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The Topsoil Test: Why We Buried Four T-Shirts in Our Vineyard
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We buried four T-shirts in our new vineyard, and we hope they come back full of holes.
In September 2026, our vineyard team buried four identical cotton T-shirts belonging to our Vineyard Manager, each one 20cm under a different type of plant cover. Over the coming months, soil life will slowly eat away at the cotton. The more of each T-shirt that disappears, the healthier and busier the soil around it.
We’re calling it the Topsoil Test. Here’s the story so far:
The Topsoil Test in brief
- What: Four cotton T-shirts buried 20cm deep
- Where: Our new vineyard, planted in spring 2026
- When: September 2026, just after sowing cover crops
- Why: To find out which plants growing between the vines support the most soil life
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What We Buried in the Vineyard, and Why
We buried four cotton T-shirts in our youngest vineyard to measure how much life the soil has under different cover crops. We planted the vines there this spring, when 16,000 new vines took root as part of our replanting project.
In September 2026, the team sowed several different cover crop mixes in the rows between those young vines. A few days later, we pulled up the spades.
Jargon explained: cover crop
Plants sown in a vineyard to protect and feed the soil, not to harvest. They keep the ground covered and give soil life a steady supply of food.
Why test the soil at all?
Living soil helps feed and water vines. Bacteria, fungi and worms break down plant material, release nutrients and keep the soil crumbly, so roots, air and rain can move through it. Soil is also a big part of terroir, the mix of natural conditions that shapes a wine.
Cover crops are one of the main ways growers support that soil life. In a three-year study in a California vineyard, cover crops nearly doubled microbial life between the rows and tripled it under the vines compared with unplanted ground (Environmental Microbiome, 2025). Closer to home, 91% of respondents to WineGB’s 2024 Sustainability Impact Report said they prioritise sustainable vineyard floor management, which includes cover crops (WineGB).
The question for us is which cover crop works best in our soil. Lab tests can measure soil life, but a buried T-shirt lets everyone see the answer for themselves.
How Does the Soil-Your-Undies Test Work?
The soil-your-undies test uses buried cotton to visually measure soil life. You bury 100% cotton fabric, wait at least two months, then dig it up. The more it rots, the more active the soil.
The idea is popular with soil scientists and farm advisers:
- In the US: The Natural Resources Conservation Service runs a Soil Your Undies Challenge, asking farmers and gardeners to bury new cotton underwear for at least 60 days
- In science: The University of Tennessee calls it a version of the classic “cotton strip test” researchers use to measure how well soil breaks things down
- Depth: Guides suggest anywhere from about 8cm to 20cm, according to the NRCS and Oregon State University
Why does cotton rot in the soil?
Cotton is food for soil life. Once cleaned and processed, cotton is about 99% cellulose, according to Cotton Incorporated. Soil microbes feed on cellulose every day as fallen leaves and old roots break down.
Jargon explained: cellulose
The main building block of plant cell walls. It gives stems and leaves their strength.
A buried T-shirt is like a plate of food left out for whatever lives in the soil. A busy, varied community will eat through it fast. A quiet soil leaves it mostly whole.
Jargon explained: soil microbes
Living things in the soil too small to see, mostly bacteria and fungi. Healthy soil contains billions of them, according to the NRCS.
Why T-shirts instead of underwear?
T-shirts gave us more fabric, and four identical ones meant every row started on equal terms. We buried them at 20cm, the deeper end of the recommended range.
Keeping the test fair
A buried T-shirt is a visual clue, not a lab result. Other things can change how fast cotton breaks down:
- Moisture: Dry soil slows decomposition, which is why the Oregon State guide suggests watering in dry spells
- Season: Soil life slows down in cold weather
- Soil type: Sandy, clay and chalky soils hold air and water differently
So all four T-shirts went in on the same day, at the same depth, in the same young vineyard. The only thing that changes is what grows above them.
Where the Idea Came From
New Zealand viticulturist Nick Paulin inspired our test by burying underwear to compare vineyard cover crops. He found “markedly greater decomposition of cotton undies buried in diverse cover crop blocks versus simple grass”, as New Zealand Winegrowers reported.
Jargon explained: viticulturist
Someone who grows grapes for wine. Viticulture is the farming side of winemaking, from planting vines to picking the fruit.
In other words, a mix of many plants fed the soil better than grass alone. Research backs up the link between plant variety and soil life. A 2024 review in Horticulturae highlights a German vineyard study that found soil bacteria diversity rose with the number of plant species.
We wanted to go one step further. Instead of comparing a mix with grass, we’re looking at which plant types make the difference.
The Four Rows We’re Comparing
We buried one T-shirt in each of four vineyard rows. Each row has a different ground cover between the vines, so we can compare them side by side.
| Row | What’s growing there | What we want to find out |
|---|---|---|
| Row 1: Legumes only | Plants from the pea and bean family | Do legumes on their own boost soil life? |
| Row 2: Grasses and wildflowers | A mix with no legumes in it | Does plant variety help without legumes? |
| Row 3: Legumes, grasses and wildflowers | The most varied mix of all | Do the two groups work better together? |
| Row 4: Bare earth | Nothing | How fast does cotton rot with no plants at all? |
Why test legumes separately?
Legumes add nitrogen to the soil, something most other plants can’t do. Small lumps on their roots, called nodules, house bacteria that convert atmospheric nitrogen into plant food. According to AHDB, legumes can add anywhere from 15 to 325kg of nitrogen per hectare each year, depending on the species.
Jargon explained: legume
A plant from the pea and bean family, such as clover or vetch. Legumes can take nitrogen from the air and add it to the soil.
Why mix legumes with other plants?
Grasses and wildflowers bring different roots and different food for soil life, so a mix may do more than either group alone. A 2024 study by Villat and Nicholas in Frontiers in Sustainable Food Systems found that on farmland, a legume grown with a non-legume stored more carbon in the soil on average than either one alone. The authors note the sample was small, and the effect is still unknown in vineyards. Row 3 is our small, hands-on way to explore that.
Why leave one row bare?
The bare earth row is our control. It shows how fast cotton breaks down without any plants, so we can see how much difference each cover crop makes.
Jargon explained: control
In an experiment, the control is the version left unchanged, so there’s something fair to compare everything else against.
What Happens Next
The T-shirts will stay underground while the cover crops settle in. Then we’ll dig all four up, lay them side by side and compare how much of each is left.
Patience will help. We sowed our cover crops in September, and AHDB notes that very little nitrogen fixation happens over winter. The legume rows may only show their full effect once the soil warms in spring, a busy time across the vineyard year.
We’ll share the photos and what we learn here on the blog. Fingers crossed for plenty of holes.
See the Vineyard for Yourself
Curious to see how a vineyard is cared for, from the soil up? Join a tour and tasting at Bolney Wine Estate in West Sussex.
Frequently Asked Questions
Can I do the soil-your-undies test in my garden?
Yes. Bury a pair of 100% cotton underwear or a cotton T-shirt flat, 8 to 20cm deep, and mark the spot. Dig it up after at least two months. Try two spots, such as a lawn and a vegetable bed, to compare them.
Why does the fabric need to be 100% cotton?
Cotton is almost pure cellulose, which soil life can eat. Synthetic fabrics such as polyester don’t break down in soil the same way, so they won’t show up in the test. Expect elastic waistbands to survive.
How long should cotton stay buried for a soil test?
At least 60 days, according to the US Natural Resources Conservation Service. Soil life slows down in cold weather, so a winter test may need longer to show a clear result.
What does a healthy soil test result look like?
Lots of holes, thin patches or very little fabric left. The more the cotton has broken down, the more active the soil life. Fabric that comes up almost whole suggests the soil could do with more life.
Sources
- USDA Natural Resources Conservation Service: Soil Your Undies Challenge: Test method, 60-day burial, billions of soil microbes
- University of Tennessee Institute of Agriculture: Soil Your Undies! How Healthy is Your Soil?: Link to the cotton strip test
- Oregon State University Extension: Test Your Soil Health by Burying a Pair of Briefs: Burial depth, watering, reading the results
- Cotton Incorporated: Cotton Morphology and Chemistry: Processed cotton is 99% cellulose
- New Zealand Winegrowers: Regenerative Viticulture: Nick Paulin’s undies test in diverse cover crops versus grass
- Visconti, López and Olego (2024), The Health of Vineyard Soils: Towards a Sustainable Viticulture, Horticulturae: Plant variety and soil bacteria in vineyards
- Rocha and others (2025), Environmental Microbiome: Cover crops and soil microbial life in a vineyard
- Villat and Nicholas (2024), Frontiers in Sustainable Food Systems: Legume and non-legume cover crop pairings and soil carbon
- AHDB: Biological Fixation of Nitrogen by Rhizobia: How legumes fix nitrogen, amounts fixed, low winter fixation
- WineGB: Sustainability: 2024 Sustainability Impact Report figures on vineyard floor management
