Introduction

Composting is a process by which biological material is converted into a soil amendment. It takes advantage of natural decomposition processes to turn certain types of biological waste into a fertile growing medium for plants. Good quality compost contains all of the nutrients a plant needs to grow healthily, as well as a balance of beneficial microorganisms which help suppress the growth of harmful soil microbes. In short, compost is a biological alternative to synthetic plant feed and inputs. It is not, however, a magic bullet. There are limits to the uses for compost, and associated challenges.

There are several different ways of making compost. The time to complete a compost heap can be anywhere from one month to twelve, even multiple years, depending on the technique used and prevailing environmental conditions. Below are the step by step instructions for the thermophilic or ‘hot’ compost making technique. It is called ‘hot’ because during the composting process, aerobic (oxygen-breathing) bacteria release heat as they break down the biological material in the heap. The ideal temperature is around 46^o centigrade (continuously for a week), which feels about the same as a hot bath.  Larger piles are able to trap more of the heat and achieve higher temperatures (up to 65^o centigrade) albeit for shorter time periods. The high temperatures are crucial for killing any pathogens which may be in the composting materials.

Input Materials

There is a lot of flexibility in the requirements for composting. At the simplest, all that is necessary is a patch of bare ground, and no tools beyond able hands. The Equipment chapter details several types of compost bins which can be made simply and easily.

In addition, the placement of the bin itself is of little consequence. The only factor to ensure is that the bin is not exposed to excessive rain, because too much water stops the composting process dead in its tracks. For a sufficiently large pile, the ambient air temperature also doesn’t really matter. The heat produced by the process is enough to keep it going in winter, as long as the pile is correctly aerated and watered.

As a rule of thumb, anything that was once alive can technically be composted, because essentially all biological matter decomposes. However, this decomposition does not occur for all materials at the same rate. Therefore, not all biological wastes are equally suitable for rapid and efficient composting. The table below details the suitability of some common household wastes for compost-making.

Suitable and Unsuitable Materials for Rapid 'Hot' Composting

In addition to the types suitable materials, it is also important to pay attention to the balance of the different material types in the compost heap. This is because different biological materials contain different amounts of carbon and nitrogen. Substances made from these two elements (together with several others) are the key to soil fertility.

Composting materials are commonly divided into two categories: ‘greens’ (nitrogen-rich) and ‘browns’ (carbon-rich). Please note the names do not necessarily refer to the actual visible colour of the material. Some nitrogen rich materials are not green, and some carbon-rich materials are not actually brown in colour. However, the system of ‘greens’ and ‘browns’ is a useful and quick way to make decisions about what materials to mix. The table below lists typical compostable materials that fall into each category.

Carbon/Nitrogen Balance of Some Typical Compost Inputs

Generally, the most commonly reported ratio is between 25 and 41 parts carbon-rich browns for every 1 part of nitrogen-rich greens. That is a C:N ratio of between 25:1 and 41:1. Keep in mind that the numbers are primarily a guide. The range in acceptable C:N ratios is wide because the process is quite flexible. The main point to remember is that the pile is made up of mostly browns and plenty of greens. The ratio can change over time as you add more material, so do not bother too much about getting it ‘perfect’.

Compost in an advanced but incomplete state of decomposition. Note the eggshells and fruit rinds still visible.

Setup

Shred up the materials as fine as you can, which will help speed up the process of decomposition. Add them all into the compost bin and mix thoroughly, ensuring a uniform composition. You can also add a few handfuls of healthy soil (or previously prepared compost), which will ‘inject’ a population of aerobic bacteria to begin the process. If you don’t have any healthy soil or compost at hand, it’s not a problem. Unless all the materials you are adding were sterilised beforehand, they will all have some small amount of the right bacteria living on them- it may just take slightly longer for the population of them in your pile to get large.

The next consideration is moisture. The aerobic bacteria that digest the material and produce the heat require moist conditions to stay healthy. They cannot survive underwater, however so it is important to ensure the water level in the compost pile is enough, but no more than that. The correct amount of water is approximately the same as in a completely damp (but not soggy) newspaper. If you squeeze a completely damp newspaper you only get a few drops of water out. The overall moisture level of the pile, one all the material has been shredded and mixed together very well, should be about as much as a pile of damp newspapers. As a general rule of thumb, it is better for the pile to be a bit too dry than the opposite, because it is very easy to correct an overly dry pile.

If you find the pile is too wet (see Challenges and Solutions), you can mix in some dry browns such as sawdust or dry greens such as dried coffee grounds or tea bags to soak up the excess moisture.

With the bin filled with a uniform mixture of moist greens and browns in the right ratio, the pile is ready to go. If there is a risk of pests and animals getting to it, cover the bin to keep them from getting in. The covering must not be airtight, however. A good supply of air is absolutely critical to the process. The lid should have many holes in it or a gap to allow in good airflow.

After a few days, the population of bacteria will be large enough for their activity to cause the temperature of the pile to increase. You should be able to feel this warmth with your bare hands. When the pile has entered this thermophilic (heat producing) phase, your only role will be to ‘turn’ the pile every one or two days. You can increase the time between turnings even up to a week, but it will greatly slow down the composting process. Much longer than that, and the pile will never properly enter the thermophilic phase, and will instead follow a ‘cold’ route which is essentially the same as unassisted decomposition or rotting.

Curing and Application

After the thermophilic phase, the compost must be allowed to cure. Curing is a vital step and cannot be skipped. Uncured compost is unfinished and could damage or even kill plants growing in it. This is because uncured compost still contains certain biochemical components produced during the process. They can cause nitrogen or oxygen starvation by competing with plant roots for air. However, once cured, compost is completely safe for plants to grow in.

Curing is generally considered complete when the compost pile has broken down to a light, soil-like material with no recognizable components of the original waste material. The pile should also not produce any warmth at all when full cured. Curing generally requires the same actions as the thermophilic phase, but consistent aeration is not as important.

Once fully cured, compost can be used whole as a growing medium for container-grown vegetables, or mixed with other materials to achieve desired properties. Mixing with vermicompost (worm manure) produces a highly nutrient dense growing medium. Mixing with a loose aggregate such as river sand produces a highly porous medium that is resistant to compaction and waterlogging. The choice depends on the needs of the plant to be grown in the container. However, adjusting the properties this way is not a requirement—it is more of an optimisation measure to maximise yield. Cured thermophilic compost is a suitable growing medium all on its own for essentially any food crop.

Cured compost can also be used to make a nutrient dense liquid fertiliser called ‘compost tea’. It is called ‘tea’ because the process for making it resembles tea-brewing. To make compost tea, simply mix 10 parts of cold, clean water for every part of compost (eg 10 litres per kg of compost), and allow to soak for several days, stirring whenever convenient. Never use hot water to make compost tea. The high temperature will kill beneficial microbes in the compost. Once prepared, compost tea can be sprayed on plants to combine watering and fertilisation.

Side by side comparison of complete compost (left) and fresh compost material. Note the unifomrly dark, soil-like colour of the finished compost.

Photo: Joi (CC-BY 2.0)

Challenges and Solutions

Although compost-making is a fairly straightforward project, and relies primarily on natural biological processes rather than human intervention, problems can still occur—especially for beginners. Below is a list of common problems encountered by composters, and ways to prevent or solve them.

The resources section lists several supplementary resources which go into significantly more detail on the theory and practice of compost-making. This chapter should be sufficient instruction for small-scale home composting, but anyone wishing to enter into commercial composting as a business will do well to familiarise themselves with the process in depth.