Life under your feet – feeding plants and the planet

If you have access to a garden, no matter how challenging you find the soil, it is hard to overlook the great things soil can do. From heavy clays to more droughty sands, healthy soils support plant growth and ecosystem services.

These gifts are mainly bestowed by the ‘soil microbial biomass’. This scientific term describes the total of all of the living things in soil that are too small to be seen with the naked eye⁵. We now understand that gardeners can work with this hidden underground community to improve the benefits that any garden soil provides.

Soil as habitat

These EPS are large carbon-bearing molecules, like sticky proteins and polysaccharides, which are manufactured by the soil microbes to co-operate and modify their habitat. This gives the microbes special abilities, such as acting in co-operative clusters to store water, and keeps small capillaries open for transport of nutrients. These microbial glues also stabilise the soil’s microscopic architecture (a network of soil pores and channels) and this is central for plant growth and environmental protection, because an ‘open’ pore-network helps to prevent waterlogging and associated greenhouse gas emissions.

The amount of EPS that soil organisms produce depends firstly on the amount of carbon made available to the microbes, but also on the soil texture², the types of plant root exudates (compounds released by roots into soil) that they have access to³, and even the presence/absence of cooperative microbial partners⁴.

Soil profiles from RHS Garden Wisley. Left to right: cultivated trails field soil, uncultivated new build soil, sandy soil under long term grassland, and loamy soil under orchard.

Soil as a focus for gardening with biodiversity

While most people consider soil microbes to be ‘out of reach’, gardening with them in mind can put us back in touch with the restorative powers of soil. Just as the way in which gardens are connected to one another in the landscape supports larger biodiversity, a complex, connected, and stable soil structure created by microscopic organisms is one of the defining features of a healthy soil and ecosystem.

Marc’s top tips on how to help the health of your soil and the planet

• Cancel your subscription to green waste collection and start composting today. This prevents needless combustion of fossil fuels. It also maximises the re-use of plant carbon and
  To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key plant nutrients usually derived from soil are nitrogen, phosphorus and potassium, while carbon, oxygen and hydrogen are absorbed from the air. Other vital soil nutrients include magnesium, calcium and sulphur. Gardeners can add nutrients by applying fertilisers (either artificial or naturally derived) to boost plant growth and improve flowering and fruiting.

  nutrients in your garden, benefits wildlife and improves soil health.

• Maximise plant cover in your garden for as much of the year as possible. Liquid carbon (food for microbes) is provided by plant roots and sustains
  Below ground level or within the soil.

  subterranean ‘hotspots’ of microbial activity. Plants that grow well generally put more carbon into the soil, but there is some evidence that the application of fertilisers may reduce carbon storage – so see what grows best without them.

• Make the most of your soil. Avoid digging and other disturbance if not needed, and consider soil analysis (e.g. the RHS Soil Analysis Service) to determine your soil type, pH, organic matter and nutrient status. Knowing your soil will help you optimise your selection of plants for your various objectives. In addition to drainage characteristics, soil
  This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.

  pH and texture (the proportions of sand/silt/clay) are often the most important properties to consider.

Using a soil testing kit

• Keep an open mind. Contrary to many gardeners’ assumptions, sandy soils are not always free draining, and clay soils can be very well drained. This is because the biologically affected soil structure (porosity etc.) is more developed and stable in healthy soils, and is often more important for drainage than your textural class (sand/silt/clay).

• Play with microscopes. You might be surprised!

Further reading

1. Redmile-Gordon MA, Gregory A, White R & Watts C. (2020): Soil organic carbon, extracellular polymeric substances (EPS), and soil structural stability as affected by previous and current land-use. Geoderma 363, 114143–114143. https://doi.org/10.1016/j.geoderma.2019.114143
2. Olagoke FK, Bettermann A, Nguyen PTB, Redmile-Gordon M, Babin D, Smalla K, Nesme J, Sørensen SJ, Kalbitz K & Vogel C. (2022): Importance of substrate quality and clay content on microbial extracellular polymeric substances production and aggregate stability in soils. Biology and Fertility of Soils 58, 435–457. https://doi.org/10.1007/s00374-022-01632-1 
3. Wang S, Redmile-Gordon M, Shahbaz M, Ge T, Zhang M, Wu Y, Liu J, Huang Q & Cai P. (2022): Microbial formation and stabilisation of soil organic carbon is regulated by carbon substrate identity and mineral composition. Geoderma 414, 115762.https://doi.org/10.1016/j.geoderma.2022.115762
4. Wu Y, Fu C, Peacock CL, Sørensen SJ, Redmile-Gordon MA, Xiao K-Q, Gao C, Liu J, Huang Q, Li Z, Song P, Zhu Y, Zhou J & Cai P. (2023): Cooperative microbial interactions drive spatial segregation in porous environments. Nature Communications 14, 4226. https://doi.org/10.1038/s41467-023-39991-4