Allotment Owners & Hot Composting – Worth it or not?
Many allotment owners reading our blog will already be keen composters. A good few of you will have your own compost recipes and secret additions all aimed at getting top quality homemade ‘black gold’ to help grow your fruit and vegetables.
We hope to answer this question; ‘Is it worth having a go at hot composting on my allotment?’
Now bare with us as it’s a long blog but we hope worth the read for anyone who wants to understand more how a HOTBIN might work on their allotment or just understand a bit more why the HOTBIN is a great way to successfully achieve hot composting at home.
HOT COMPOSTING – WORKING AT 40-60C
We define hot composting as working at 40-60C and we are really looking for the waste to be at this temperature for 3-10 days at a time.
Many of you will occasionally notice your compost is warmer to touch than the soil and occasionally you may see steam (well water condensation) coming of the compost heap. We always suggest a degree of care when using these as indicators as it is quite hard to judge temperature unless you use a temperature gauge.
The following table will help:
||No decomposition. Typically your heap will be inactive from October to March – it is little more than a storage bin.
||The average temperature over a year in the UK is 10C. Do not get mislead by a steam coming off as an indication the heap is hot – you can see steam on a cold autumn morning when the temperature of the heap is 20-30C
||Temperature of the average shower
||Temperature of the hot water in your hot radiator. It is very hard to get compost heaps on an allotment to these temperatures (see below).
Take care to monitor how long your heap stays hot – you may miss that the heat you saw one day has dropped and gone the next. You also need to look at the temperature in the center of the heap as well as the edges to determine how much of the heap is hot.
Why should allotment owners consider hot composting?
- Kills weed seeds
- Kills plants diseases & pathogens
- Kills deep rooting and spouting stems such as couch grass and dandelion
- Produces compost faster
- Can convert your autumn clear up into compost for spring
- Frees up space for more growing (reduces 4 compost bins to 1)
- Potential for more compost – you can add all food waste to a well designed hot compost bin
- No turning
We have a more comprehensive illustration at the FAQ on hot composting benefits
What do you need to create a Hot Compost Heap?
The basic laws of nature governing the amount of heat produced by bacteria from waste and the heat loss from waste via conduction and convection are fixed and immovable. These laws mean that most compost bins do not retain enough heat to rise above ambient (air temperature) never mind get into the hot composting 40-60C range.
There are two main methods of achieving hot composting; building large compost piles and turning them often (minimum 1m3, more typically 2m3), or using specialist composting bins that are insulated and regulate air flow.
Big heaps – big effort
The total amount of heat per Kg of waste is fixed. If you lose heat fast (e.g. in an open windy swept heap) you need more new waste each day to keep it warm. A big heap has the potential to stay warm as the outer layer of compost acts as insulation to the waste right in the middle. The inner core heats up and turning the waste brings new waste to the centre. Lots of waste is needed to get a heap hot – typically 0.5m3 (or a half full 1X1X1m pallet frame box) and typically another 50Kg of waste a week plus turning once a week is needed to keep it hot. A group of allotment owners may well have the amount of waste but who’s got the muscle to turn this often? The big guys use tractors, on the allotment it’s a fork and your back.
This large scale hot composting system was defined by Sir Howard back in 1930s and it forms the backbone of the ‘New Zealand 3 bin rotation method and most ‘organic’ compost systems.
The issue is few gardeners have the volume of waste or the time and energy to turn such large heaps so they end up with a cold compost heap that takes 18-24 months to generate compost.
Commercial Hot COMPOSTING BINS
A cup of hot tea always cools to room temperature. Leave it outside in the snow and it will eventually freeze. A compost heap follows the same law – all heat produced by the bacteria moves from the hot centre out to the cooler outside air. You cannot stop this; but you can slow down the rate of loss by insulating. A Compost bin made from a thick waterproof insulated material will slow down the conductive heat loss from a compost heap.
Heat is also lost by convection. Air moves within the compost (at last we hope it does otherwise it will go anaerobic and smelly very quickly!). Moving air transfers heat very fast. Winds can cool things 100 times faster than still air. Go out in a cold wind with no clothes on and you will suffer hypothermia very quickly due to ‘wind chill’. Your compost heap is the same; it loses heat as cool air flows over it and carries warm air out.
We have discussed conductive and convective heat loss and the need to minimise it to have a reasonable chance of achieving hot composting by using insulated walls, sealed lids and airflow valves.
There is one other important factor – you cannot get more heat energy out than you put in. There is a bit of chemical engineering science that looks at amount of energy available from bacteria composting each Kg of food waste, how much is lost as heat due to convection and conduction and how much needs to be retained to keep the compost above ambient (i.e. at 60C rather than air temperature at 10C. Most households do not have enough waste (i.e. energy going in) to keep the compost hot due to the rate heat is lost (i.e. heat goes out).
The average house produces 5Kg of food waste a week and maybe 10Kg -20Kg when including summer green garden waste, and allotments users will be at the higher end of the scale. The HOTBIN will stay hot with 5-10Kgs of waste a week.
The maths are straight forward – most other compost bins lose heat so fast they need 50-100Kgs per week going in to keep warm.
So is it worth spending hard cash on a HOTBIN for your allotment? Only you can determine this. But why not find out what allotment users say about their HOTBIN.
Benefits of hot composting over cold composting
|Hot composting kills weed seeds which means all your hard weeding does not go to waste
||Many seeds will survive in ‘cold’ composting heaps. All your hard work weeding can be wasted as you replant viable seeds in nature’s best growing medium – humus/compost.(Ever had a bunch of melon, tomato, or weeds grow from your newly composted plot!
|Hot composting kills plant diseases.Compost at 60C for 1 hour equals good sanitisation.Compost bacteria and fungi are not the same bacteria and fungi that attack living plants.
||You need to leave cold compost a long time (12-18 months) to achieve the same level of sanitisation and bacteria die off.
|Herbicides and pesticides are broken down by composting bacteria. Bacteria work 32 times faster at 60C than at 10C – so the final product is far less likely to have residual pesticides in.
||The risk of pesticide infection via use of domestic compost is low (because the concentration and volumes used are low). However the extra security of fast destruction in hot composting is an extra level of security
|The right hot composting system will work all year round. You can undertake a major autumn clean up and have compost in spring.
||Most allotment compost heaps ‘stop’ working as soon as the temperature falls below 5C. Anything collected in autumn often just sits there until spring when the sun warms up the waste and the bacteria start breaking down.
|You can add all food waste. Not all allotment users will be able to transport food waste to the allotment – but given that the average home generates 250Kg of food waste, that if you do use a proper hot compost container that will not attract rats and vermin. You can make an awful lot more compost (50Kg is about 100L which is two large sacks extra).
||70% all household food waste is not added to ‘cold’ compost bins as it is likely to cause issues with odour, rats and flies.Cold compost bins tend to be constructed with open vents and hatches – so any odour not only attracts it also allows entry to food – creating the infamous ‘swarm’ of flies when lid taken off, or even worse finding the nest of rats when the heap is broken open.
|Speed is not important to everyone – gardeners on the whole are a patient group.However if you are short of space speed means less bins. It is common to see 4 or 5 dalex bins per plot.You want to grow vegetables not add compost bins – then composting down quickly is a benefit.
||A UK ‘outdoor’ heap average the same temperature as the UK ambient air temp – i.e. 10C. At 10C the heap it is 32 times slower than at 60C. If a soft waste like grass takes 6 months in a cold hep it will take about 6 days to reach same state in hot heap. If the branches and bit take 18 months then at 60C they would take 18days. It is never quite this simple as there as mixes and ranges of temperature – but work on 30 days for mulch and 90days for good humus. .
|A hot bin will kill fly eggs & larvae – so no swarms of flies or maggots in a hot compost bin.It’s too hot in the top for ants, rats, and most things you don’t like to see in a heap.
||Flies will lay eggs and larvae (e.g. maggots) are present, and often swarms of flies can come out when lid open.Often hardy allotment owners do not worry about flies swarming from bins – but for some this is an issue.
|The quality and amount of compost you use on your allotment is everything.You need to add rich humus. It is very difficult to assess if the composting you are adding is any good. Many of us have been taught to look for friable light brown compost. This is a sign of partially broken down organic matter. What you want is to add is humeric substances and it appears not all compost has same amount of humeric substances in!
||Producing humus does not appear to be anything to do with a hot or cold composting. But many notable organic authorities past and present have noted that hot composting produces top notch, sticky colloidal humeric substances. Try it and see – great rich compost.
|A correctly managed HOTBIN will require no laborious turning – but it will require the waste to be chopped up – which can also be time consuming if there is no shredder
||A large heap can be created that does not need turning to created aeration, but most of the outer waste will stay cold and not compost. Turning brings the outer cold into the inner centre
For advice and tips on allotments, try these sites:
NASLG: The National Society of Allotment & Leisure Gardeners
Allotment.org: allotment diaries, photographs, advice about growing vegetables, fruits and herbs with a forum for chatting on the plot.