Along the Broadwalk we are always trying to improve the health of our soil and we have developed a soil improvement plan to this end. We try and listen to our plants and in a few beds in particular our plants have been telling us they are experiencing difficulties in cultivation or establishment due to poorer soil health. Thus, we have prioritised these beds for soil regeneration.
Green manures are used as part of organic and regenerative agriculture to improve soil health and we wanted to trial these in a horticultural setting. They are notable as a good example of the ethos of plants providing solutions to anthropogenic problems, and also as antithesis to highly synthetic modes of monocultural production.
Green manures provide a significant resource for a variety of pollinators. This was seen during the Wallace garden trial and also in this year’s trial. We would like to develop the scientific study of this in the future, but anecdotally this year the clovers and the vetches have been highly attractive to pollinators, in addition to their nitrogen fixing capacities within the soil.
Mixes of green manures have been made from selections trialled in the Wallace garden in the 2022 growing season. These have been sown in 3 beds with the poorest soil conditions. These beds have experienced plant mortality, both of mature specimens and establishing plants. As a consequence, they also have more numerous available niches. Green manures form part of a larger project of soil regeneration designed with the generous help of a visiting scientist from the University of Cambridge, Raffi Hull.
The initial Broadwalk sowing trial comprises of:
| Helianthus annuus | Sunflower |
| Linum usitatissimum | Flax |
| Medicago sativa | Alfalfa |
| Onobrychis viciifolia | Common sainfoin |
| Phacelia tanacetifolia | Blue tansy |
| Sinapis alba | White mustard |
| Trifolium alexandrium | Egyptian clover |
| Trifolium incarnatum | Crimson clover |
| Trifolium michelianum | |
| Trifolium pratense | Red clover |
| Trifolium repens | White clover |
| Vicia sativa | Common vetch |
| Vicia villosa | Hairy vetch |
The content below is credited to Raffi Hull:
Regenerating the soil
A resilient soil is at the heart of a garden. Healthy soils are alive with earthworms engineering tunnels that improve soil structure, mycorrhizal fungi shuttling nutrients between plants and capturing carbon in the soil, and bacteria-producing chemicals that promote soil aggregation. Here are some ways we promote rich soil biodiversity at the Botanic Garden.
Provide living roots
Soils determine plant growth but are also the products of the plants that grow in them. In between plantings at the Botanic Garden, we sow green manures to provide extra roots for the soil. Roots feed beneficial soil microbes, increase soil organic matter when they break down and are home to beneficial mycorrhizal fungi. Green manures can be used as a mulch or incorporated into the soil during planting to increase soil fertility.
Add organic matter
Our clay soils here at the Botanic Garden benefit from regular additions of organic matter. We add well-rotted organic matter to the soil when we are planting to provide food for soil microbiota and improve fertility. We don’t use any pesticides or herbicides, as these damage wildlife, both above and below ground.
Use mulches
You will see that many beds at the Botanic Garden are mulched with a layer of bark. Mulch is extremely useful. It helps to suppress weed growth and feeds the topsoil, holds moisture during dry periods, and keeps the ground warmer during cold periods. We don’t use bark mulch around vegetables because it deprives them of the high levels of nitrogen they need for vigorous growth. Occasionally, unusual fruiting bodies pop up in the bark mulch, so keep an eye out for fungi growing around the plants.
