Following a recent article in which I expressed concern regarding the lack of evidence-based practise at the core of biodynamic winemaking, I spoke with Keith Edward’s of Mise en abyme. We discussed in more detail what exactly I’d like to see result from the discussion sparked around the legitimacy of biodynamics. My answer was simple. I’d like to see the wine industry champion rigorous, evidence-based farming centred around maintaining healthy soils and fostering diverse, resilient ecosystems. Though still not throughly understood, it is commonly accepted that microbiome is essential in upholding the fabric of life. Our gut, mouth and skin each host their own unique microbiome community whilst healthy soil microbiome remains crucial for the growth and longevity of crops and wildlife. Nurturing this symbiotic relationship between a community of bacteria, archaea, viruses, fungi and protozoa is a core tenet of biodynamics. However, a number of studies have shown biodynamic preparations to be ineffective in improving soil health metrics. In this article, I explore microbiome in more detail and discuss working, evidence-based practises aimed at strengthening and diversifying soil microbiome.

The microbiome

Root microbiome is the diverse, dynamic community of microorganisms associated with plant roots. Plant roots provide unique environments for a diverse collection of soil microorganisms, including bacteria, fungi and archaea. The microbial communities around the root and in the rhizosphere are distinct from each other and from the microbial communities of bulk soil, although there is some overlap in species composition. Research suggests that plants may, in fact, self-select their own microbial communities, different plants planted in the same soil only metres apart display entirely different microbial communities.

Beneficial soil microorganisms include bacteria which fix nitrogen and promote plant growth including mycorrhizal fungi, protozoa and certain biocontrol microorganisms. The mycorrhizal mutualistic association is an astonishing display of natures self-sufficient potential. The mycorrhizal relationship provides the fungus with nearly constant and direct access to carbohydrates, such as glucose and sucrose. The carbohydrates are translocated from their source to root tissue and on to the plant’s fungal partners. In return, the plant gains the benefits of the mycelium‘s higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hypha, which is much longer and finer than plant root hairs, and partly because some such fungi can mobilise soil minerals unavailable to the plants’ roots. 

Unaided plant roots may be unable to take up nutrients that are chemically or physically immobilised; examples include phosphate ions and micronutrients such as iron. Immobilisation can occur in soil with high clay content, or soils with a strongly basic pH. The mycelium of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonise. Immobilisation also occurs when nutrients are ‘locked up’ in organic matter that is slow to decay, such as wood. Some mycorrhizal fungi act directly as decay organisms, mobilising the nutrients and passing some onto the host plants.

Early research indicates that a strong, diverse root and soil microbiome is paramount to sustainable and rewarding agriculture. Until recent advances in sequencing technologies, root microbes were difficult to study due to high species diversity, thus understanding is in a stage of relative infancy. However, existing peer-reviewed research clearly indicates that healthy and diverse root and soil microbiome increases nutrient uptake, disease, drought and salinity resistance, resistance to toxicity and resistance to insects.

In summary, robust soil and root microbiome supercharge the plant’s natural ability to survive, prosper and yield quality fruit. This symbiotic relationship supports sustainability and longevity thus reducing the need for pervasive, costly and ongoing interference. For a rigorous exploration of available understanding and future direction of the role of soil microorganisms in plant mineral nutrition, Jacoby et al. have covered the topic in great detail.

Harming microbiome

Pessimistic estimates vary, the UN claims there to be around 60 years of harvests left whilst UK ministers have placed the figure at around 30-40 years. Much of this is sensationalism, soil experts agree that setting an end-point for agriculture is nigh on impossible and any estimates are likely unrealistic. That being said, most experts agree there are real threats to many agricultural soils around the world.

Reviews show that prolonged, intensive and indiscriminate use of agrochemicals adversely affects soil biodiversity, agricultural sustainability, and food safety, bringing in long-term harmful effects on nutritional security, human and animal health. Most of these agrochemicals negatively affect soil microbial functions and biochemical processes. The alteration in diversity and composition of the microbiome can be unfavourable to plant growth and development either by reducing nutrient availability or by increasing disease incidence.

In recent years, conscious farmers have ushered in a shift away from widespread, long-term and over-application of pesticides and herbicides. Studies have shown severe effects on soil ecology that may lead to alterations in, or the erosion of, beneficial plant probiotic soil microflora.

Industrial agriculture can be particularly troubles, mono-cropping, the practice of growing the same crop on the same plot of land, year after year, depletes the soil of nutrients, reduces organic matter in soil and can cause significant erosion. Mechanical tillage and the use of heavy farm equipment can cause both soil compaction and soil erosion. Soil compaction is often caused by heavy farm machinery and tilling when soils are too wet. Compaction leads to poor water absorption and poor aeration which further lead to stunted root growth in plants and smaller yields.

Erosion can be caused by many different factors, but poor soil management, including tilling, has been known to cause significant erosion over time, as can practices such as not planting cover crops in winter and not mulching. In a recent article, I discussed the efforts of Piedmontese farmers working to negate erosion. 

It’s all too easy to pontificate over ‘modern agriculture’, and to denounce it based beliefs of what ought to be. As Stephen Skelton MW points out ‘since the 1920s, the use of modern farming methods, machinery and improvements in planting material has vastly improved yields, thus lowering the price of food and increasing the availability, in both type and season, of what we eat. In 1960 we spent 17% of disposable income on food, today it is 9.5%.‘

This achievement is only a touch short of a miracle, many millions of people have been lifted out of absolute poverty over the last 8 decades and whilst one can certainly point to an array of unforeseen circumstances (long-term soil fertility being one) arbitrary moral relativism and blaming science is poor sportsmanship. Modern agriculture solved a profound humanitarian problem, sure there are practices which require review, but we ought to express more than a little humility pre-judgement for it can be argued our very existence and prosperity is in part owed to modern agriculture.

The failure of biodynamics

I’ll spend as little time as possible talking about biodynamics, given that I have already expressed my thoughts in great detail. However, in his brief rebuttal to my piece, Craig Camp mentioned soil microbiome a total of 8 times, it’s clear that this topic is at the heart of his affinity with biodynamics. Craig spoke of ‘the secrets of naturally building the microbiome‘ of how biodynamic preparations are intended to ‘rebuild soil microbiome‘ and of soil microbiome and mycorrhizae being the ‘hot topic of modern agri-science‘

There’s little debate about whether soil microbiome should be at the forefront of viticultural discussions, whether the topic is yield, grape quality, soil health or sustainability, microbiome appears to be the glue which holds it all together. But let’s be clear about one thing, if the aim of biodynamics, in any form, is to improve soil microbiome it fails miserably.

In summary, peer-reviewed research provides little evidence that biodynamic preparations improve soils, enhance microbes, increase crop quality or yields, or control pests or pathogens. Reviews also establish that the additional costs associated with formulating and applying the preparations represent an economic loss when compared to organic farming.

Studies found no significant differences between soils fertilized with preparations 500–508 vs non-biodynamic compost. Other studies confirm a lack of efficacy on soil fertility from preparations 500–507. Organic matter in organically treated soils has been shown to be higher than that in unmanured soils treated with biodynamic preparations 500–504. Similarly, studies reported enhanced soil life in organically managed fields compared with those under biodynamic management.

Studies observing the effect of biodynamic preparations 502–507, specifically meant for use on the compost, reported a consistently higher pile temperature and more nitrate in the finished compost using these preparations. In contrast, researchers have found that biodynamic preparations reduced both compost pile temperature and nitrate concentration. Craig entirely misunderstood the importance of these findings, it was not that I misunderstood the intention of the preparation it was to highlight that no consistent outcome was achieved.

Researchers have consistently found no differences in microbial activity, biomass, or fungal colonization in biodynamically treated soils compared with organically managed soils. A single report of greater dehydrogenase activity in biodynamically treated compost linked to greater microbial activity has been observed. When added to organically grown crops, biodynamic preparations have been uniformly ineffective. Compared with organically managed systems, additions of biodynamic preparations did not affect yields of cover crops, forage grasses, lentil, rice, spelt, sunflower, or wheat. 

Based on both available literature and rational deduction it seems evident that any improvement in soil health observed under biodynamic farming ought to be attributed to the organic elements of biodynamics, increased time spent in the vineyard, self-selection and uncontrolled variables. If we are to take soil microbiome seriously we ought to stop romanticising pseudoscience and look to the established literature and serious work of scientists and farmers who have established practical, sensible methods to improve soil microbiome.

Practical farming for healthy soil microbiome

Opposed to seeking to discredit existing literature in the hope of future studies affirming a priori beliefs. Or seeking obscure, abstract reinforcements to support outdated, pseudoscientific beliefs it seems to be infinitely, and immediately, more beneficial to promote and share an understanding of practical, evidence-based solutions shown to reinvigorate and reinforce soil health.

Cover crop is a crop of a specific plant grown primarily for the benefit of the soil rather than the crop yield. Cover crops are commonly used to suppress weeds, manage soil erosion, help build and improve soil fertility and quality, and control diseases and pests. Cover crops are typically grasses or legumes but may be comprised of other green plants.

A meta-analysis of current research (60 field studies) up to 2019 show that cover cropping significantly increased parameters of soil microbial abundance, activity, and diversity by 27%, 22%, and 2.5% respectively, compared to those of bare fallow. Further research indicates that multi-mix treatments, as opposed to single-mix cover crop, are better at maintaining overall microbial composition and diversity. A plethora of literature has reinforced the importance of cover crop, not only in viticulture but in tree crops and other plant life.

Tillage is the agricultural preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. There is a growing movement toward reduced or zero tillage in an effort to promote healthy, nutrient-rich fertile soils. In a three-year study, researchers demonstrated that tillage decisions played a major role in shaping microbial community structure, and in influencing additional environmental, ecological and agricultural soil parameters. Numerous other studies have demonstrated similar results. That being said causal mechanism is not well-established and the benefits of reduced tillage could be related to reduced compaction.

The most relevant human-induced causes of soil compaction in agriculture are the use of heavy machinery, tillage practice itself, inappropriate choice of tillage systems, as well as livestock trampling which all reduce pore space and aeration. Use of large and heavy machinery for agriculture often causes not only the topsoil but subsoil compaction. Compacted soil is economically damaging to farm business efficiency, as it results in reduced crop yield and increased fertiliser and energy input requirements. Several studies have also shown compacted soils to lack beneficial microbiology, particularly ectomycorrhiza, a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. Less compacted soils have also been shown to be more resistant and resilient.

There are a number of ways to reduce soil compaction (covered in detail here) including but not limited to avoiding wheel traffic and tillage of wet soils, using wider tires, minimising tractor weight, maintaining the minimum tire pressure needed for acceptable tire life, avoiding using oversized equipment, and combining field operations to make fewer total passes.

Conventional agriculture may target plant pathogens through the use of pesticides/fungicides, with a potential side effect of reducing soil microbial community diversity and evenness. Whereas, organic agriculture may seek to control plant pathogens through competition and/or antagonism by utilizing treatments that promote a more diverse and even microbial community such as the addition of varying types of organic matter or more creative practises such as vine skirts. Further work has explored available research on practical farming practises and their effect on soil microbiome.

Conclusion

For plants to access recalcitrant soil-borne nutrients, they are dependent upon the metabolic activities of soil microbiota. Despite the experimental challenges, scientists argue that many important factors have been identified that will enable us to understand the mechanisms governing dynamic plant-microbe interactions. We know that although the soil is the major determinant of the microbial community associated with plant roots, plants have a significant effect on taxonomic assembly. Continued progress in our capability to collect and analyze exudates will be important for assessing the molecules plants use for communication with soil microbes, and also the pathways the microbes use to decrypt these signals.