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Soil

The Soil EcosystemThe soil is an incredibly complex system made up of microscopic and macroscopic organisms. For example, a spoonful of healthy soil contains many millions of different organisms including beneficial species of bacteria, fungi, nematodes and protozoa. These organisms perform vital functions such as keeping disease-causing organisms in check, recycling and storing nutrients and making them available to plants, allowing healthy root growth, and providing a highway for air and water to pass through. The more diverse the soil foodweb, the healthier the soil ecosystem.

Soil structure is a complex system of aggregates, pores and channels, all created by the activities of soil organisms. Bacteria glue the clays, silts and sands together into what are called microaggregates.

Microaggregates are bound together by fungal hyphae (underground fungal vegetative growth), root hairs and roots. Larger creatures such as arthropods, insects and earthworms burrow larger passages through the soil. Only when all the organisms are present and active can roots and water move into (and through) the soil with ease.

The soil foodweb is fueled by plant primary production, which provides organic matter (both living and dead) to support the other organisms. Plants also actively release large amounts of carbohydrates, produced by photosynthesis, into the soil around their roots. This process provides support for both bacteria and the essential symbiotic fungi that form a net around plant root hairs, protecting them from disease-causing fungi and greatly extending their ability to absorb nutrients.

Bacteria and fungi, along with the organic matter they produce, function as a storehouse for most of the nutrients in the soil. These in turn are eaten by protozoa, nematodes, and micro-arthropods. It is the by-products of these organisms that provide the soluble nutrients that plants need in small but steady amounts for optimal plant health. Many species of larger creatures, such as earthworms, also eat these smaller soil organisms and they create the larger aggregates and pore spaces in soil. Similarly their waste become nutrients that get recycled into the storehouse by bacteria and fungi and made available to plants.

This intricate food web not only provides nutrients, but also serves as an "environmental protection agent." The multitude of pathways used by this diverse soil life allows it to break down pesticide and hydrocarbon pollutants, bind heavy metals into immobile forms, and convert soluble fertilizers into complex stored organic forms. Breaking down, binding and converting these contaminants in the soil is known as "biofiltration" or "bioremediation" and is what keeps these pollutants from entering ground and surface waters. All of this happens naturally in soils with adequate organic matter.

Natural Soils
Click on the image to the left to get a larger view of what Native Soils should look like. (62KB)
Over four billion organisms live in a teaspoon of healthy soil. Together these organisms bind soil particles, fetch nutrients and water for plant roots, churn the soil particles creating spaces in the soil for air and water, and breakdown leaves and other dead plant material. Characteristics: Many air and water pockets Numerous micro and macro organisms Deep plant root growth High evapotranspiration and surface water infiltration and detention Low surface water runoff and erosion

Modified Soils
Click on the image to the left to get a larger view of what Developed Soils typically look like. (62KB)
Soil can be changed by various factors. Typical construction practices in the Northwest remove 2-3 feet of native topsoil, and then ultimately cover it again with a few inches of soil and sod. The subsoil underneath is usually so compacted that it essentially acts like concrete. Fire can volitilize organic matter and destroy the porous structure of the soil. Characteristics: Few air and water pockets Limited micro and macro organisms Shallow root growth Low evapotranspiration and surface water detention and infiltration High surface water runoff and erosion

Excerpted and edited from: http://dnr.metrokc.gov/swd/resrecy/composting/soilfunctions.shtml

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