alternative pathways for colonisation do however include soil dust contamination by wind or rain splash, and human interference during harvesting or other vineyard management tasks. The phyllosphere, or leaf zone of nearby plants may also be the source of aerial or insect based distribution of microbes, which suggests that local biodiversity may impact which species of microbes are found on the grapes at harvest.

The vine is not a passive participant in this process, nor is any plant with photosynthetic activity, as root exudates actively encourage the growth and distribution of microbial species in the rhizosphere. These in turn greatly influence nutrient availability for the plant, playing a key role in its physiology. From a vineyard management point of view, the link between soil based microbes and the microbiota making its way into fermentation suggests that regenerative farming practices which aim to build soil health by creating advantageous conditions for microbial soil life can actively shape the flavour profile of the wine, not just by giving the plant the best possible chance of creating flavour and aroma precursors, but also by shaping the microbial environment in which these precursors are developed during the fermentation process.

A study by Rosado et al. from 2022 actively explores the impact of adding vermicompost, a mixture of worm castings and decomposed organic material, as a soil supplement to an Albariño vineyard in the northwest of Spain (O Rosal, near the Portuguese border). This vermicompost, derived from grape marc from the same vineyard, was added to one part of a vineyard block, with the other part being used as a control. While the study goes on to make some anecdotal claims as to the resulting flavour profile of the resulting wines, noting a distinct difference, and indeed organoleptic improvement in the wines made from the vermicompost treated vines, it the heart of the study focuses on the microbiota of the respective grape musts prior to fermentation.

Vermicompost in the making

Using RNA sequencing methods to analyse the musts, they found that the musts originating from the vermicompost-treated soils exhibited a distinctly different microbial composition compared to the control, both in terms of abundance and the overall community structure. It represents not just a marginal change, but a measurable difference in the bacterial and fungal communities that are present at the point of crushing. Differences in the microbiome at this stage, essentially the starting line of the fermentation process, is critical because it sets the initial conditions under which microbial succession unfolds during fermentation. The fermentation process is after all a highly competitive, sequential takeover in which early colonisers modify their environment, using available oxygen, depleting available nutrients and producing metabolites in a way that influences which organisms can follow. While alcohol tolerance ultimately leads to S. cerevisiae coming out on top, these earlier stages are crucial in shaping the flavour profile of the wine, and do not depend entirely on alternative yeasts.

It is worth noting here that the fact that the vermicompost was derived from grape marc originating in the same vineyard creates a form of microbial feedback loop in which the worms, effectively recycle and amplify elements of the vineyard’s existing microbiota, even if in a transformed and stabilised form. The vermicomposting process is known for its ability to enrich microbial diversity and functional activity, creating a community that is both more abundant and more metabolically versatile than the original substrate on which the worms feed. It is this that makes it such a powerful soil amendment, not only giving a direct nutrient boost, but also supercharging the rhizosphere with a much stronger microbial community.

Venn diagrams showing the number of shared bacterial (A) and fungal (B) ASVs between control and treatment groups of the must and wine Albariño microbiome. Source

Once in the soil, the now-altered soil community will disperse, with some eventually making their way up through the rest of the vine. By the time the grapes reach maturity, the microbial populations present on their surfaces will bear the imprint of the soil environment in which they were grown.

Management Options

While more research is clearly needed on this subject, the fact that vignerons can actively influence the microbial starting point for wine fermentation through soil amendments opens the door to a host of possibilities. Though the paper by Rosado et al. showed vermicompost to be an effective tool for altering the soil and vine microbiome, there are a range of other amendments that can be employed and which neatly align with organic and regenerative practices. In essence it is a matter of introducing biologically active carbon and microbes, and for the sake of ease, let’s break some of them down by their defining properties, namely whether they are solid or liquid. This is a distinction that matters mainly in terms of practical application and how they may be produced.

Solid Amendments

Solid amendments are perhaps what we are most familiar with when it comes to soil amendments. They form the foundation of any attempt to reshape the soil microbiome, and have the advantage of functioning on timescales that allow ecological processes to take hold. These materials include compost, vermicompost, mulches and cover crop residues, which introduce both organic carbon as well as living microbial communities to the soil. They also create habitat for these microbial communities to exist in. They decompose gradually, feeding microbial populations over time, and encourages the formation of more complex, stable consortia. Their function is largely distinguished by the degree to which they are broken down at the time of application, and the method of their preparation. Compost will introduce a broad range of microorganisms and partially stabilised organic matter, while vermicompost tends to be more biologically active and considerably more concentrated. Mulches such as wood chips or leaf litter on the other hand act more as substrate than inoculants, but aid in shifting the local ecosystem toward fungal-dominated pathways as they slowly break down. Biochar is another solid amendment which, in addition to its ability to retain moisture and bind with nutrients in the soil severs as extensive and protected microbial habitat, which helps stabilise, and indirectly grow, microbial soil communities.

Fungally dominated mulch: Photo by Julietta Watson on Unsplash

Cover crops occupy an important middle ground, functioning as a living amendment whose root exudates continuously shape the rhizosphere before ultimately contributing biomass back into the soil. All these approaches are united in that they don’t simply feed the vine, as one might do with synthetic fertilisers, but rather build and sustain a microbial habitat. Furthermore, increased organic matter improves water retention, glomalin and other protein rich exudates from fungi and plants aid in soil aggregation and structure, moderates temperature fluctuations and feed the entire soil food-web. The increased abundance that results increases the likelihood that a broader range of microbes will participate in the migration from soil to vine, and eventually to grape.

Liquid Amendments

Liquid amendments on the other hand work in a different manner. Rather than building structure and providing habitat, they act more like pulses, either introducing microbes directly or stimulating those already present. These can take the form of compost teas and vermicompost extracts, which would fall into the former category, effectively concentrating and dispersing microbial populations in a form that can rapidly be applied to the soil or canopy. They are short term inoculations, which can shift the microbial balance and introduce beneficial species that can aid in kickstarting a broader ecological cycle of microbial feeding and regeneration.

Fermented inputs such as plant ferments, fish amino acids preparations or cultures of lactic acid bacteria tend to function more as stimulants. They do so by supplying readily available carbon and nitrogen compounds that drive short-term microbial activity. This can also be achieved to some degree by simple additions such as molasses which accelerate microbial grown without necessarily changing the underlying composition of the community. More transient, they need more frequent application and are perhaps best used in combination with solid amendments which can establish the ecological framework for these preparations to have the greatest impact, amplifying and directing the natural processes that are already underway.

Liquid amendments have the secondary benefit of being very cost effective to produce, even at scale, on site. Solid amendments tend to require substantial amounts of biomass that in many cases may not be readily available, creating a reliance on externally sourced material. If you’re interested, there is a substantial amount of YouTube videos out there dedicated to this sort of thing, the key search term being Korean Natural Farming, which is its own little sub-genre of amendment focused regenerative farming.

While it has been established that such amendments can influence the microbiota in the resulting grape must, their primary function should still be considered in aiding soil fertility and plant health. A diverse and well functioning soil ecosystem has significant benefits in terms of increased disease tolerance and resilience for the vine, which not only has a positive impact on yield but improves fruit quality.

Final Thoughts

There are clear limitations to the idea that we can shape the flavour of the resulting wine through microbially active soil amendments. A key constraint is the fact that we are not able to control the final composition of the microbiome present on (and in) the grapes at the time of harvest. Secondly, organoleptic studies evaluating the impact of various starter cultures, as it were, are lacking. There is a fair amount of anecdotal evidence out there suggesting that farmers that have switched to organic or regenerative farming practices have seen the profile of their wines become fuller, richer and possibly more complex, but there is no work out there that I have been able to find which with confidence links any particular soil amendment practice, or soil bacteria or fungi with specific organoleptic properties. It is definitely research I hope someone will undertake however.

In terms of controlling the presence of specific bacteria on the grapes, foliar applications and sprays may work, but without clear evidence of what the effect on the flavour of the wine will be one would effectively be taking a shot in the dark. At present, I think this is an interesting element to the already mind-bogglingly complex concept of terroir which growers ought to be aware of. While we do not know the exact consequences of such amendments on flavour profile, we are however able to gauge plant health quite well, and I think that is where current focus should be, and where the conscious fostering of a beneficial habitat for soil microbes becomes critical.

This also opens up the question of whether terroir associated flavours can be transferred through inoculation. Could one achieve flavour profiles that resemble famed vineyards through heavy propagation and subsequent inoculation of soil bacteria from one location to another? I doubt it, but it could be an interesting experiment as well.

I hope you’ve enjoyed this, and look forward to your comments and thoughts on the matter.

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