FungiFungi are comprised of microscopic cells that grow in long threads called hyphae.  Fungal hyphae bind soil particles into macro-aggregates which increase the soil’s ability to absorb and retain water.  Fungi also play an important role in the breaking down of organic matter into useful forms and assists in nutrient retention and disease suppression.
 BacteriaBacteria also play a vital role in breaking down organic matter, maintaining the soils ability to retain nutrients, assisting in disease suppression and helping improve soil structure by forming micro-aggregates.  Certain types of bacteria also undertake a range of other functions including the breaking down of toxic compounds and assisting with nitrogen fixation.
active fungal and bacterial biomass At SFI we measure active fungal and bacterial biomass in addition to total fungal and bacterial biomass.  Active biomass is a measure of the organisms that are metabolising or “doing the work”.  Fungi and bacteria are active when food resources are available and conditions are favourable (eg. moisture, temperature, etc).  If activity is low then organisms may need feeding. To assess active organisms a dye is used that the active organisms take up so that they become easier to identify (pictured).
 ProtozoaProtozoa are single celled organisms that primarily consume bacteria and release nutrients in forms that can readily be absorbed by plants.  There are three different groups of protozoa in soil – flagellates, amoebae and ciliates.  If a soil contains a large number of ciliates this indicates it is in an anaerobic / compacted condition.
 NematodesNematodes are classified in a number of functional groups based on their food source – fungal, bacterial, or root feeders.  There are also predatory nematodes that eat other nematodes.  The majority of nematodes are beneficial as they form an integral part of the soil food web.  They convert nutrients into plant available forms by consuming either fungi or bacteria.  Beneficial nematodes also consume disease causing organisms, and are an important food source for larger organisms.  The presence of root feeding nematodes in high numbers is an indication that the soil food web is in a degraded state.
 Mycorrhizal fungi Mycorrhizal fungi colonise the roots of most plant species, forming a mutually beneficial relationship that has a significant role in nutrient uptake and protects against root feeding nematodes and pathogens.  Mycorrhizal fungi are especially important in supplying plant available phosphorus.  Many studies have documented a decrease in mycorrhizal colonisation when soluble phosphorus levels are high, usually as a result of excessive use of inorganic fertilisers.

Pictures from top to bottom: Fungal hyphae; Bacteria (cocci); Active Fungi on leaf surface; Protozoa – amoebae bottom right of picture and two amoebae cysts; Fungal-feeding nematode; Vesicular Arbuscular Mycorrhizal fungi (VAM).