Teaching Organic Farming and Gardening

Soil Biology and Ecology

Unit 2.3 | 13
Students’ Lecture Outline

3. Functional classification

a) Microorganisms

i. Colonial growth forms (cells about 1/25,000 inch wide)

• Bacteria and yeast

• Adapted to high surface area (SA)/volume

• Colonize surfaces, crevices, pores

• Teaspoon of soil contains 100 million to 1 billion bacteria

• Biomass equivalent to 2 cows per acre

• Many functions (N fixers, nitrifiers, denitrifiers, decomposers, pathogens, promote
aggregation)

ii. Mycelial growth forms (hypha length ranges from a few cells to many yards)

• Fungi and actinomycetes

• Penetrate organic matter

• Translocation of nutrients

• Functions: decomposers, mutualists, pathogens, nematode-trapping fungi

iii. Algae

b) Microfauna

i. Protozoans (1/5000 inch to 1/50 inch wide)

• Small animals (acellular) living in water films

• Feed on bacteria, other microorganisms, detritus

• Encystment: Distinctive response to drying out

• Inhabit transitory environments

• Colpoda divide once or twice per day at 12oC

• Several distinct types

• Ciliates have fringe of small hairs used for locomotion


• Amoebae have an amorphous body shape


• Flagellates have a whip-like tail for locomotion

ii. Nematodes (1/500 inch in diameter, 1/20 inch in length)

• Global distribution

• Soil abundance = million/m^2

• Outer cuticle protects; resistant to toxins

• Include microbivores, omnivores, predators, some parasites (10%)

• Abundant at sites with high organic matter concentration

iii. Rotifers, Tardigrades

iv. No comminution, i.e., they don’t shred organic matter into smaller pieces

c) Mesofauna

i. Potworms (Enchytraeida)

• Small annelids (related to earthworms), 1mm to 5 cm long

• Tolerate pH < 4

• Thousands/m^2 in high organic matter soil

• No burrows

• Feed on hyphae, microorganisms, feces

ii. Collembolans (springtails)

iii. Mites (acari)

iv. Insect larvae

v. Regulate microorganisms by grazing (feeding)