Healthy Soil

Soil Food Web

The soil food web is a very diverse collection of organisms that spend some or all of their time in the ground. They range in size from single celled to insects and worms

It is called a food web due to the delicate interdependence between its multitude of inhabitants. Many of the microbes present still can not be cultivated in the laboratory outside of their complex ecosystem. The more complex the web is the more complex the structure of the soil is in terms of its ingredients and its physical form.   

Organisms live in the micro scale environment between and within soil particles. In a healthy root zone there can be over 100,000,000 beneficial bacteria in a single teaspoon of soil.

How It Works

Living plant roots exude many types of complex high energy nutrient molecules into the surrounding soil. These nutrients provide the initial energy to kick start the webs evolving diversity. Lichens, moss, photosynthetic bacteria and algae, like plants, also convert gaseous carbon dioxide, using energy from the sun, into carbohydrates that feed other elements of the ecosystem.  

Other than a few specific bacteria all other inhabitants of the soil food web get their nutrients from the consumption of the waste by products of these photosynthesizers or from the consumption of their bodies.   

A key starting point for the formation of any soil is the presence of specific bacteria and fungi that convert minerals/rocks into an available organic form through the release of acids and enzymes.

The diversity, quantity and quality of organic matter and inorganic matter determines the diversity, quantity and quality of bacteria and fungi that form the base of the food web. Bacteria feed on the simplest  organic compounds such as root exudates and fresh organic matter whilst fungi can work on more complex forms such as wood, and plant fiber.

The bacteria, fungi and other soil dwellers that make up the "microherds" occupying the area around the plant roots, the rhizoshere, transform and release nutrients from organic and inorganic material which feeds the plants. They also secrete auxins  enzymes, vitamins, amino acids, indoles and antibiotics that are absorbed by the roots and used by the plant. Plants through their specific root exudates have learnt to foster a particular food web beneficial to their specific needs. 

Web Diversity

As discussed the base of the soil food web is occupied by bacteria and fungi. When tending a landscape how the soil food web should be nurtured will vary with what species of plant is being cultivated. This is because some plants, for example grasses, prefer a bacterial dominant soil, whilst others, for example woody shrubs, prefer a fungal dominant web.

In a well tended soil the bacteria and fungi can out compete pathogenic microorganisms providing protection for the plants.

One specific fungi that has been extensively documented for its beneficial symbiotic relationship with plant roots is mycorrhizae. In return for the plant feeding it through its root exudates the mycorrhizae solubolize and provide phosphorus as well as supplying nitrogen, micronutrients and water.

Bacteria and fungi are in turn fed on by the next level of the food web, the protozoa, nematodes, and fungal feeding mites.

This level of the web in turn becomes prey to arthropods including, millipedes, centipedes, predatory mites, ground beetles, spiders, ants and to earthworms.

Finally birds, reptiles, amphibians and mammals feed on the secondary level consumers.

The predatory activity in the soil food web, as in the food chain we are familiar with, plays a role in keeping lower members of the web in balance. Therefore any activity that impacts the higher order organisms will effect the overall health of the soil food web and in turn the health of the plants. Soils treated with chemicals can lack a healthy soil food web and become dependent on excessive chemical fertilizers, herbicides and pesticides, further destroying the ecosystem.