Our "soil your undies" experiment has become a talking point in New Zealand viticulture the last few years, but behind the novelty factor is serious science about soil biology and what it means for fine wine production.
When Chloe, a consultant from Quebec working with us at the time, first suggested burying cotton underwear in our vineyard to test soil health, I thought it sounded a bit mad. But she'd seen similar trials in California, and the concept made sense from a scientific perspective. Cotton is a natural fibre that breaks down through microbial activity, so the rate of decomposition should reflect the biological activity in the soil.
This was around 2019, and we were already several years into experimenting with diverse cover crops at our Low Burn Estate in Central Otago. We'd moved from simple two-species mixes to our current 12-species blend including grasses, cereals, brassicas, chenopods, and legumes. But while we could see visual differences in the vineyard – more earthworms, better soil structure, healthier vine growth – we needed a way to demonstrate the biological differences that was both scientifically valid and immediately understandable.
‘Soil your Undies’ test briefs, 2019. L: Cover crop row (ryecorn, oats, mustard, lipins, plantain clovers, phacelia. R: Grass row (fescue, rye mix)
The experiment itself was pretty simple. We took two pairs of identical cotton undies and buried them in side-by-side rows of the vineyard. One pair went into a row with our traditional grass sward – the kind of neat, mown grass cover you see in most vineyards. The other went into a row with our diverse cover crop mix that we'd grown through winter and then crimped in spring rather than mowing.
We buried them about 15cm deep, marked the spots, and waited. The plan was to dig them up after about a month and compare the decomposition.
When we dug up the undies after four weeks, the difference was obvious. The pair from the grass row was largely intact – you could still clearly see it was underwear, with just some minor degradation around the edges. The pair from the diverse cover crop row was a different story entirely. It had been almost completely broken down by soil microbes, with only the elastic waistband remaining recognisable.
This gave us something concrete to point to when people asked about soil health differences. The diverse cover crop system was supporting a much more active microbial community than the simple grass system. When we followed up with soil tests that winter, we found a 0.5% difference in organic matter between the two systems – 4% in the grass rows versus 4.5% in the cover crop rows. That may not sound like much, but it represents a significant improvement in soil health achieved in just one season.
National Viticulturist Nick Paulin (The Vine Gardener) in the Low Burn Estate vineyard with the 2022 cover crop row test undies
The science
What the undies test actually measures is the activity of soil microorganisms – bacteria, fungi, and other decomposer organisms that break down organic matter. Cotton is primarily cellulose, and its decomposition requires specific enzymes produced by soil microbes. The more active and diverse the microbial community, the faster the breakdown.
In our diverse cover crop system, we're feeding the soil biology in multiple ways. Different plant species contribute different root exudates – the sugars, proteins, and other compounds that plants release through their roots. These feed different microbial communities, creating a more diverse and active soil ecosystem. When those plants die back or are crimped, they provide varied organic matter for decomposition.
The grass-only system, by comparison, provides a much more limited diet for soil microbes. Single-species systems tend to support less diverse communities, which means less biological activity overall. This shows up in the undies test – fewer active microbes means slower decomposition of the cotton.
The undies are just the visible part of what's happening. These same microbes are responsible for nutrient cycling, soil structure formation, and supporting plant health. A more active soil biology means better nutrient availability for our vines, improved water infiltration and retention, and greater resilience to environmental stresses.
Not just novelty
While the ‘Soil your Undies’ test makes for great storytelling (I've even pulled pairs out from my vest pocket to surprise vineyard visitors), the real value is in what it represents about our approach to viticulture. We're not just growing grapes; we're cultivating an entire ecosystem.
This thinking has changed how we manage our vineyards. Instead of seeing the inter-row space as something to keep clean and tidy, it’s an opportunity to build soil health. Our cover crop program now includes species specifically chosen for their ability to support beneficial soil biology. We've moved from mowing multiple times per season to crimping, which leaves organic matter on the soil surface to feed microbes rather than removing it.
The results extend beyond soil tests too. In blocks where we've implemented these practices, we're seeing improved vine resilience during dry periods, better fruit quality, and reduced need for inputs. During the 2023 season, our Snake's Tongue block – where much of this experimentation has taken place – needed no irrigation during a very warm six-week period between flowering and veraison. The improved soil biology and organic matter helped the vineyard hold moisture and support the vines naturally.
Spring 2025 cover crops at Low Burn Estate vineyard
From a fine wine perspective, this matters because soil health directly impacts fruit quality.
Healthy soils with active biology provide more consistent nutrient availability, which leads to more balanced vine growth and better fruit development. The improved water-holding capacity means less irrigation stress and more consistent ripening. The enhanced microbial activity also supports the development of complex flavour compounds in the grapes.
We've seen this play out in our winemaking. Huw has seen improvements in fermentation health and wine quality from blocks where these soil health practices were implemented early. The fruit comes in with better natural balance, and the wines show more complexity and expression of place.
While the experiment has become something of a calling card for our regenerative approach, it's really just one tool in a much larger toolkit. We're now running 28 different trials across our properties, testing everything from different cover crop species to varying crimping techniques to subsurface irrigation systems. Each experiment builds on what we've learned from previous trials.
The trial also showed us that simple, visual demonstrations can be incredibly powerful for communicating complex concepts. Soil biology is invisible to most people, but everyone understands the difference between intact and decomposed underwear! It's opened conversations about regenerative viticulture with everyone from fellow growers to wine lovers to our own staff.
Kenichi Ohashi MW and Anne Krebiehl MW find out what Nick’s capable of pulling from his vest pocket
The ‘Soil your Undies’ experiment has proven its point about soil biological activity, but it's also raised some new questions. We're now interested in understanding how different cover crop species affect decomposition rates, how seasonal timing impacts results, and whether we can use similar simple tests to measure other aspects of soil health.
The beauty of the undies test is its simplicity and accessibility. Any grower can try it with minimal cost or equipment. But like all good experiments, it's most valuable when it leads to deeper questions and better understanding of the systems we're working with.
As for what's next, we're focused on converting these soil health gains into better vine health. We're using sap testing to fine-tune nutrient status and we’re reducing irrigation inputs – we may even be able to dry farm the young Snake's Tongue block this year. The trials are really about trying to make better wine through more natural processes and fewer external inputs.
Nick Paulin
National Viticulturist (aka the Vine Gardener)
Pyramid Valley, Smith & Sheth, Lowburn Ferry
Aotearoa New Zealand Fine Wine Estates
Read more on our regenerative programme in the latest New Zealand Winegrower Magazine