The Vegan Patch: Woodchip to build fertility

Sarah Oliver of Flowering Bean Organic Gardens talks veganic gardening innovation with Iain Tolhurst, founder of one of the UK’s longest-running organic farms.

The unimaginable number of living flora and fauna that toil away unseen in an endless cycle within the soil are crucial to what happens to all life above. However, as we overfeed, underfeed, or completely kill off the wildness below, we risk the beauty and richness of life above. Understanding and supporting the fertility that this undersoil life provides is central to our food system.  

I came across Iain Tolhurst’s work when I first googled veganics back in 2016. Tolhurst grows certified organic food on leased land for a box scheme and farm shop on a property in the UK. He has been growing without using animal inputs such as manures or blood and bone on just under 20 acres (about 8 hectares) for 35 years. His farm was recently described in George Monbiot’s 2022 book Regenesis, as “genuinely regenerative” using no artificial or livestock inputs and creating a space for diversity and wildlife to return. 

Tolhurst produces about 100 different varieties of vegetables on the farm. He uses a system of long, seven-year rotations and times the planting of the different crops to fit their differing fertility needs. When a plot is at the end of five years, he will take it out of production for two-years to build the soil fertility using a green manure of legumes and wildflowers. To this part of the cycle, he also adds in ramial chipped wood.

It was his use of woodchip that I was particularly interested in when I asked for an interview. Woodchip offers a good addition to the growing toolkit of alternative solutions to using manures, other animal inputs, or artificial fertilisers in gardens and growing systems. 

Woodchip at Tolhurst Organics

While we are used to seeing woodchip around fruit trees, on ornamental perennials beds, and in forest gardens, using it in vegetable beds often raises questions around nitrogen lock-up and soil health. Nitrogen lock-up can occur when a carbon rich material, such as woodchip, creates an imbalance in the soil. In fact, Tolhurst agreed to the interview as he wants to highlight the potential of using woodchip in horticultural growing systems.

Tolhurst sources the woodchip for the farm from local arborists, coming from a mix of trees, usually from local gardens. Using local material also reduces the carbon footprint of transport, an issue with many fertilisers. Tolhurst explains that the woodchip is very different from wood shavings or sawdust — using these would cause issues as they are very difficult to compost. 

At Tolhurst’s farm they use woodchip in three different ways; as a propagation substrate seedling mix, applying it directly to the plots to build fertility, and as a weed suppressant on pathways in tunnel houses. However, he explains, the key to the system working and avoiding nitrogen lock-up is using different forms of woodchip, the way it is applied, and the timings of the harvest of the woodchip.

Composted woodchip

Once the woodchip arrives on the farm it is composted in a long pile called a windrow. This is turned four times in the first year and then applied directly in their tunnel houses as compost. For seed raising mix, they keep a specific amount aside and continue to compost this for at least another six months and up to two years. At this stage it becomes very crumbly and there is not much visible woodchip left. They can then grade this, add a little vermiculite and, in their experience, at this stage it acts as well as any peat-based material.

Finding an alternative to peat-based material has been an important aspect of their search for alternative fertility building materials, especially in potting mixes.