By definition, soil health means the continued capacity of soil to function as a living ecosystem – a viable community of organisms that sustains plants, animals and humans. From a gardener’s point of view this would mean a substrate that sustains healthy and flourishing plants with minimal input like fertilization, raking and weeding.
Soil is a natural habitat for numerous micro-organisms – most of which live in air and water pores among the solid mineral and organic particles, and most of which are beneficial for the soil and plant health, although numerous forms of pathogenic soil micro-organisms also exist. Soil micro-organisms include different species of bacteria, fungi, protozoa and nematodes, of which the beneficial ones stabilize the soil and they help the water and nutrient cycling, for example by decomposing organic matter. Along with bigger organisms – worms, arthropods, birds and mammals they form the soil food web. A healthy soil food web maintains itself and recycles nutrients and materials effectively.
Bacteria are an essential part of the soil food web, as they function as decomposers as well as food providers for other organisms. A teaspoonful of soil contains a billion bacteria, most of which are beneficial. Bacteria feed on organic matter compounds by decomposing them with extracellular enzymes. This releases phosphorus and other nutrients from organic matter into a plant-available form. While doing this, they create biofilms on the surfaces of the soil particles – this slimy film is excellent in holding moisture and increases the water holding capacity of soil.
Fungi (over 100 000 diverse species) accompany bacteria in solubilizing minerals for plants to use. They consist of microscopic threads called hyphae, that form a network (mycelia) to transport water, nutrients and other resources. Fungi are especially good at decomposing woody material high in lignin. They glue soil particles together into macroaggregates and decrease the risk for soil erosion. Mychorrizal fungi live in symbiosis with certain plant species: In a symbiosis, the fungus receives carbon compounds from the plant, while giving the plant nutrients and water.
Protozoa are single-celled organisms that live in the waterfilm and pores between soil particles, which makes them vulnerable to soil compaction. Protozoa feed on bacteria and fungi, thereby releasing back into the soil the nutrients that had been stored inside them from decomposing. At the same time while feeding, the protozoa reduce the population of pathogenic bacteria and fungi.
Nematodes eat organic matter, bacteria and fungi and take part in releasing nutrients into plant soluble form. They are also food for many predator species, and thus an essential part of the food chain. Where there are nematodes, there is a continuous buffet for insects, worms, and lastly, birds and mammals.
Soil micro-organisms thrive in a natural ecosystem in which energy and nutrients are recycled endlessly without any additive fertilizing. The best way to create a successful soil food web in a garden or a green area is to try and imitate the circumstances of natural habitats. The diversity of micro-organisms in a soil can naturally be conserved and/or improved by planting diverse, various plant species rather than monoculture: biodiversity above ground helps biodiversity below ground and vice versa. Hanna Williams suggests that it might be a good idea to start with a diverse set of plants and rely on natural selection, as this guarantees the best opportunities for specialist species to survive as well. The diversity of micro-organisms in a soil could also be manipulated artificially by adding microbial-based biostimulants. To do this successfully, the present microbial composition and its activity in a soil sample can be visualized by laboratory techniques and the community can be supplemented with artificial cocktails of one or more micro-organisms (biostimulants). Roxanne emphasizes this artificial manipulation of soil ecology is complex, its success depends on many factors, such as the physical and chemical conditions of the soil, the community of micro-organisms already present, and the plant species that will grow on the soil.
In order to ensure a viable soil community, the soil should be disturbed as little as possible after planting. Any digging or deep tillage will disrupt life cycles and tear organisms apart – although the more diverse the system is from the beginning, the more resilient it will be at any future disturbance. A high-quality substrate that has a good ratio of sand and organic matter decreases the need for soil improvers, and so does favoring microbe-filled compost as raw material. Hanna has great news for all lazy gardeners: leaving grass clippings and fallen leaves on the ground protects the soil from drought and erosion, and provides food for microbes and animals.
For more information please watch the two very topical presentations on biodiversity in soil:
- Biodiversity within the soil / substrate by Hanna Williams, Agronomist with focus on soil health, Splendor Plant, and
- Microbial ecology of soil and growing media by Roxanne van Rooijen, Researcher, Kekkilä-BVB.
Hanna Williams works for Splendor Plant – a Swedish nursery that produces trees, bushes and perennials for wholesale. Focusing on soil health is an essential part of Splendor Plant’s operations, since the plants are produced in open-field cultivation. In order to maintain the production fields and understand the crop plants’ needs, Hanna Williams has taken a deep-dive into the life below the surface.
Roxanne van Rooijen works as a Researcher for Kekkilä-BVB in the Innovation department. Kekkilä-BVB’s Innovation is interested in manipulating the microbial ecology and soil health of a substrate to enhance the substrate’s quality and therefor its value for horticulture.
Hanna Rummukainen works as the Product Quality Specialist in Kekkilä-BVB, Landscaping & Recycling. Through our landscaping solutions we can contribute to green and biodiverse urban areas.