Compostation, humification, or assisted aerobic digestion is the decomposition of green manure (sometimes with feces) into humus using aerobic bacteria. Humification using ie bins, ... is a way to speed up the aerobic digestion process by creating a suitable environment for bacteria and other microorganisms. The resulting humus looks, feels and smells like fertile garden soil. Humas can make a soil rich, able to retain (more) water, and encourages a healthy ecosystem of soil microorganisms.
Purpose of composting
The purpose of composting is two-fold:
- It reduces the amount of "waste" (plant matter, feces) considerably. It is thus useful as a "waste management technique"
- It creates humus from the plant matter, which is a very useful compound in farming. It is useful for this as
- it improves soil structure, improves aeration, water-retention, erosion problems, and makes the soil easier to work with.
- it adds nutrients to the soil. The organic acids in compost make nutrients more available. Compost is mostly humus, which is negatively charged so nutrients that are positively charged such as Calcium, Potassium, Magnesium, Sodium and most trace minerals attach to it. Because plant roots are surrounded by hydrogen ions that are also positively charged, the plants "bargain" with the humus exchanging some of their positive hydrogen ions for positive nutrients.
Compost piles need:
- Air & moisture: composting only works with a minimum of water, but bear in mind that it is still an aerobic process: when there is too much water, too little air is present for the bacteria to develop and the plant waste is little converted. As a general rule of thumb the pile should be as moist as a squeezed out sponge.
- Food: organic matter containing carbon and nitrogen
- Warmth: decomposition will continually slow down as the temperature gets below 70 degrees Fahrenheit and will stop altogether if it freezes.
Composting is done trough the digestive action of several living organisms:
- Small animals, e.g. worms - these break down organic matter and soil further and make plant nutrients available.
- Very small organisms, "microbes" or microorganisms.
In the event that manure is used in the compost pile (not essential; only carbon, nitrogen and water is essential), we could consider large animals (e.g. cows) part of the process, forming the first chain.
Compost piles are made according to a specific ratio of carbon materials to nitrogen. This ratio should generally be 30:1, C:N. Smaller ratio's are also possible, but not larger ones than 30:1 or 35:1. The carbon/nitrogen ratios of different materials are explained in In-depth explanation of bin composting.
Although it is true that to make a suitable compost pile, we need to alternate "green" with "brown" layers, these colored layers however do not differ in the carbon or nitrogen ratio. Instead, green layers are just needed as they contain more water, which is needed for the composting to occur at all (it's thus not essential as we can simply add additional water aswell). The "brown layers" refer to layers high in carbon content (ie dried leaves, dried stems, dried roots, ...); -they don't refer to feces however-. We best remember of each element what it contains:
- manure aswell as urine contains very high amounts of nitrogen
- green manure (freshly or recently cut plant matter) contains water and carbon
Note that the C/N ratio can be varied a bit depending on the type of composting you want. If you want a hot, rapidly composting pile, use a relatively higher amount of nitrogen (ie 25:1), if you want a cooler, more slowly composting pile, use more carbon (ie 35:1).
Composting always heats up the soil. Hot composting however means that you make the pile in such a way that it attains higher temperature and thus kill bacteria and weed seeds (which is useful if you want to break down a lot of weeds). Hot compost is more difficult to make but will decompose much quicker. Hot composting releases a lot of carbon which means the end result is higher in nitrogen compared to a cold compost. To make a hot compost have the Nitrogen to Carbon ratio slightly higher, insulate of a pile if you live in a cold area, and monitor the temperature stirring the pile when it starts to drop. To kill weed seeds and pathogens a pile should get to about 150 degrees Fahrenheit for 10-15 minutes. This is not hard to accomplish especially in the center of a pile where you will want to place any possibly harmful materials, such as weed seeds or meat. The optimum temperature for decomposition is between 105 degrees and 130 degrees. Keeping a compost at a temperature higher than 130 degrees Fahrenheit, will kill off microbes that are necessary for decomposition. If a pile gets too hot, around 170 degrees, there is danger of it spontaneously combusting.
Sometime called "lazy person's compost". Tends to be slower to decompose, but is used for convenience or when materials are scarce. These piles have a higher carbon to nitrogen ratio. They are slower and the end result is higher in carbon.
Types of compost
Pile or bin composting
Some people prefer compost bins while others choose to make a pile. Both are however fairly identical, as even bin composting is done directly unto the soil (so without a non-permeable layer in between). The compost always needs to have direct contact to the soil as worms, ... need to be able to reach the pile.
With both methods, we need to layer the pile by alternating the previously mentioned "brown" and "green" layers. The ideal way to layer materials is by first loosening the ground on which the pile will be located to allow for drainage. The first layer should be a stalky material to allow for air flow and drainage, sunflower stalks, corn stalks, small branches or twigs work well for this. Then add a significant amount of dry vegetation or carbon such as leaves or straw. After that comes the nitrogen layer, which is green waste or kitchen scraps topped with a thin sheet of soil. The process is then repeated leaving out the stalky layer. If there is enough material, it is best to make the whole pile at once. For really cold climates a pile should be 4x4x4 feet to insulate it, but in a mild climate 3x3x3 feet is sufficient. A pile will shrink to about a fourth of the size and usually takes 3-6 months to fully break down. You will know that a pile is done when it is rich in color, falls apart easily in your hand, and you can not discern any of the original contents.
When you wish to use a bin system, the 3-bin system is often the easiest. In this system, one bin is used for the layering, the others are used when the bin needs to be rotated (turned over). Two bins are used for this to allow storing the humus in once bin, and having one left for when the first bin needs to be rotated. See 3-bin compost system
Specialised bins also exist, ie the geobin system -- a sheet of polymer material rolled into a cylinder and stood upright: Cost is in the $20-$40 range commercially.
This takes place where animals are kept. Making compost from the bedding, manure and urine.
To sheet compost you lay uncomposted organic matter on a garden bed and turn it under. Doing this ties up the land and nitrogen for about 3 months. It may be good to do in the winter, if you are not planting a cover crop, so that nitrogen in the soil does not leach away.
Another approach is to lay the organic material on top of the soil and place heavy porous material such as old carpet (organic, or course) over it. The carpet ensures that water is retained and that weeds or other plants do not have the opportunity to grow while the organic material is composting. This method allows use of even the toughest material such as freshly cut bamboo stalks to be used as composting material.
The added bonus of this method is the cheapness of the main material; old carpets are easily collected for free during any hard waste dump. The use of carpet also allows extra organic material to be added with minimal effort, just pull back the carpet and replace it after the organic material is added. Regular watering of the compost will assist the breakdown of the matter.
Once the matter has broken down, there is no need to remove the carpet; simply cut holes in it and plant the plants of choice directly into the mulch. The carpet will continue to provide protection against weeds and will aid water retention. If the carpet is organic, such as wool on a hessian backing, it will eventually break down and add to the soil, so it does not even need to be removed.
Troubleshooting a pile
- Make sure you increase the amount of air regularly by turning it over.
- Increase surface area of materials by chopping them up.
- If your compost is smelly it is not getting enough oxygen (going anaerobic).
- If space is an issue (ie if you are living in a house or apartment with not much of a yard), you can use vermiculture (i.e. using a worm bin).
- Animals (ie bears, raccoons, rats, and other animals) may become a problem if there is something in a compost pile they would like to eat. Access to compost must be restricted (ie by using a high, fully closed bin); avoiding the scraps that attract them too can help, especially meat, fish, ... scraps. As you still need to discard these somehow, you can opt to compost them anaerobically (using a biogas digester).