Soils & Soil Physical Properties
Unit 2.1 | Part 2 –9
Lecture 1: Soils—An Introduction
D. What Is in Soil?
- 40–50% mineral. Generally almost half of the soil is
made up of non-biological particles of different sizes.
The sizes present depend on the history of the soil,
including the forces that formed it, how long it has
been forming, and the parent material.
a) Rock particles too big to be soil: from gravel, to
stones, to boulders
b) Large soil particles: Sand (0.05–2.00 mm)
c) Medium soil particles: Silt (0.002–0.05 mm)
d) Small soil particles: Clay ( < 0.002 mm) - 0–10% biological (See u Table 2.2, Soil Fauna and their Eating Habits, and u Table 2.3,
Common Populations of Some Soil Microorganisms). A small fraction of the soil is made
up of biological organisms, or parts of organisms. The percent present depends on similar
factors from the history of the soil, including how long it has been forming and the parent
material, and is strongly influenced by environmental conditions.
a) Includes plants, animals, algae, bacteria, archaea, and fungi
b) Organisms may be alive or dead (when dead they become “organic matter”)
c) This includes both macroscopic organisms (organisms you can see with the naked
eye, such as plant roots, rodents, earthworms, insects) and microscopic organisms
(organisms you can see only with assistance, such as some fungi, bacteria, archae) - ~50% pore space
Pore space consists of the “empty” spaces in the soil. This is a critical part of the soil because
it is filled with either:
a) Air, which allows gas exchange for organisms (particularly CO 2 or O 2 for respiration)
b) Water, which is key for organismal function, and is especially important for plants via
uptake by roots- • • • • • • • •
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t FIGURE 2.1 | SOIL COMPOSITION:
AN IDEALIZED SOIL
ORGANIC MATTER 5%
MINERAL
45%
25% SOIL AIR
25% WATER
u TABLE 2.2 | SOIL FAUNA AND THEIR EATING HABITS
MICROPHYTIC FEEDERS CARNIVORES CARNIVORES
SECONDARY CONSUMERS TERTIARY CONSUMERS
ORGANISM MICROFLORA PREDATOR PREY PREDATOR PREY
CONSUMED
*feed on live plants/plant residues, and/or soil organic matter
Springtails Algae*
Bacteria*
Fungi*
Fungi
Algae
Lichens
Mites Centipedes
Earthworms*
Insects
Centipedes
Ants
Beetles Spiders
Mites
Beetles*
Spiders
Mites
Centipedes
Spiders
Centipedes
Mites*
Scorpions
Mites Springtails*
Nematodes*
Enchytraeids
Springtails*
Nematodes*
Snails*
Slugs*
Aphids*
Flies*
Protozoa
Nematodes
Bacteria and
other microflora
Bacteria
Fungi
Fungi
Moles
Termites