Every year, since 2007, I have grown vegetables and recorded the harvests from dig and no dig beds, each with a surface area of 7.5 m2 (81 ft2).
. Yields over 16 years, from 2007 to 2022, totalled:
- 1327 kg (2926 lb) from the dig beds
- 1471 kg (3243 lb) from the same area of no dig beds
These weights are of trimmed vegetables, and from the same plantings on both beds (at Lower Farm and then at Homeacres), with the same amount of compost added to each bed.
- Compost is spread on top of the no dig bed.
- Compost is incorporated into the dig bed, as in traditional gardening practice.
(See Lesson 5 for a trial where compost is on top for both.)
Two identical trial beds
I started this trial at Lower Farm in 2007 and ran it for six years. Since then I run a similar one at Homeacres for another 10 years so far, including 2022.
The Lower Farm trial consisted of four smaller beds, each measuring 1.5 × 2.5m (5 × 8ft). When I moved to Homeacres, I simplified it to one bed of each method, with the same total area.
When I began the comparison in 2007, I had no idea whatsoever how growth would differ between dig and no dig. I had used a tractor rotovator in the early 1980s, but never dig as part of annual soil preparation.
The harvest totals for Lower Farm (2007–12) were:
- 376 kg (829 lb) from the dig bed
- 400 kg (882 lb) from the no dig bed
(Follow the link at the bottom of the page for more information on the Lower Farm trials.)
Strong differences in results
As well as higher yields from no dig, I observed other notable differences. The soil was much easier to weed and water on the no dig beds at Lower Farm, and I noticed that parsnips rooted more deeply!
The results at Homeacres have been broadly similar. Soil at Lower Farm was clay, and at Homeacres is silt over clay.
Over the two sites, and in 14 years out of 16, the no dig bed has been higher yielding.
Mostly there is not a huge difference between the beds, but a common variation is stronger growth in early spring in the no dig bed. In the cold spring of 2018 this was especially marked – seedlings started strongly in surface compost, compared to loosened soil into which the compost was incorporated while digging.
In 2022, there was a huge growth difference from April to June, of peas and onions in particular. Every year there are standout differences, and not of the same vegetables.
Of all the vegetables I grow for comparison, potatoes give harvests which are the most similar between each bed. Other vegetables show more variations between dig and no dig, for whatever reason.
To a gardener who habitually digs, the dig bed’s soil would appear absolutely brilliant – soft and loamy. Until you see the difference in growth. I suspect that, among other things, there is damage to fungal networks (see Lesson 14).
In spring 2018 there was more slug and wireworm damage on the dig bed, which contradicted the common assertion that ‘digging allows birds to eat pests’. What this statement should mention is how digging allows birds to eat worms, and other beneficial inhabitants of the soil. This disrupts the natural and fragile balance of pest and predator, for example ground beetles (Carabus violaceus) that eat slugs. Other beetle species climb plants to feed on aphids and caterpillars.
There is still plenty to be discovered about how soil organisms interact, and I reckon most of it will highlight the value of no dig/no till. My trial results point to that, as do the photos below.
- This 2022 YouTube video shows the growth differences in that spring plus summer, and over 10 years.
Growth differences – 2018
These beds are a microcosm of the garden, with so many different vegetables in a relatively small area. They illustrate two important and often misunderstood aspects of vegetable growing:
- Many vegetables grow in half a season and, after clearing them in early to mid-summer, there is time to grow another harvest for autumn, winter and spring. See the 2018 and 2019 results tables below for ideas of first and second plantings.
- A four year rotation is not necessary, unless your soil has been abused over decades (or longer), by digging and careless growing. For example on some allotments there are soil diseases, such as clubroot of brassicas and white rot of onions. Most soils do not have these long-lasting problems and need less attention given to rotation – especially when the soil is healthy no dig.
During 2022, I had feedback from two different allotment holders who were successfully growing brassicas in soils where clubroot had been prevalent three years earlier, when they took over the plots. Since then they have been no dig and results keep improving.
Watering test
This video is from October 2018; we had already done a watering test of the same nature in August 2018. I was shocked by the difference, when imagining the effects of heavy rain on cultivated fields.
Harvests – 2018 example
Towards the end of summer, growth usually looks more even than in spring and early summer. Harvests are another matter however, for example the red beetroot and leeks we pulled in autumn, which looked similar when growing but weighed 15-50% more on no dig. The results of 2018 show a wider difference than I ever recorded, around 30% more harvests on no dig throughout the season. Perhaps it’s to do with a stronger response to the dry conditions, although we water both beds the same amount.
Harvests – 2019
For the seventh time I dug the dig bed in December, and incorporated two large barrows of compost. On top of the no dig bed I simply spread the same amount, which settled to a layer of 2.5 cm (1 in). I also spread 25% of homemade compost and 25% of mushroom compost, not mixed together but each batch scattered evenly on top.
Spring growth of the dig bed’s potatoes was fine, but its spinach and beetroot were desperately slow, as in 2018.
Measuring the soil differences – October 2019
I sent samples of soil from each bed, taken at a depth of 15 cm (6 in), to a microbiologist in Norway, Katelyn Solbakk.
The results of her study resulted in this paper, which included the table below.
Solbakk’s paper illustrates and explains the differences she saw in a microscope analysis of each bed’s soil, and can be summarised as follows:
‘There was a very noticeable difference in the physical condition between the dig and no dig samples. All samples did appear to be rich in organic matter, but the no dig samples were exceptionally clean and well aggregated. The structure was significantly weaker in the dug soil, with more loose, unbound debris and fewer large aggregates. The no dig samples had a very clean appearance with large, strong aggregates and very little unbound material.’
One hears little about protozoa, but here is what Katelyn has to say about them on her website:
‘Protozoa are important but often under-appreciated members of the soil food web. The main ecological role of soil protozoa, as it is currently understood from our perspective, is to free up nutrients that have been immobilized by bacteria, completing what is known as the “microbial loop”. Different species of protozoa are known to selectively hunt particular types of bacteria, so if there are many types of protozoa, it could be a sign that there are many types of bacteria as well. Protozoa have also been found to provide benefits to plants that extend beyond the release of nutrients, such as improved growth and disease resistance.’
Of the samples taken from each bed, Katelyn wrote:
‘In these samples, there was a considerable improvement in protozoa abundance and diversity in both the no dig trial bed and the full garden sample, compared to the dug bed.’
Time and weed differences
The dig beds need more time than no dig:
- Around two and a half hours for digging
- An extra half hour through the season for weeding
Disturbing soil causes weed seeds to germinate (see Lesson 9 and the photo comparison below).
The classic reason given for this is that cultivation exposes some weed seeds to the light, triggering their germination. I think another reason is that disturbed soil needs to heal, and weeds are part of the healing process. We see an example of this with chickweed, which grows so fast after the passage of a rotavator – I know this from experience!
The photo of surface conditions between the parsnips (in July 2010), is after four weeks with just 27 mm (1.1 in) of rain, and afternoon temperatures of 21–24 °C (70–75 °F). No water was given, and the dug clay is cracking along the lines where my spade had entered, allowing a lot of moisture to escape.
Trials and uptake elsewhere
Following my work to promote no dig, I am heartened to see its uptake by official bodies, who, until recently, were following well established but old fashioned Victorian methods. For example, the current gardens manager at RHS Wisley heard me lecture on no dig at Kew Gardens in 2011 and decided to give it a try on her allotment. The results were so consistently good that she has introduced the method to students at Wisley in Surrey, and to apprentices studying there. Wisley’s edibles garden is now no dig.
Kew Gardens in London give each of their students a bed, to grow vegetables and then be examined on. Since 2018, these beds have been no dig. Many National Trust gardens are now no dig too.
Another significant trial started in 2016, at Beechgrove garden in north east Scotland, near Aberdeen. The garden is run entirely for the BBC television series ‘The Beechgrove Garden’. They set up two areas side by side – one dig, and the other no dig – and initially were sceptical of how the no dig plants would grow. Each year the no dig vegetables have grown strongly, to the point when, in November 2018, they decided to convert all of their vegetable growing to no dig.
You are in good company!
Further reading and viewing
- More details of the Lower Farm trial.
- More details on the dig/no dig trials from 2013 – 2018.
- I’ve put together two videos of my no dig bed at Homeacres in 2017, to view them click on ‘first plantings’ and on ‘second plantings‘. See exactly how I managed to grow over 120 kg on 7.5 m2 (265 lb on 81ft2).