Jackie Maxwell
Geography 3080
Chapter 11 Summary: Organisms and Ecology of the Soil
Many of the central concepts and themes in Chapter 11 are best explained by Figure 11.1, which is
summarized below:
Abiotic Factors
S
O
I
L
C
O
L
U
M
N
Primary
Producers
Inorganic/Organic
Inputs
Primary
consumers
Secondary
consumers
Higher level
consumers
Solar Energy
CO2
Heat energy loss (from radiative loss
and metabolic activity)
Plants, algae, lichen and bacteria
Plant Debris/Feces/Dead
Bodies
Soil humus
Mineral Nutrients
Saprophytic bacteria and fungi, actinomycetes, mites, earthworms, nematode
root feeders and mycorrhizal fungi
Protozoa, bacteria, fungi, actinomycetes, nematode microbial feeders, mites,
springtails, earthworms
Amoebas, earthworms; mites, beetle, spider and ant predators; nematodes;
mammal and bird predators
Figure1: Summary of Figure 11.1 (Brady and Weil 2008)
As shown in Figure 1, there is a constant exchange between the different types of soil organisms, organic
detritus and abiotic factors within the soil column and release of heat from metabolism and re-radiation.
The soil ecosystem is incredibly diverse. This is due to the variety of microclimates, which exist in and
around each soil aggregate and in the soil pores (micropores, macropores, biopores, intraped and interped
pores). Diversity of soil organisms is an indication of soil health, including the state of its physical and
chemical properties and processes, which in turn influence the amount of resources available to the soil
organisms. Some ecosystem engineers, such as the earthworm, change the soil profile by inverting it,
digging into it or chemically altering compounds within the soil or secreting compounds that change the
physical properties of horizons; perhaps even the entire profile. The importance of a certain species of soil
organism is determined by 1) number of individuals in the soil, 2) biomass per unit volume or area, 3)
metabolic activity and 4) their ecological niche. Keystone species may be used to give a relative view of
the other species present in the soil (such as density and ecological role) and they may indicate the
relative diversity of the soil ecosystem. Since organisms tend to concentrate in areas of favourable
physical, chemical and biological conditions, the presence and diversity of soil organisms is a relative
measure of the distribution of nutrients within the soil. Microflora (e.g. bacteria, fungi) represents 80% of
the soil metabolism; however, they are intricately influenced by the activity of microfauna (e.g. protozoa,
nematodes). Above ground plants alter the soil physical and chemical properties through root activity.
The rhizosphere is the area within the soil that is significantly influenced by plant roots and extends 2 mm
out from the root surface. Prokaryotic organisms (e.g. bacteria) can improve the condition of soil that has
been polluted by hydrocarbons. Soil organisms have the ability to be a positive or negative influence on
the soil; they are actively managed and utilized by humans to increase soil fertility, decrease pests and
disease, improve the quality of polluted soils and remediate the effects of degrading human activities.
Therefore the health of the soil ecosystem is intricately linked to above ground processes. Maintenance of
soil ecosystem will therefore be of great concern now and into the future.