Compost is the result of the natural rotting process that occurs with all organic material. Compost contains valuable nutrients and is rich in humus. Humus is long-lasting in the soil and can be beneficial in providing for improved physical, chemical and biological conditions.
Commercial producers and on-farm production of compost speeds up the natural process. Aerobic composition is the rapid decomposition of organic material into humus-rich product ideally suited to soil improvement. High temperatures are naturally generated during the composting process resulting in the destruction of any weed seeds and pathogens that may be present in raw organic materials.
Physical requirements of compost
The basic ingredients of making compost are quite simple:
- a supply of organic materials
Microorganisms, moisture and oxygen are universally available and practically free, however, watering costs must be considered in every climate. Compost is a net user of water.
Sourcing composting material
Organic material can be sourced from anywhere:
- garden or green waste
- animal manure
- kitchen waste
Successful producers in Victoria are composting:
- green waste
- grease trap sludge
- food waste
- wool scour waste
There are strict regulations governing the siting and operations of composting enterprises. Contact EPA Victoria for assistance early in the planning phase of a facility.
Depending on your method of production, you may also need to add some other materials such as:
You also need microorganisms to do the composting. Microorganisms have three basic needs (oxygen, moisture, food supply) and when these are provided the composting process will proceed and the mix will heat up.
Machinery to make compost
The other requirement is machinery.
Depending on the scale of production and the types of waste accepted, composting requires the use of heavy machinery.
As a minimum, commercial scale operations require the use of a front-end loader to load, unload and turn compost.
Screening equipment for the end product is also typical for most operations.
Production of compost
If composting is done properly, you can minimise the loss of nutrients into the air and enhance your end product. Like every production process, the quality of your compost largely depends on the quality of the organic materials that you use.
There are many aspects to quality compost such as its:
- organic matter content
- nutrient content
- C/N ratio
- salt content
- suitability for use with particular plants
The Australian Standard for composts, soil conditioners and mulches (AS4454-2012) is the industry standard for determining compost quality.
The two most critical factors to consider when mixing organic materials are the C/N ratio and moisture content.
Ideal C:N ratio of compost
The ideal C:N ratio is about 25 to 30:1. This means the composition of the raw material has about 25 to 30 times as much carbon as nitrogen. If there is too much carbon relative to nitrogen, composting is slowed down or even stops completely. If there is too little carbon relative to nitrogen, there is a loss of the nitrogen (as ammonia gas). This produces unpleasant odours and valuable nutrients are lost. The C:N ratio will gradually fall to 10-20:1 as the carbon in the pile is digested.
Examples of C/N ratios of commonly used materials are:
- animal manure (5 to 12)
- weeds (20)
- leaves (60)
- lawn clippings (20)
- paper (170)
- straw (100)
- pine needles (70)
- sawdust (450)
- and seaweed (25)
Seaweed can be used as long as you wash off the surface salt before adding it to the compost heap. If nitrogen levels need to be boosted, it is preferable to add other organic materials that are high in N (for example, manures and grass clippings). Inorganic fertilisers such as urea can also be used.
The next thing to consider is the moisture content. The optimum moisture content for composting is between 50 to 60% on a wet weight basis (50% = 50g water in 50g dry matter). Below 40%, the compost is too dry for the microorganisms to work at maximum efficiency. Above 60% there is a high risk of odours developing because of a lack of oxygen in the compost.
Temperature also is important. The composting process naturally produces heat, which helps the microbial activity. However, you don't want it to get too hot, otherwise the microbes slow down. When temperatures exceed 65°C, the activity and diversity of microrganisms drops off markedly. The most rapid composting occurs between temperatures of 45°C and 55°C.
In colder areas, you may need to insulate the heaps in winter to retain the warmth. Heaps that are too small (for example < 1 m3) will not heat up.
Heaps that are too large become too hot and risk catching fire. The optimum size of a heap depends on the materials being composted, space and equipment available. Correct temperatures are also important in getting rid of pathogen, parasites and weed seeds that may be in the compost.
Composting can occur at a broad range of pH. Adjustment may only be necessary if the starting materials are extremely acidic or alkaline, or if a certain pH range was required in the end product. Also, there is a greater risk of loss of nitrogen as ammonia causing odours when the pH exceeds 9.0.
The most common form of composting is the windrow system. Organic materials are placed in windrows of varying dimensions depending on the waste being composted, space available and equipment used.
- For manure based operations, the recommended size is around 1.5m high and up to 2.5m at the base.
- For green wastes, heaps can be as high as 2.5m or more and 4 to 5m at the base.
- The length of windrows only depends on space available, and can vary between 20m and 100m.
Windrows are turned by either front-end loader or specialised windrow turners. Turning is used to:
- mix the compost
- control temperature
- control moisture
- provide the aeration needed for composting
Capital outlays for windrow type systems are relatively small (unless a concrete pad is installed). Operating costs can be high because they are usually labour intensive.
Some types of wastes are more difficult to handle and must be composted with specialised aeration and leachate control equipment. These systems can be very expensive, and may not be an option outside urban areas.
Another simple and cheap option is the passively aerated windrow or pile system. In this system, pipes are laid on the ground and the heap is formed on top. The pipes extend outside the heap to provide adequate airflow to the compost. A layer of finished compost is placed over the pile to reduce the incidence of odours. This system does not require turning. It has been used to compost manures, but careful preparation and monitoring is required to ensure success.
Marketing your compost
Markets can vary from:
- home gardeners
- nursery suppliers
- topsoil manufacturers
- landscape suppliers
- potting mix manufacturers
- local government
- commercial horticulture
It is recommended that compost producers become certified to the Australian Standard for compost (AS4454-2012).
Alternatively, compost producers may choose to get organic certification for their compost National Association for Sustainable Agriculture Australia (NASAA).
Certification to one of these standards should give your product a marketing edge.
Most producers are small and sell through local markets and outlets. There are opportunities with larger organisations but they will want a consistent, regularly supplied and professionally packaged product.
Financial aspects of composting
There are considerable capital outlays for urban based composting operations. However, on-farm based composting can be done relatively cheaply if the right equipment is available.
Organisations and contacts
Phone: 03 8626 8700
Research & training programs in compost production and use
Phone: 03 9210 9222
- Designing, constructing and operating composting facilities. EPA Publication 1588.1* June 2017
- Composts, soil conditioners and mulches. Standards Australia (AS 4454-2012)
- Managing effluent — Composting spoiled hay
- How to compost food and garden waste, Sustainability Victoria
- Cahill, G. (1998) Guide to best practice, composting green organics.
- Cahill, G. (1996) Environmental guidelines for composting and other organic recycling facilities. Best practice environmental management series, publication 508. EPA, Victoria.
- Australian Standard for composts, soil conditioners and mulches (AS4454-1999). Standards Australia, Homebush, NSW