Author Topic: Introduction to Soil Biology  (Read 1947 times)


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Introduction to Soil Biology
« on: April 14, 2011, 03:45:09 PM »
Introduction to Soil Biology

State university soil testing labs can provide a percentage of organic matter present in the soil. This gives you a rough estimate of the quantity of SOM but no qualitative information.

If you are interested in the biological properties present in your soil (percentages of bacterial and fungal communities and which ones are dominant), you need to have a separate test performed by a private laboratory such as one of the soil foodweb laboratories. These tests will give you an indication of the soilís biology and how best to amend the soil for turf or any other plant community

Soil biology can be amended either by supplying the organisms themselves, such as in a compost tea, or by supplying the food they like, such as compost. Here are descriptions of some of the useful microbes in healthy soil.

Bacteria are microscopic single-celled organisms. They do most of the work in consuming organic matter and releasing nutrients. They generally work fast when conditions are right: soil pH in the mid-sixes to mid-sevens; soil temperature in the 80s or 90s; and adequate aeration but with adequate moisture.

 Bacteria consume the simple organic materials such as sugars and simple starches. They do not make much use of proteins, cellulose or more complex materials.

 Bacterial populations increase very rapidly when sugars and starches are made available and conditions are favorable. When these materials are depleted, the populations drop rapidly; most of the bacteria die and are themselves decomposed.

Soil fungi are also tiny, but they are more complex than bacteria are. Most soil fungi are filamentous, meaning that they produce string-like mycelia that spread out. Fungi as a group are active over a much wider soil pH range and decompose organic matter in acid soils, where there is little bacterial activity. Fungi work much more slowly than bacteria do, but they can consume complex materials such as proteins and cellulose.

Mycorrhizae are a specialized group of fungi that form a mutually beneficial association with plant roots. The fungi infect the roots and obtain sugars from the plant. In exchange, the mycelia explore the soil, taking up nutrients, which are then made available to the host plant. This relationship extends the effective size of a root system by up to ten times, increasing the plantís ability to obtain water and nutrients.

Actinomycetes are a group of microbes that are neither bacteria nor fungi. They give off volatile chemicals called terpines, which are responsible for the earthy smell of newly plowed soil. Actinomycetes are active in soils with pH levels down to the mid fives. They tolerate dry soils better than bacteria or fungi do. Like fungi, they work slowly and decompose difficult materials, even waxes and lignins.

Feeding the Microbes
Microbes feed on organic matter. To encourage high populations of microbes, we must supply organic matter. As noted above, the various microbes thrive on different components of organic matter. If we want to feed all of the different microbes, we must supply organic matter that is 
not fully decomposed, or we must apply biologically active compost tea. Well-aged compost has had most of the sugars and starches consumed during the composting process, so it will not stimulate a high level of bacterial activity. Nevertheless, well-aged compost is an excellent soil amendment because it improves soil structure, increases waterand nutrient-holding capacity and provides nutrients. Lawn clippings are a complete microbial food that will stimulate bacterial as well as other microbial activity.

Microbes consume a lot of carbon (C), but also other nutrient elements such as nitrogen (N) contained in organic matter. They generally consume about 25 to 30 Cís for each N. If the soil contains C and N in this ratio (25:1 to 30:1), everything is fine. If the ratio is lower (e.g., 18:1), there will be more N than the microbes need and the excess will be available for plant uptake. If the ratio is higher (e.g., 50:1), there will not be enough N for the microbes and they will take what they need from the soil, creating an N deficiency for the plants that will require the addition of more N. Generally, green materials have a surplus of N (they are low in C) and brown materials are low in N (high in C). Most finished compost has a C:N of 15:1 to 20:1 and will supply N.
« Last Edit: April 14, 2011, 04:01:04 PM by admin »