Written by Sarah Jones
Made from kitchen scraps, compost is well known to gardeners as a rich growth medium to improve soil quality. But the fascinating biological and chemical processes involved in its production get far less attention.
So, to celebrate World Soil Day, we'll take you through how compost is made and the numerous benefits associated with it.
Why is compost such a valuable resource?
As plants grow, necessary nutrients can be stripped from the soil. By incorporating compost into your soil, you can replenish it with nutrients that will be released slowly over several months. On top of that, by directing your food waste towards your compost heap, you’re helping to reduce the amount of food waste that ends up in landfill every year.
We throw away 7.4 million tonnes of food waste every year in the UK
But how does food waste become mature compost? A wide variety of organisms play a part in the complex chain of processes that lead to the formation of this gardeners’ must-have.
There is such a thing as a free-meal
What you throw on the compost heap – peelings, clippings and yesterday’s salad - may look like bin-worthy waste to you, but to the creepy crawlies in your back garden, it’s a veritable feast. Amongst the first diners at the buffet are macrofauna (organisms bigger than 2mm), such as earthworms and ants; and mesofauna (0.1-2mm in size). Mesofauna includes nematodes and springtails – invertebrates you can just about see with the naked eye, but you might have to squint!
These early consumers help to get the degradation process moving by breaking down bulk biowaste into smaller particles. Read more about them here:
Although macro- and mesofauna can help turn bulky waste into bite-sized pieces, the biggest players in compost production are those that are invisible to the naked eye: the microbes. Each gram of compost contains as many as one billion microorganisms, including bacteria and microscopic fungi.
Microbes and fungi are crucial in breaking down organic matter
There are three phases of microbial compost production: the mesophilic, thermophilic and maturation stages.
During the first stage, mesophilic -mid-temperature loving - microbes break down the most easy-to-degrade organic compounds in the waste heap. The mesophiles include a wide range of microbes that thrive at temperatures in the 20 to 40ºC range. A side effect of the mesophiles’ work is heat generation: temperatures rapidly increase, leading to the initiation of the second stage within a matter of days.
Once temperatures in the compost rise above 40ºC, thermophilic microbes begin to act. This is the active composting phase, which can last anywhere from several days to several months. The thermophiles and their heat decompose the tougher, woodier elements of waste, such as cellulose. During this phase, temperatures can reach up to 65ºC. However, if temperatures rise too high, decomposition rates will slow down as other organisms die. Turning of the compost at this stage can help to maintain both an effective temperature and appropriate levels of aeration.
Composting can get hot!
Aged like a fine wine
What does compost have in common with whisky or wine? Much like a high-quality tipple, it needs to be 'aged' to reach perfection. The final stage in composting is the maturation or 'curing' stage. If not adequately matured, the quality and stability of the compost will be significantly impaired.
Luckily for the home gardener, the maturation stage of compost production doesn’t take quite as long as a malt whisky! This stage typically lasts between six to nine months, during which, we see a drop off in temperatures and a return of the mesophilic microbes, which help to produce biologically stable substances.
Using compost before it is properly cured could potentially expose plants to toxic levels of organic acids.
A breath of fresh air: getting the conditions right
Microbes can be aerobic (need oxygen) or anaerobic (able to function without oxygen). Anaerobic composting is possible, but the process isn’t efficient and releases gases like hydrogen sulfide and methane. Cultivating the right conditions for aerobic microbes to dominate is key to at-home composting success!
One of the most important ways of ensuring your compost remains an aerated environment is periodic turning. Turning is the process of mixing compost and should be carried out thoroughly to ensure aerobic microbes can keep working throughout the pile. How often you should turn is dependant on the exact conditions in your heap, but roughly once a week should be suitable for most set-ups.
Another way you can help aerobes to flourish in the compost heap is by getting the mix of wastes right. The types of waste material you use will impact the porosity, particle size and moisture content of the heap – all of which affect how aerated the heap is.
For the best outcome, include a mixture of green (plant-based kitchen waste, grass clippings) and brown (shredded paper, woodier plant stems, hedge clippings) wastes. Food waste that contains meat and other animal products should stay off your compost heap – unless of course, you’re looking to attract pests!
Like many aspects of gardening, with composting you get out what you put in: the only nutrients present in the final compost are the ones that were there at the start, albeit transformed by some microbial magic.