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.
Abiotic soil components
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.
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.
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
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
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.
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.