Lesson 1 - Compost and organic matter
Homes and gardens tend to produce quite an excess of organic matter, from kitchen waste and coffee grinds, through to lawn clippings and tree leaves. As we have learnt already, when organic matter decomposes it provides nutrition for plants and improves soil structure, so using it in the garden makes a lot of sense. But organic materials decompose at different rates. The larger and woodier the particles are, the slower they are to break down. Greener, herbaceous materials will break down more rapidly. Kitchen scraps, and some garden waste such as leaves and weeds, can also be used as a supplement to chickens. Aside from the recycling of the organic materials and a supply of eggs, you also benefit from access to poultry manure – an excellent source of nitrogen, phosphorous and potassium.
However, the best use of organic waste from the home and garden is to accelerate the natural decomposition process through composting. Composting utilizes billions of microbes that work away to break down the different parts of plants and animals that make up organic matter. As this material is broken down it is transformed into mineral compounds and other byproducts, such as humus, providing nutrients to plants and improving soil structure.
Microbes in composts work by digesting organic matter and transforming this to mineral nutrients. To do this they need the right balance of carbon, as the main source of energy, and nitrogen, to synthesise and build proteins. This is known as the C:N ratio and refers to the types of materials you need to make your compost, regardless of the method used.
The aim for most composts is to ensure you have a C:N ratio of around 25-30:1, that is about 25 or 30 parts of carbon for every part of nitrogen. This is the range where microbes will work most effectively at decomposing. A C:N ratio that's too high (excess carbon) will be slow to decompose and mature, while a ratio that's too low (excess nitrogen) can also be slow to decompose, but it will also be slimy and have a distinct ammonia-smell (losing valuable nitrogen from the compost). A well-balanced C:N ratio at the time of making will ensure that the finished compost will be a good source of nutrients and humus. High carbon materials or ‘browns’ as they are referred to are those that are dry, woody and higher in lignin, such as dry leaves, wood chips, sawdust, shredded paper and straw. While high nitrogen materials or ‘greens’ tend to be green plant tissues and moist waste such as kitchen scraps, lawn clippings, coffee grounds and manures.
One way to accelerate decomposition is to reduce the size of some dry organic components. A small mulcher can chip up woody branches, stems and leaves into smaller pieces, producing chips of between 2 cm to 5 cm in size. The resultant materials can be used as a mulch or better added to a compost as a ‘brown’.
During decomposition, the microbes working away generate heat, which is why composts are hot, especially in the early stages. The temperature inside a compost early in the process can be as much as 70°C, although the ideal temperature is between 32°C and 60°C. The temperature will drop every time the compost is turned, and over time it continues to fall and stabilizes, an indicator that the compost has matured and is ready for use. As the compost develops and then matures the microbes present in the compost also change, particularly with changes in temperature. This also results in a change in the C:N ratio. On maturity a good quality compost will have a C:N ratio of around 10:1 (i.e. 10 parts of carbon to 1 part of nitrogen), due to the carbon gradually being used by the microbes and converted to CO2.
Composts need moist, but not wet, conditions to mature successfully. Dry composts are very slow, while wet composts can also be slow and have an unpleasant smell. When made, a compost should feel damp to the touch, but no water is able to be squeezed out. In dry conditions it can be useful to wet down materials when making a compost, while in prolonged wet conditions some coverage of the compost will be helpful to prevent saturation.
The other thing that composts need is oxygen. And that’s because the microbes that work quickest in a compost are those that are aerobic, requiring oxygen-rich environments. This can be achieved by ensuring you have a mix of particle sizes in the compost and that there is plenty of air movement. This is aided by open sides to the compost and regular turning of the compost. Some methods even use slotted pipes into the pile to increase aeration.
| Carbon (C) to Nitrogen (C) ratios | |
| Browns = high carbon (C) | |
| Cardboard, shredded | 350/1 |
| Corn stalks | 75/1 |
| Leaves | 60/1 |
| Newspaper, shredded | 175/1 |
| Peanut shells | 35/1 |
| Pine needles | 80/1 |
| Sawdust | 325/1 |
| Straw | 75/1 |
| Wood chips | 400/1 |
| Greens = high nitrogen (N) | |
| Ashes, wood | 25/1 |
| Lucerne | 12/1 |
| Coffee grounds | 20/1 |
| Food waste | 20/1 |
| Garden waste | 30/1 |
| Grass clippings | 20/1 |
| Manures | 15/1 |
There are many different types of composts. Many have ‘recipes’ and can be completed in a few weeks, while others are slow and will take several months for maturation. One method is known as trench composting which includes digging a small trench directly in the ground and then filling this with kitchen and green waste before covering with the trench soil.
The most common method of composting is to build a heap directly on the ground and bordered on two or three sides by timber or a wire mesh. As noted earlier, the best mix of organic materials will have a C:N ratio of about 30:1, gained by mixing one part, per volume, of ‘browns’ to one part, per volume, of ‘greens’. The parts used can be buckets or even wheelbarrows if you want to make something bigger. Look for many of the online compost recipes to check for different materials and the relative proportions of each. Make sure that the different materials are mixed thoroughly when making the compost pile or added in layers to be mixed on turning. checking for moisture as the compost is made.
There is generally no need to add anything further, other than perhaps some handfuls of soil, particularly clay, or some previously made compost. Small additions of poultry manure are a useful source of phosphorus, especially if you have a compost with a high proportion of leaves. If you have lots of kitchen scraps or manures, then adding some soil or a light sprinkling of lime and sawdust can be helpful. This increases the pH and provides additional carbon to balance the excess nitrogen present. Avoid using commercial accelerators or inoculants, they are of no benefit in home compost making.
Once the compost is made, you should leave the heap for a few days, during which time the pile will heat up considerably. It can then be turned by forking it over to an adjoining space or to another bin nearby, checking the moisture level as you do this. The heap should be turned every few days to a week. As you do this you will notice elevated temperatures inside the heap and over time a significant drop in the overall volume. These are both good signs that compost microbes are hard at work. Depending on the original mix of materials and the amount of turning or ‘exercise’ the compost receives, the compost will be mature and ready for use in between 6 to 10 weeks.
Many home gardeners use bin systems for composting as they are useful where small, regular amounts of organic material are available. The most common are around 200 litres in volume, made from high-density polyethylene (HDPE) or polypropylene (PP), with a lid at the top but connecting directly to the ground. Similar products are rectangular in shape and sold as kit constructions often with removable timber planks as sides. Bin composting works best when you have two or even three bins, reflecting the different stages of decomposing and compost making. Tumbler composters are a variation where the bin is a cylinder mounted on a steel frame, enabling the bin to easily rotate. This frequency of turning of the compost increases aeration, accelerating the composting process considerably.
Anaerobic composting decomposed organic matter in the absence of oxygen. Mostly used in commercial composting, perhaps the most common home process is the Bokashi system.. This fermentation method uses small, sealed containers (15-20L) and a special bran additive, to rapidly break down kitchen scraps. Care is needed during making, especially to drain liquid from the bin and regular mixing to aerate the fermenting compost. The end product, made after two to four weeks, benefits from further decomposition outdoors, mixed into soil or added to a compost bin. But, if you don’t produce a lot of waste at home and don’t have access to a large garden, then Bokashi could be for you.
Vermicomposting uses earthworms to break down organic waste and turn it into a compost. There are a range of smaller ‘worm farm’ products available that use kitchen and other organic wastes, such as shredded paper and leaves. This end product from worm digestion, known as worm castings, is a particularly rich and high nutrient compost. It’s important to use the right worm species for composting, they are not the same as garden worms. Composting worms are available when you purchase a worm farm kit.
It can be difficult to identify a quality product when buying compost, but some bagged composts are produced to the Australian Standard for Composts, Soil Conditioners and Mulches (AS 4454-2012). The standard ensures that the product meets minimum requirements for its physical and chemical properties, the label on the bag will identify these, including pH and organic carbon content. The other benefit of buying an Australian Standard product is that it’s safe to use in terms of free from weed seeds, pathogens, pesticides and heavy metals. Always look for the AS logo when purchasing bagged compost.
Mushroom compost is a product that many home gardeners like to use. Produced as a byproduct of the mushroom industry, the ‘compost’ is the spent material used to grow mushrooms and is made from a mix of straw, manures (especially horse and poultry), gypsum, wood shavings, gypsum and other organic wastes. It’s a material that can be quite rich, partly because it is still ‘composting when it is delivered. And because of this there are two risks associated with its use – elevated pH (alkalinity) and high levels of soluble salts, including sodium and chloride. While the salinity and pH will fall over time, typically within weeks of delivery, some care is needed in how mushroom compost is used. It is best to avoid it’s use in alkaline soils and never use it fresh around acid-loving plants as this can lead to lime-induced chlorosis in these plants. Because of these issues, its best to use mushroom compost as a soil conditioner, top-dressed into existing soil or turned and mixed in to beds well before any planting. Watering it in well after incorporation helps age the compost further, improving its qualities.
Well-made compost is a source of plant nutrients and can improve soil structure by increasing aeration, moisture and nutrient retention. It can also boost beneficial microbe activity in the soil, particularly if it is used regularly. Compost will be ready when it no longer feels warm, has a pleasant, earthy smell, is dark in colour, moist and crumbly to touch, and is soil-like in texture. This can take anywhere from 8-12 weeks to 8-12 months, depending on the materials used and the method followed.
The best way to use compost is to mix it directly into a garden bed prior to planting, adding up to 10 cm and then incorporating this well into the existing soil. You can also add a layer of compost directly around growing plants, particularly annual vegetables and flowers, with a 5 cm layer being adequate. Some less-mature composts can even be used as a nutrient-rich mulch around woody plants, such as fruit trees, with a 10 cm layer being sufficient. Composts can also be added to home-made potting mixes or sieved and added as a lawn topdressing. Some even make ‘compost tea’, by soaking compost in water for several days, straining the solution and using it as a weak liquid fertiliser.
While even poorly made composts can have some benefits for garden soils, there are some problems you need to look for. Be careful adding any diseased plant materials to your compost. This material needs to be properly composted (i.e. ≥55°C for 3 days) to make sure any pathogens present are killed. During the process of decomposition, toxins are produced that can be injurious to plants. While these phytotoxins are transformed during composting, they can still be present in freshly dead plant materials and in some poorly made composts. Always wait some time before planting into soils where freshly dug plant materials have been added to. And when inspecting your compost, make sure that the original organic materials used are no longer readily identifiable. The other common problem in some composts is nitrogen deficiency, caused by excessive amounts of high carbon materials, such as sawdust or paper. Make sure you have a balanced C:N ratio to start with to avoid this.
Further learning
Further learning
- The Compost Coach by Kate Flood (Murdoch Books, 2023)
- Gardening Australia: Compost 101