The Biology of Compost

Many people think of compost as a source of nitrogen and other plant growth promoting minerals.  While those materials are important, they do not maintain their efficacy for long in soil nor in compost.  


What actually creates these plant growth promoting materials in the first place?.  They are created by the bacteria, fungi, protozoa, nematodes and micro arthropods that make up the soil food web communities.  So, actually, what we want to be adding to our soils is the correct soil micro biological community, as these species will then ensure the ongoing creation of more of the enzymes and hormones that your plants need to thrive. 

If you are growing plants in soils where the soil biology is not in a healthy or balanced state, you can only achieve plant growth by using toxic chemicals (to try to overcome the diseases that will attack the stressed plants) and by using chemical and salt based inputs (to try to feed the plants the inorganic nutrients they need).  However these plants are then not healthy, they are stressed, and the nutrients the crop produces are not at optimum levels.  As such, when consumed by humans these crops are not delivering the high potential levels of nutrients that are possible.  By adding chemicals, we are able to increase production of plant material in unhealthy systems - an example of this being crops grown in conventional hydroponics systems.  But at what cost to water quality, soil structure, human nutrition or the quality of our lives?  The long-term impacts of this method of farming are already being seen in environments around the world. can help you rehabilitate your soils from chemical dependency and equip and guide you towards restoring  a natural balance.  This ultimately leads to improved long term productivity and sustainability.

Plants depend on beneficial micro-organisms in the following ways:

  1. to protect them from pathogens,
  2. to retain nutrients in the soil so they are not lost from the root zone through leaching, 
  3. to cycle nutrients into plant available forms (both predator-prey and mycorrhizal fungi work to achieve this), 
  4. to increase the uptake of soil and foliar nutrients, 
  5. to break down any pollutants present in the soil or on above-ground plant surfaces, and 
  6. to build the air passageways that enable air and water to infiltrate into the soil and also be retained.  In this way the plant roots can grow as deep into the soil as physiologically possible enabling them to obtain water and nutrients all year long, regardless of conditions.

If the organisms that perform these key functions are missing, they need to be replaced.  A failure to do so will make plant production and soils totally reliant on chemicals and the need to utilise scarce water resources in large quantities.

Compost biology

Compost organisms perform a number of important processes during composting.  But their relevance doesn’t stop there – these same organisms survive and live in soil, on leaf surfaces, and around roots, leaves, stems, blossoms, etc. They create a protective layer on leaves, stems, blossoms, fruit and any above or below ground plant surface.

Bacteria and fungi – retain nutrients in the compost, and ultimately, in your soil too.  They also perform the same function on leaf surfaces, if you could somehow get compost to adhere to the leaves.  That is possible if you turn the compost into compost tea – refer to our sections on compost tea to learn more.

Bacteria and fungi also build structure in the compost while the protozoa and nematodes help build the larger pores in the compost.  So within a week or so, if you have the right biology present in the compost, air passageways and water infiltration hallways will have been built by these organisms.  Turning compost becomes less and less critical as the biology grows and forms structure which in turn aerates the compost for you.

Protozoa and nematodes – mineralise the retained nutrients held by the bacteria and fungi.  In compost, these mineralised nutrients serve to help other organisms grow and utilise the carbon sources in the organic matter present in the compost pile.

The dynamic living system present in compost is influenced by the ingredients that you choose to put into the compost pile, the biology present in the organic matter going into your pile and by the effects of rain, wind, heat, sunlight and pollution that occur while you are composting.

There are numerous factors that have an effect on compost quality including the starting materials, moisture, aggregation, and temperature / turning.  Additionally, different plants or crops may require a more fungal or bacterial dominated compost.  It is imperative that the environmental conditions in compost are managed so that a high quality, pathogen-free compost is produced.

GroundGrocer can support and equip you to develop the compost that your soil and plants need.