Biodiversity in soil
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.