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?

Structure + Process -> Pattern

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.

STRUCTURE

Abiotic soil components

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. The example shown here (10 percent clay, 70 percent sand and 20 percent silt) is a sandy loam. 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.

In the presence of both bacteria and nematodes primary productivity is enhanced.

	Management practices significantly alter soil ecology.

Organic matter. Soil organic matter (SOM), though usually comprising less than five person 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.

Patterns

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.

Additional reading

Coleman, D.C. and D.A. Crorsley, Jr. 1996. Fundamentals of Soil Ecology. Academic Press. N.Y. 205 pp.

Doran, J.W., D.C. Coleman, D.F. Bezdicek and B.A. Stewart. 1994. Defining soil quality for a sustainable environment. Soil Science Society of America Special Publication Number 35, ASA, Madison Wis.

Paul, P.A. and F.E. Clark. 1996. Soil Microbiology and Biochemistry. Academic Press, N.Y. 340 pp.

Soil and Water Conservation Society. 2000. Soil Biology Primer. Published in cooperation with the USDA Natural Resources Conservation Service. For more information visit their website at www.swcs.org.