The process of composting is 'decomposition due to microbe metabolising/digesting organic carbon compounds to derive the energy that these organisms require for reproduction'. To make a good compost you first have to look at the technical side.
PART 1: Technical Aspects And Processes
What is the purpose of composting?
- to achieve physical breakdown of organic materials
- to destroy plant disease organisms (heat up to 60 degrees C generally kills plant pathogens)
- to destroy pathogens from materials of animal origin
- to break down components of the organic materials that may be toxic
It inevitably results in loss of carbon to the air as carbon dioxide, the process of composting may result in the loss of 25% of volume (or more) and also loss of nitrogen to the air. For this reason, composting time should not be 'too long', just enough to achieve the points listed above.
Things manufacturers to take into consideration:
The amount of heat produced is influenced by the speed of microbial metabolism and the dimensions of the compost heap, larger higher heaps of compost accumulate more heat.
The graph below illustrates a large compost pile going through the composting process. At this scale, the temperates reach 60 degrees before being turned which is indicated with the sudden drop. Piles are turned at least three times until it has completed the full process which also helps kill off any seed.
An example of a potting mix temp probe below.
The speed of microbial metabolism is influenced by the C:N ratio and the moisture content of the mixture of organic carbon composition of the compost.
The C:N ratio is used to predict the effect that the compost is likely to have on plant available nitrogen in the soil and as an indicator of the rate of decomposition. A C:N ratio of about 30 is considered nitrogen neutral - lower ratios will release nitrogen and act as nitrogen fertilisers and higher ratios will immobilise nitrogen as microbial breakdown of the carbon component occurs. In the long term, the nitrogen content will become ‘available’ as carbon decomposition occurs.
Typical C:N Ratio
- Poultry manure 7 (indicative of a fast rate of decomposition)
- Bio-solids 8
- Poultry litter 10
- Cow manure 12
- Green Waste 20
- Corn Stalks 33
- Dead Leaves 60
- Straw 100
- Bark 500 Sawdust 550 (indicative of a low rate of decomposition)
High C:N materials such as bark, sawdust, or post peelings are often used as mulch due to their slow rate of decomposition when spread on to the soil surface. However, when such high C:N materials are cultivated or mixed into the soil, plant available nitrogen may be reduced in the short term.
Low C:N materials tend to have a fast rate of decomposition when applied to the soil and act as nitrogen fertilizers.
The types of microbes growing in compost
The microbial populations on the cool outside of a hot compost pile are different from those in the hot centre of the compost pile. Turning or mixing compost at regular intervals mixes these up, exposes the compost to air, and is required to produce a uniform composted material.
The aim is to produce a friable soil with good water infiltration, good water holding capacity that retains good physical structure in all seasons.
PART 2: Your Questions Answered
What makes a great compost and how to determine the quality of your compost.
Should a compost smell, if so how much?
It will have some smell. If compost has a bad 'sour' smell or a smell rather sour or acrid, that is an indication of inadequate decomposition or lack of air causing anaerobic microbial activity. This is typical of a pile of grass clippings left to 'rot' where there is no air in the centre of such a pile, the centre is generally dull green, gooey and stinks.
What are the signs of well-composted compost?
The signs of a 'well-composted' material is when you have achieved the points above in "The purpose of composting".
Should a compost be warm or cold?
The C:N ratio of the compost is a strong influencing factor on compost heat accumulation. Compost loaded onto a trailer from a large pile or bin will be warm/hot and will stay that way until it is spread out. If it is loaded cold to a trailer, even 0.3 of a cubic metre will most likely start to generate some heat. That is not a problem, it is an inherent characteristic.
How long do nutrients last in compost?
Commercially made compost provides approximately 1 year of nutrients into the soil. The amount of nutrients and value that brings to the soil is determined by the compost inputs. A high quality compost will have beneficial nutrients past one-year however this is generally when they start diminishing.
When is Compost ready?
Compost is ready when it has cooled, turned a rich brown color, and has decomposed into small soil-like particles.
Any other notes/tips that a home gardener might find beneficial?
Compost is an excellent source of potassium for growing plants. It is also a source of nitrogen. For more specific nitrogen levels a potting mix is recommended.
Compost vs Soil vs Potting mix. When to use what?
Soil: Generally a soil is used for building up to larger areas with a fill that doesn’t require a large nutrient value. The soil will compact the most of these three so is excellent for building up areas.
Compost: As a landscaping media it’s purpose is to provide nutrients to larger garden or non-contained areas. Compost will have a slightly varied range in PH levels due to the natural process of composting. Most compost products aim to be neutral or slightly acidic in PH which is well suited to the majority of plants. Some plants require specific PH levels however this is generally for exotic or niche plant types
Potting Mix: If you’re working with a contained area or need a specific requirement for your growing media, potting mixes are the best option. Creating a media from a bark fine base and adding in extra supplement media and fertilisers means you can create the exact environment your plants thrive in. This is well utilised in the nurseries sector as well as glasshouse market gardeners who can operate a controlled environment. For residential users, potting mix is useful in garden beds, potted plants and other plants with specific requirements.
Information written in collaboration with John Turner of Hill Laboratories.