|
Since the 1990’s there has been a profound new way of looking at the soil as a living entity. It has been discovered that the chemical and physical properties of healthy soil are the result of the Soil Food Web (SFW), a term used to describe the incredible diversity and interaction of organisms living part or all of their lives underground. These organisms include bacteria and fungi, single celled animals, microscopic multi celled animals, insects, worms, even small mammals, and reptiles. Research in 2004 reported that every soil examined contains enormous amounts of total nutrients, but they are unavailable to the plants without the beneficial organisms of the SFW to make them available.
The Soil Food Web (SFW)
The Soil Food Web (SFW) is fueled by the primary food producers. Plants and other photosynthetic organisms use energy from the sun energy to fix carbon dioxide from the atmosphere, converting it to sugars, amino acids, and proteins. Up to 80% these compounds are given away by the plants as exudates to mutualistic microbes living on its roots in exchange for food, water, and protection from pathogens and pests. These Fungi and Bacteria consume and store (immobilize) these exudates in their own bodies. Higher orders of microscopic organisms are needed to immobilize the nutrients in those microbes and release (mineralize) in a form available to plants, as well as other members of the SFW. This process is called Nutrient Cycling.
Another way the primary food producers (plants) fuel the SFW is to supply decaying organic matter in the form of its dead tissues. Part of the SFW is comprised of organisms that consume only dead tissue (saprotrophs). The plants use their roots to extract minerals deep from the subsoil and rock below, then deposit leaves and other dead tissue on the surface of the soil. The plant litter serves as both home and food source to SFW members cycling the nutrients back to the plant.
A single gram of soil may contain:
Bacteria: 100 million to 1 billion 10,000-40,000 species
Fungi: Several yards to several miles of hyphae 1,000-5,000 species
Protozoa: Several hundred thousand 100-500 species
Nematodes: Several hundred 10-50 species
And in a square foot contain:
Arthropods: Up to 25,000 500-1000 species
Earthworms: 10 to 50 One or several species
Food Web Structure
The Structure of the Food Web describes the relative number and types organisms in the soil. Each type of eco-system has a characteristic food web structure. In a healthy food web structure there may be 50,000 different species of microorganisms per teaspoon of soil. Each has a preferred temperature, moisture, pH, food source, habitat, and stress tolerance. Only a small percentage of total species are active at one time, and each can remain dormant for long periods of time until their desired set of parameters is met.
The ratio of Fungi to Bacteria (F:B) Gives fundamental structure to the soil food web by determining the nutrients and organisms in the rest of the food web. Grassland and agricultural soils typically have bacteria dominated food webs that is to say; most of the bio mass is in the form of bacteria. Highly productive agricultural soils tend to have an F:B ratio of 1:0.75 - 1:1 or roughly equal mass of fungi and bacteria. Forest food webs have a higher ratio of fungi to bacteria. The F:B ratio may be 5:1-10:1 in a deciduous forest, and 100:1 to 1000:1 in a mature coniferous forest food web. The F:B ratio determines the types of predators that feed and proliferate. Where bacteria dominate, Protozoa, Flagellates, and bacterial feeding Nematodes are abundant. When fungi are more prevalent, there will be a greater percentage of arthropods and fungal feeding nematodes.
Management Practices Change the food web structure. In an agricultural system where there is a lot of tillage (soil disturbance), organisms sensitive to soil disturbance are destroyed and weakened. Fungi are broken apart each time the soil is turned over, making for a more bacterial dominated system. Loss of fungi means some essential nutrients will no longer be held in the soil, and less structure building ability results in soil compaction and decline of many soil organisms. With a reduced tillage system, fungi are able to grow raising the F:B ratio to a more favorable level. At the same time, earthworms and arthropods become more plentiful. Even the method of plant waste management has an effect on SFW structure. If the waste is left on the surface, the fungi are encouraged because they are the only organisms able to bridge the soil surface with their thread-like hyphae. Where the waste is incorporated, the bacteria are encouraged because the organic matter is brought to them under the surface.
Inorganic fertilizers are nutrients in the form of water soluble salts, often with a higher Salt Index than table salt. Many organisms, sensitive to osmotic shock are destroyed. The effect extends to other organisms that depend on them for food. Since the nutrients are water soluble, they are easily washed out, into ground water or water courses with an imbalance of nutrient salts. Other food web structures miles away may be affected. The disruption in the food web structure inhibit the natural complex cycling of nutrients from one organism to another, meaning there are fewer nutrients available to the plants, requiring more inorganic fertilizer, creating more voids in the food web structure, meaning fewer nutrients available to plants, requiring more inorganic fertilizer, creating more voids in the food web structure, meaning fewer nutrients available to plants, requiring more inorganic fertilizer, creating more voids in the food web structure, meaning fewer nutrients available to plants…
Nutrients are only made available to plants by the organisms that inhabit the soil. A disruption in plant available nutrients can create stress in the plant creating metabolites sensed by, and attracting pathogens and insect pests. Pesticides may be required.
Pesticide use has effects on the food web structure. Pesticides destroy a much larger group of organisms than the target for which they were meant. A chemical application designed to kill a single insect type will drift onto plant, soil, and water surfaces killing thousands of species of bacteria, fungi, nematodes, protozoa, arthropods, earthworms, insects, birds, amphibians, reptiles, and mammals, creating large gaps in the food web structure. This weakness creates new habitats for pathogens. The proliferation of pathogens will require the further use of more chemical pesticide, creating more voids in soil food web structure, requiring the use of more chemical pesticide, creating more voids in soil food web structure, requiring the use of more pesticides...
A damaged Food Web Structure has consequences. When a part of the food web is damaged or destroyed, there is an imbalance of activity, and the food web structure may suffer. A damaged food web allows more opportunity for pathogens to find food and habitat. A complex and healthy food web contain numerous organisms that out compete the disease organisms either by consuming them, by out competing them for food and space, or by producing metabolites that are toxic or inhibitory to pathogens. Pathogens exploit weakness in an organism. When an organism is healthy and robust, pathogens are unable to proliferate.
For example, if fungi are damaged through tillage or pesticide use, there may be far reaching effects on the food web:
Fungi are the principle immobilizers of Calcium. Fungi deposit crystals of calcium oxalate on the surface of their hyphae. If the fungi are damaged and are unable to re-establish, there is no longer the mechanism for holding calcium in the soil. It will wash away.
The structure-building ability is lost. There is no longer the formation of large soil aggregates. The pore spaces collapse, and lead to compaction and thus less air into the soil possibly leading to anaerobic conditions. The by products of anaerobic respiration are toxins (alcohol and organic acids) which may further damage to the diversity of other food web organisms. The absence of particular organisms in the SFW will deleteriously affect other organisms that depend on them as food or habitat.
Where the food web experienced an episode of depleted oxygen, there may be a high number of Ciliates which are aerobic microbes, but which feed on anaerobic bacteria.
|
