Compost is an essential component of organic gardening. It creates the humus essential for maintaining that healthy soil ecosystem responsible for the recycling of nutrients in a form accessible to our plants.

Compost is the end-product of the biological decomposition of a selected mix of organic materials through a controlled aerobic or anaerobic and, ideally, a thermophilic (hot) process.

Compost delivers nutrients slowly through the growing season and they are less likely to be lost through leaching as with soluble fertilisers and manure. Incorporating compost in an organic garden by digging or layering also:

  • improves soil structure by aggregating soil particles that in turn maintains good aeration;
  • improves the moisture-holding capacity of the soil;
  • inoculates the soil with beneficial soil organisms and helps suppress harmful organisms and pathogens;
  • increases the tolerance of plants to changes in acidity and alkalinity pH); and
  • assists in preventing absorption of heavy metals and other toxic substances by plant roots.
  • Composting contributes to sustainable organic gardening practices by recycling our plant waste and kitchen food scraps back to the soil as usable nutrients rather than being dumped in land-fill.

There are different approaches to composting:

Hot (thermophilic) composting:

A hot compost heap on a cold July morning in Canberra.

A mix of organic materials is contained within a pile of sufficient bulk and proper composition to generate a minimum internal temperature of 55 degrees Celsius. The pile is turned regularly for at least six weeks to expose this heat generated through the bacterial decomposition to the entirety of the material to enable the destruction of harmful microorganisms and weed seeds. [More detail on this process below.]

Cold composting

For cold or cool composting, a mix of raw organic matter is piled up and left to decompose without turning for what needs to be a very long period of time. With the right mix of materials it will ultimately produce useful compost but without the richness of beneficial bacterial produced from hot composting.

In-ground composting

There are two basic approaches. “Trench composting” involves burying vegetable waste in a pit, usually about one spade deep, and covering it with soil progressively in layers. It will take two to three months for the material to sufficiently decompose for you to plant into the bed. “Sheet composting” is a term used to describe various methods whereby large quantities of raw organic matter (eg straw mulch or manure) is applied directly to the soil over a wide area (eg a garden bed). The Australian Certified Organic standard requires a sheet composted area using animal manures to be followed by two successive green manure plantings before the area is used for cops intended for human consumption.

Anaerobic compost bins

Closed bins have the advantage of enabling layers of organic material to be added progressively as it becomes available and prevent the access of rodents to kitchen waste. A bin must have access to bare soil and be managed to be partially anaerobic; that is, the contents should still be turned using a screw aerator or similar tool. The addition of lime may be necessary to correct the high acidity these closed processes produce.

Arguably the most effective method of composting for organic gardeners is the hot composting (or Indore) method. It was devised by Sir Albert Howard, the founder of the organic movement, and can be readily adopted in home gardens where there is an availability of sufficient raw organic waste material, such as leaves and grass clippings.

The primary objective in composting by the Indore method is to construct a bulky, layered collection of organic matter that will provide an environment favourable to the high temperature aerobic bacterial decomposition that converts this matter to humus that can be used by growing plants.

Compost requires a mix of carbon and nitrogen as well as air and water. Not all compost is the same and the quality of the end-product will depend upon what materials have been used and how well the process has been managed. Consult a good composting guide (ref) for details of the value – including the C:N ration – of common compostable materials. This list will give you a start and you will learn from experience:

  • grass clippings, weeds and spent green vegetable plants;
  • dry leaves, leaf mould and soft hedge clippings;
  • woody prunings and straw;
  • kitchen scraps;
  • animal manures.

All these materials together will provide a good balance of carbon and nitrogen. Animal manures are not essential if you have a high volume of fresh green growth and/or kitchen scraps. Layering with garden soil provides inputs of minerals, including trace elements. Materials to avoid include meat and fish scraps, pet poo and noxious perennial weeds (eg nutgrass and oxalis) or plants infected with virulent disease (eg brassica club root – a pathogen that can thrive in moist, acidic conditions – and blight affected tomato plants).

For an effective Indore composting process:

The heap needs to be set on bare ground (in sun or shade, but protected from strong winds) and be at least one metre deep by two metres wide to generate and contain the heat necessary for decomposition. Covering the heap with a layer of straw will help insulate it from cold air or drying winds.

Shredding coarse material will help with speedier decomposition and turning.

The heap should be continuously damp but not wet; that is, around a 50-55% moisture content. If the compost dries out the bacterial decomposition will cease with the risk of re-infestation of pathogens.
Natural activators (such as blood and bone, lucerne and seaweed extracts) or commercial products can accelerate decomposition. If the initial mix of materials is sound then such additives may be unnecessary.

Do not add lime. Aerobic conditions means this is unnecessary and lime can cause loss of valuable organic nitrogen. Any correction to pH should be addressed in the soil.

Turn the material at least every two weeks to keep it aerated and expose all the material evenly to the high temperature. In summer, compost will take six to eight weeks to be usable but in winter it will take much longer.

The finished product should be dark brown or black and have an earthy smell. The best time to use the compost is when it is mostly, but not completely decomposed, and about a month before planting. Be generous. Heavy feeders (eg brassicas and solanum) and early cropping plants will need more.