You’ve mentioned that you and the team at Rothamsted are working on a new theory of how soil works. Can you expand on this theory, and how it’s different to existing understanding? What was the old way of thinking?

For a long time, theory regarding soil function was directed solely at maximising yield from crops. Until recently our understanding of soil was largely based upon physical (the behaviour of water in soils, water holding capacity etc.) and chemical (pH and plant nutrition) understanding. So, the “old way of thinking” neglected the needs of soil as a living system in its own right. The consequence was routine soil disturbance through cultivation, prioritisation of crop over soil needs, and a reliance upon synthetic, inorganic approaches to fertilisation. Soil has effectively become starved of organic matter in the process. With the new approaches to sequencing DNA extracted from soil, it becomes possible to incorporate biological information in soil theory. Rather than focus upon soil biodiversity, we are focussing on incorporating the close relationship between soil architecture (the pore space of soil) and the metabolic activity (function) of microbes into a whole-system theory. This treats soil as a living porous medium, responsive to flows of energy in the form of carbon and other nutrients through soil. The theory describes soil’s multifunctionality—including water holding capacity, hydrodynamics and oxygen availability—which control nutrient cycling and use efficiency in productive agricultural systems.

Is your theory of soil as an extended composite phenotype of any practical use to New Zealand’s wine industry? Walk me through it. How do I apply this thinking in my vineyard?

Our aim is to develop a “unified theory of soil”, so we would be very disappointed if it was not of practical use to New Zealand’s wine industry. Although we’ve given the theory a rather grand title, in fact our evidence suggests that continual addition of organic matter to soil results in improvement of various aspects of soil performance (water holding capacity, building of biomass, retention of nutrients), and I think this will not be a surprise to attendees at the Beyond Vineyard Ecosystems conference. Applying this thinking to vineyards is about finding ways to drive organic matter through soils in a way that suits each manager/vineyard combination. There are many ways to do this: maintaining permanent plant growth between vines using herbal leys; regular addition of mulch or compost; incorporating livestock as part of the management. There were good practical examples of how to achieve this presented by growers Richard Leask and Robbie Holdaway.  It’s about learning from each other and taking aspects that you feel comfortable working into your methods of management. Anything that increases organic matter throughput in the soil will start to accrue benefits.

During your talk, you spent quite some time arguing that biodiversity is not simple to understand. Are you suggesting that we shouldn’t be worrying about soil biodiversity? If not, what should we be focussing on?

Yes—that’s exactly what I am suggesting. There is a great deal of interest in the general relationships between biodiversity and environmental “health”, with the assumption that more is always better.  The evidence we have gathered from long-term field experiments is that there is no consistent loss of biodiversity in soil microbial communities associated with management, even in soils that have been degraded intentionally by regular cultivation and organic matter starvation. A recent review of the scientific literature supports this observation. It is also the case that many of the existing microbial measurements are not easy to interpret and may not necessarily provide credible inferences about soil health.

Is there something that we can add to our soils to reduce the dead ends and increase porous connectivity? How do we connect understanding of soil and soil health with practical evidence of farming?

There are now several products and services which promise to measure the biodiversity of soils. I would suggest that these will always be largely a waste of money as diagnostic tools. Rather than fretting about soil biodiversity, I would suggest that vineyard managers will see greater benefits from identifying practices which support the continual input of organic matter into soil and fitting them into their current management approaches. Evidence indicates that over time, the continual flow of organic matter into soils creates a more porous, and more connected soil architecture (fewer dead ends) which stores more water. Here, the microbial metabolism shifts towards building biomass and storing nutrients within the soil, rather than losing them either to the atmosphere or groundwater.

So, I get it that you don’t think soil biodiversity is a useful way to measure soil health, but you haven’t really suggested anything practical that we should be measuring instead. So, I’m going to put you on the spot – what on earth should we be measuring? Can we simply test soil pore connectivity, and could this be a useful index for viticulturists?

Yes—I share your frustration. Our evidence suggests that the pore architecture in soil is an important emergent property that accounts for many of soil’s functions. We use X-ray computed tomography to visualise pores smaller than 100 micrometres. From these three-dimensional images, we can measure the degree of connectivity between pores, this we call connected porosity. Based upon our evidence, I think that this connectivity between pores would make an extremely useful index, not only to viticulturists but everyone concerned with managing soils in productive systems. However, measuring the porosity at this scale is very challenging, and there currently are no tools to measure connected porosity in the field. We are working to identify approaches to estimating connected porosity without using computed tomography, including infiltrometry (measuring the rate of water infiltration into soil) and handheld seismology approaches. But for now, these remain experimental.

Science is all about being precise and perfect, whereas farming is about getting stuff done. Of all of your years of research, what’s the one thing that you would say to farmers globally and viticulturists in New Zealand?

As you now know, I can never say just one thing when three will be better. So, I would say: do you really need to disturb (cultivate) your soil; always consider the “diet” you are providing your soil—the more complex the better; and collaborate with your peers so that our collective knowledge increases.

In all the research that you have undertaken, Andy, what’s the most detrimental farming practice in terms of soil health?

Soil disturbance (cultivation).

What happens when you disturb the soil?

Before discussing what happens when soil is disturbed, it’s worth thinking about how organic matter is sequestered in soil. Like many biological phenomena, soil is a hierarchical assemblage of interacting processes, stabilised and actively maintained at different timescales. Soil is best regarded as a process rather than a material. Organic matter is the fundamental causative agent generating structural complexity, as it acts to bind mineral particles and colloids together. This is predominantly via calcium (in water-limited, alkaline soils), or iron- and aluminium-oxyhydroxides (with increasing moisture availability and acidity) bridging between organic molecules and soil mineral surfaces, thus generating hierarchical structure. Plant root exudates may be sequestered directly, but plant and animal derived complex organic polymers are processed by microbes before joining the soil organic matter pool. This pool may take the form of microbial polysaccharide and protein exudates, as well as necromass, and is chemically structurally diverse. In effect, organic matter sequestered in soil is a continuum of progressively more extensively oxidised compounds, trapped between soil particles and within aggregates where it is inaccessible to further microbial activity. When soil is disturbed, this inaccessible organic matter becomes accessible as structure is lost. Once accessible, this organic matter may be metabolized by microbes and lost as carbon dioxide to the atmosphere. So, the effect of disturbance is a loss of organic matter and structure. If the lost organic matter is not replaced, gradually, the sequestered organic stock is reduced with significant consequences for structure, water holding capacity, nutrient cycling and, ultimately, crop performance.

What does compaction do to soil health, specifically soil’s ability to provide plant nutrients?

Compaction will cause a loss of soil porosity—both large-scale pores that are visible to the naked eye and responsible for water infiltration, and small-scale pores which I referred to in my talk. Thus, compacted soil is more likely to flood during periods of high rainfall. It is also more likely to become anaerobic (losing organic carbon and nutrients) as methane and nitrous oxide, both potent greenhouse gasses. The loss of small-scale pores will restrict plant access to nutrients as pore connectivity is reduced (there are likely to be more dead ends). Another recently identified issue with compaction is that it prevents diffusion of the plant hormone ethylene away from the root, which limits root growth.

The term “soil health” was only coined about 20 years ago, and even “biodiversity” was coined about 50 years ago. In terms of science, putting these concepts together is still relatively new. Now, you spent a lot of time telling us what healthy soil is not. So, how would you now define what soil health actually is?

I am not alone in being uncomfortable using the term “soil health”. Some argue the absence of a clear, quantifiable and agreed-upon definition of soil health makes it difficult for practitioners, particularly farmers, to know what to do practically to maintain it. Others accept that while it is not a useful concept, it does have value as a communication tool. It is interesting that similar discussions are associated with defining human health. But we don’t need to concern ourselves with semantics, because it is easy to identify what we require of our soil. In viticulture as well as agriculture, soil is required to provide access to water and nutrients for crop growth and provide stable—rather than diverse—microbial communities. This limits the establishment of populations of soil-borne plant pathogens. As I explained in my talk, the throughput of organic matter results in a soil state that provides greater water holding capacity (providing plants access to water during periods of limited precipitation) and a more oxygenated environment where nutrients such as nitrogen are turned over as biomass rather than being lost to the atmosphere or leached to groundwater. In addition, although I didn’t cover this in my talk, our evidence suggests that microbial communities in disturbed soil are subject to greater randomness as the community assembles, providing the potential for pathogen establishment. So, for the purposes of growing crops, a “healthy” soil has a high organic component; is porous, and the pore network contains both water and oxygen; and physical disturbance is kept to a minimum.

What’s your take on products and technologies that claim to manage one organism? What about adding bacteria to the soil, and what is the efficacy of it?

As someone who considers soil as a whole system, I spend very little time considering the fate of individual species in soils. Metagenomics reveal that soil communities may contain tens of thousands of individual species. I am sceptical, therefore, that individual species added to this complex community will thrive. Much of the evidence supporting the effects of microbial inoculants are derived from controlled laboratory experimentation but there is little evidence that they are effective in the field. For mycorrhizae, it is likely that if a plant host is present, they will also be present, as the presence of hosts is likely to be the most important determining factor.  If the issue is mycorrhizal associations with vine roots, I would imagine that root drenches at planting would be most effective. Regarding “soil conditioner” and “microbial activator” products, most of the products I am aware of are really composted organic matter, very similar to what could be produced on farm or vineyard. Since adding organic matter to soil is often likely to have positive effects, the products probably do more good than harm, but home-made compost will be cheaper and there are plenty of approaches which can be adopted to increasing organic matter inputs into soil. I am largely sceptical of these products.