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There's more to dirt than you might think.
Last modified: 2012-11-05 18:24:09

There's more to dirt than you might think.

Soil Ecology-

Key concepts and questions:
-What is soil?
-What are the abiotic (non-living) components of soils?
-What are the biotic (living) components of soils?
-How do abiotic and biotic soil components interact?
-What is soil quality?
-How can biological diversity and crop rotation benefit soil quality?
-What is soil?

What is soil?
-The outer most layer of our planet.
-A regenerative living system.
-An ecosystem (Biotic, Abiotic, Boundaries).
-A place where energy and matter and transformed and transported.


-Abiotic soil components include mineral matter (clay, silt, sand), water, air and organic matter. Air and water percentages vary significantly with soil texture, weather and plant water uptake.
-Mineral matter is composed of various proportions of sand, silt and clay particles. Sand particles are 0.05 to 2 mm in diameter, silt particles are 0.002 to 0.05 mm in diameter and clay particles are less than 0.002 mm in diameter. Because clay particles have a very large surface area to volume ratio, they can hold much more water and nutrients than larger particles.
-Soil texture is the proportion of sand, silt and clay in a soil. The soil texture triangle, shown here, is used to classify a soil into one of 11 different categories, each of which has different physical and chemical properties. Soil texture affects nearly every aspect of soil use and management, but is not affected by management unless significant soil erosion occurs.
-Water and air. Since each size particle confers different physical and chemical properties on a soil, soil texture is an important determinant of water retention, bulk density, aeration and fertility. The aeration and water status of a soil, in turn, have important influences on soil biota activity.

Soil Processes-
Many types of chemical and biological processes exist in soils. These include mineralization of organic matter and fixation of atoms of mineral matter into organic compounds. The processes take place within ecosystems (groups of organism interacting with their abiotic environment). Specific organisms such as nematodes and protozoa are involved in the mineralization process. Primary productivity is enhanced in the presence of both bacteria and nematodes.

Management practices significantly alter soil ecology.

Soil organic matter (SOM), though usually comprising less than five percent of a soil's weight, is one of the most important components of ecosystems. SOM strongly modifies soil organism habitat and provides a food source for much of the soil biota. When soil microorganisms feed, they change the form of SOM and in the process release inorganic nutrients, especially nitrogen, phosphorus and sulfur. This process is called decomposition and is an important process in all healthy ecosystems. Because soil microorganisms are continually consuming the SOM portion of their home, SOM must be continuously replenished to maintain soil quality.

In addition to obtaining inorganic nutrients and water from soil, the root system serves as a host for various herbivores, including fungi, bacteria, nematodes, arthropods and insects. Decomposers, including fungi, bacteria, actinomycetes and earthworms, mineralize labile and resistant substrates (soil organic matter). These are referred to as first-order interactions. In second-order interactions, organisms feed on organisms involved in first order interactions. Numerous species of soil-borne organisms including nematodes, insects, mites, fungi, bacteria, and protozoa feed as carnivores, bacterivores or fungivores on the organisms involved in the previous activity level. Soil ecosystems seem to function very much the same as the aboveground pastures with which we are all more familiar.

Soil ecosystems function in accordance with the Second Law of Thermodynamics, which states that "in any energy conversion, the final product will consist of less useable energy than the original product, because of the inevitable loss of energy in the form of heat." The amount of biomass, therefore, is less in each subsequent interaction order or trophic level.


Ecosystems: Places where organisms interact with each other and their abiotic environment-
Soil organisms interact in many ways. For example, protozoa eat bacteria and some fungi feed on protozoa or nematodes. Other fungi are consumed by protozoa or parasitized by nematodes. Interactions among soil organisms may be very complex. They are crucial to the functioning of soils. An understanding of the nature of the organisms that live in soil is essential for understanding soil ecology. The Soil Ecology and Soil Biology components of the website(http://www.safs.msu.edu/) are designed to complement each other. One should remember that Structure + Process ->Pattern.

Nematode Patterns as Indicators of Soil Quality-
Soil, air and water, are basic natural resources that provide important ecosystem services. For example, soil is a carbon and nutrient cycling site and also helps clean both water and air. Much of our drinking water in Michigan is filtered through soil as it moves into ground and surface waters. Poorly managed, soils can serve as a pipeline for pollutants, such as nitrate into groundwater, silt into surface waters and nitrous oxide into the atmosphere.

Soil quality is a measure of a soil's function, specifically, a soil's ability to:
-Accept, hold and release nutrients and other chemical constitutents.
-Accept, hold and release water to plants, streams and groundwater.
-Promote and sustain root growth.
-Maintain suitable soil biotic habitat.
-Respond to management.
-Resist degradation.
-A system of nematode community structure analysis has been developed as
an indicator of soil quality.

While soil cultivations can result in soil degradation, including loss to erosion and decreased soil organic matter content, a sustainable agriculture, by definition, does not decrease soil quality. While there is currently no consensus on which set of measures to include in an assessment of soil quality, scientists generally agree that measures of both abiotic and biotic soil components will have to be integrated in a holistic manner to assess soil quality. Balanced biodiversity is increasingly seen as an essential component of soil quality.

Soil characteristic patterns important to soil quality:
-Soil organic matter
-Water holding capacity
-Water infiltration rate
-Microbial biomass carbon and nitrogen
-Bulk density
-Electrical conductivity
-Nutrient availability and release
-Balanced biotic diversity

Management goals for maintaining or improving soil quality include:
-Using renewable soil components (such as organic matter and nutrients) no faster than they can be renewed.
-Using nonrenewable soil components (such as soil particles) no faster than substitute resources can be developed.
-Generating or applying potential pollutants associated with soil management (such as manure or pesticides) only as fast as the soil system can assimilate or transform them.
-Management options that increase soil quality include crop rotations and cover crops. These options can increase soil organic matter, organic nitrogen and protect against soil erosion.
-Ecological pest management strategies decrease the need for agricultural pesticides and also reduces soils' exposure to toxic compounds.

Three structural factors are currently recognized as indicators of soil quality and there are three major ways to approach soil quality management.

Soil Quality Structure:
-organic matter amount
-organic matter diversity
-organic matter quality

Soil Management Trilogy:

Source Soil Biology

Source Soil Ecology

Keywords: Soil Biology, Soil Ecology
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