Teaching Organic Farming & Gardening

Soils & Soil Physical Properties

Unit 2.1 | Part 2 –9

Lecture 1: Soils—An Introduction

D. What Is in Soil?

1. 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)


2. 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)


3. ~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