The Long and Short of it

Q. What is composting?
Composting is the decomposition of essentially carbohydrate and nitrogenous organic matter by means of biological activity to make it as acceptable to the soil as possible from the point of view of growing plants successfully. Precisely defined, composting means ‘enhancing the consumption of crude organic matter by a complex ecology of biological decomposition organisms’.

Q. What are the key factors that control the rapid and safe aerobic composting of typical garden compost heap constituents?
The major factor is the raw constituent mix, primarily its C:N (Carbon to Nitrogen ratio). The bacterial, fungal and enzyme content; texture, pH, moisture and oxygen content during the process are important, as are the imposed environmental factors e.g. climate, weather and the size, shape and compactness of the heap and degree of insulation and containerisation.

Q. What are the key indicators for monitoring the composting process?
Monitoring the temperature at regular intervals is one of the best, although simply observing when the heap ‘slumps’, the dampness and condition of the constituents and any smells e.g. excess ammonia or worse, that ‘orrible’ rotten egg smell. Oxygen, carbon dioxide and water content might also be monitored in large scale commercial composting.

Q. What is ‘good’ finished compost?
It’s a balanced humus like mixture that acts as an active nitrate fertiliser. It should be capable of being added to the soil safely, knowing that seeds and plants will grow well immediately, without the soil being initially depleted whilst having to complete nitrogen fixation or cellulose breakdown. It will be ‘live’ and contain a panoply of beneficial organisms; maybe 40 million active bacteria per gram and over 10,000 different species. It should have a wide mineral spectrum and contain from 1 to 3% Nitrogen. A very good compost might have an NPK of 3:1:2 and a C:N of 10:1 The original constituents should be effectively unrecognisable and it should of course be weedless and seedless.

Q. What is an aerobic process.
One that requires oxygen. Aerobic composting is characterised by the generation of heat and the rapid multiplication of specific oxygen loving species of microorganisms, bacteria and fungi. It is natural, but usually requires intervention to ensure that air (oxygen) and water do not become exhausted within the heap. Conversely, an anaerobic process is one devoid of oxygen. (Totally anaerobic decomposition processes are used on an industrial scale in highly controlled conditions for rapidly breaking down almost any organic materials e.g. car tyres.)

Q. Why aerobic?
Typically, an ‘open’ mix of compostable materials with sufficient bulky fibrous matter (e.g. ‘long’ manure) will start off with a strong aerobic action. Oxygen loving organisms multiply rapidly if sufficient nitrogen is available, producing carbon dioxide, water and heat. At about 50C these go to sleep or die off, and bacteria called aerobic thermophiles take over – but even these have their limits (65 – 70C). Too high a temperature and a restricted oxygen supply can lead to rapid cooling as these organisms die, run for cover or mutate – leading to anaerobic conditions, or cessation if the constituents dry out. Degrees of both processes may co-exist in typical garden heaps but predominantly anaerobic conditions are cooler, slower, odiferous and are best avoided for horticultural composting. Rapid and safe composting is predominantly a high temperature aerobic process. This tends to kill most weeds and seeds, speeds decomposition, avoids dangerous by-products and sets the scene for nitrogen fixation in later stages.

Q. How do I ensure a predominantly aerobic composting process?
By choosing the constituents carefully (overall mix, texture and C:N ratio), maintaining the moisture level correctly (damp but not wet) and by turning the heap, mixing it up and keeping it open and aerated. ‘Good’ heaps shouldn’t smell or attract flies. Ammonia smells indicates nitrogen is being wasted, rotten eggs (hydrogen sulphide) that anaerobic processes are taking over (smelly, slimy and sludgy) – add dry spacing matter and turn it, breaking up any sticky masses!
N.B. Note that ‘turning’ implies moving the heap in its entirety, it being impossible to mix it up properly in situ; apart from the physical strains, the outer layers must be incorporated into the centre of the new heap. Never step or walk on the heap, or deliberately compress it, it’ll slump soon enough!

Q. What is the ideal starting C:N and final C:N of good compost?
25 to 35:1 by weight is an ideal starter mix. The aim is to get this down to about 10:1 in 3 to 4 months, with the original nitrogen content maintained and fixed. Single constituent heaps are not generally practicable, but good ‘long’ manure (including the wetted bedding) is fine and can be an excellent base constituent. Strong fertilisers like chicken droppings can be down to 8:1, seed meals 5:1. Between these extremes, one can match and mix, trying to get the starter to average about 25:1, giving a ‘finished’ compost close to the 10:1 required by most soils.

Q. So what else can I use?
The list is endless. Wood chips can be higher than 100:1 with little if any nitrogen, leaves from 15 to 100, highly dependent on species and bark content (avoid sawdust unless its been used as animal bedding). Kitchen waste is quite nitrogenous, maybe in the 15 to 20 range, so often goes anaerobic – add spacing cellulose matter (dried late grass cuttings, annual weeds etc) or mix into your main heap. If you’re a home espresso fan, those grouts are superb, high in nitrogen and minerals, teabags and eggshells too. Always try to incorporate a little earth (sieved and weedfree) – it holds the nitrogen and supplies bacteria. Top the pile with a thin layer of earth and dampen it in scorching weather, mixing it in at each turn. Use a pail at home for teapot dregs, cooking water, grouts etc and re-hydrate the heap with it. If you really do want to compost any dairy, meat or fish waste, ensure its well mixed into an active (hot) aerobic heap.

Q. What is the best temperature/time and turning profile?
A typical profile** for summer temperatures is that of the Indore process, where every facet was tightly controlled. A suggested schedule suitable for a few cubic yards of horse manure is outlined below ***.
Ideally, a typical horse manure heap should peak and stabilise within a few days at no more than 70 C, start slumping and be turned after 1 or 2 weeks, again after a month (about 50C), two months (about 40 C) and then left to mature for another 30 to 40 days whilst cooling to ambient temperature. After that, you are just watching the nitrate nitrogen dissipate.
Maturing (ripening/curing) is important as this is where further nitrogen is being fixed by the likes of azobacteria. The temperature drop normally averages out at about a degree C every 3 to 3.5 days. With difficult high C:N starchy constituents and a winter climate, this profile could be stretched out to a year, but should still follow a similar pattern.
Turning and mixing can stabilise a hot heap or re-heat a cool anaerobic heap, but do gauge the overall dampness carefully and try to adjust the C:N if it seriously wrong. Aerating and hydrating the whole mass thoroughly is always the best way to restore and maintain aerobic activity. It’s almost impossible to turn it too frequently, commercial operations might turn every other day for 2 weeks! N.B. Thoroughly mixing compost in small conical proprietary bins can injure you, don’t box yourself in – better still don’t use them!

Q. What is the best shape and size for a compost heap?
This depends on the constituents and environmental factors. Depth is important. There is a ‘critical mass’ for each combination but generally reckoned to be about 1 cu yd.; using ‘good’ manure (25:1), an oblong with a starting depth more than 2½ ft may overheat in summer. Later in the process or in winter it can profitably be deeper. It is important to keep the temperatures as even as possible out to the edges, which will be cooler and also subject to drying out or conversely, water-logging. A container can help prevent rapid drying out and cooling at the edges– breathing holes or slots may be necessary to prolong each phase of aerobic activity between turns. Although a conical heap is more likely to burn out its core in summer, it’s more likely to retain heat and shed cold rains in winter. Very large quantities of less strong materials (C:N greater than 30) are sometimes composted in longitudinal mounds, called windrows, but are turned and hydrated frequently.

Q. How can I ensure a good temperature rise without using strong animal manure (C:N too high)?
Thoroughly mix in comfrey, spring grass, nettles, vegetable kitchen waste etc. If dry, dampen with a comfrey or urea solution. Ideally, the moisture content should be like a wrung out sponge. If the volume is on the low side it may not ‘go critical’, heap it up a bit higher, make it more cubic. Ensure it contains enough spacing material, is loose and aerated and consider containerisation or coverings – a container with at least three insulating walls might help (usually having holes or slots for aeration), possibly with some form of cover. Whilst thick PVC retains all the moisture it might encourage fungal growth, a well fitted floral patterned Axminster might be just what you’re looking for! … Ensure heavy rains will run off any covering to avoid waterlogging the heap.

Q. Can I keep adding compostable materials to the heap?
Yes, with provisos! Whilst adding to the heap early on is usually OK, particularly to re-balance the mix, the assumption here is that a heap is assembled from raw materials in one go. A continuous (e.g. domestic kitchen waste) composting process rarely follows a reliable or satisfactory aerobic profile, but several approaches are taken, one being the successive ‘layering’ of balanced mixes of materials, it usually being necessary to incorporate dry vegetable matter and earth at regular intervals. Various ways of separating out or even sieving and recycling unfinished material are used, or simply starting a new heap and leaving the previous one 2 to 3 months to finish before using it. Alternately, constituents can be stored until sufficient are available for building a ‘critical mass’, some being dried and others retarded with earth or soaked in water (slurried). Another approach is to use a wormery (vermi-composting) – not considered here.
If possible, always aim at building as large a heap as possible in one go with a good mix.

Q. Are there any other tips and tricks?
Yes, thousands no doubt! But a good one is to add up to 10% weed free sieved soil in stages to help preserve the fixed nitrogen – covering the heap with a thin layer is a good idea, then again after each turning. Also, inoculate a new heap with up to 5% from an existing heap. Some add a trace of lime to keep the pH sensible (often a common practice in cattle stalls, to keep the floor and bedding sweet and sanitary, so maybe its there already) but it tends to precipitate ammonia. ‘Fractious’ woody materials, if used, are best chopped, mashed or distressed first – a great stress reliever – use your imagination!
If the heap is slowing down too early, ensure it is not too damp or too dry and re mix. If it seems to be reverting to an anaerobic heap frequently then force some holes into it or add some ‘longer’ cellulose matter whilst turning it.

Q. What is wrong with just leaving my manure heap to breakdown and compost in its own good time.
This is OK if you don’t mind waiting up to 18 months. If the heap has dried out prematurely or never been turned then a large amount of woody cellulose material may be present, particularly near the surface. When tilled into soil this will ‘tax’ it for a while. There will also be precious little fixed nitrogen, which again requires the soil to work hard at the same time, balancing this up to its native C:N of about 10. Also, the potential for perennial weed growth and undesirable spores increases the longer it is left, particularly as an unmanaged heap is more likely to have overheated locally. (Get manure supplies as fresh as possible, so you can control it’s composting.)

The quality of manure is determined by feed, bedding, health and working nature of animals – more work, more nitrogen content! It is too easy to throw away all this goodness to the wind, sun and rain or even encourage unfavourable stagnant decomposition. Among other things, a well managed compost process can actually increase the nitrogen content above that of the original constituents, and fix it too, whereas left for a year or more after first cooling off, there will be next to no nitrogen remaining.

Q. My heap is full of red worms, is it finished and ripened?
Maybe, but probably not! Brandling worms are not earthworms. Native (grey/brown) earthworms will not populate the heap until it is a very good match to soil, whilst brandlings tend to die pretty quickly in normal earth. They indicate that the heap has cooled but there is further work to be done (read about vermi-composting). Root rhizomes, azobacteria and other beneficial soil flora cannot multiply rapidly if immature compost is added to soil, unless ground temperatures are high. Seedlings particularly, may have a hard time of it.

‘Most importantly, humus is the last stage in the decomposition of organic matter. Once organic matter has become humus it resists further decomposition. Humus rots slowly. When humus does get broken down by soil microbes it stops being organic matter and changes back to simple inorganic substances. This ultimate destruction of organic matter is often called nitrification because one of the main substances released is nitrate – that vital fertilizer that makes plants grow green and fast.’ – Steve Solomon ±

Q. Any dangers?
If grey/white powdery spores or strange looking fungi occur, then things have gone wrong, probably overheating and rapid cooling. Don’t breathe in any dust, damp down first and get them into the core and re-activate the heap. Wash hands of course after dealing with any compost, whether manure based or not.

Q. And the short of it?
Well, if ‘the long of it’ is the wet animal bedding then the ‘short of it’ should be obvious. The two mixed together are ideal as the principal compost heap constituents.


* ‘Whatever its varied chemistry, all humus is brown or black, has a fine, crumbly texture, is very lightweight when dry, and smells like fresh earth. It is sponge-like, holding several times its weight in water. Like clay, humus attracts plant nutrients like a magnet so they aren’t so easily washed away by rain or irrigation. Then humus feeds nutrients back to plants. In the words of soil science, this functioning like a storage battery for minerals is called cation exchange capacity’ – Steve Solomons ±

** Typical temperature/time profile

*** Suggested method for rapid composting of horse manure (4 turns, 13 weeks):

  • Its best to turn, re-mix and re-hydrate as soon as 2 days to 1 week after delivery, using maybe 5 to 15 gallons of water or kitchen slops. At this time, incorporate some older compost from a previous ‘clean’ heap (if available) or a little soil and any other suitable waste one wants composted (into the centre of the heap). In summer particularly, make the heap wider and less conical; top with a thin layer of soil.
  • After 2 weeks or so, or when the temperature drops noticeably (a few days after the heap slumps), turn, re-mix and hydrate thoroughly, adding some more older compost and re-top with soil.
  • Repeat the last step after 4 weeks and 7 to 8 weeks. The necessary degree of re-hydration should drop somewhat at each turn, and the soil content slowly increase as the top covering gets mixed in each time. The heap shape can be made relatively higher to maximise activity, bearing in mind the volume will now be noticeably less.
  • The last turn at about 7 to 8 weeks should be onto a spot where the compost can be left to mature (which begins as the temperature drops back to ambient after the last turn). After 13 to 14 weeks the majority of the compost should be fully broken down and ready for use. The surface layer could be sieved to remove anything requiring recycling to a younger heap.
  • NB. If the weather is very hot, wet or cold, bear in mind the provisos and tips in the Q&A above, particularly temporary coverings for the heap. When turning, take time to get any dry, matted material re-hydrated and into the core, and break up any sludgy or obviously resistant masses.

± References/Links: All you really need is a web browsing computer and a link to Steve Solomon’s soil and health website, looking for his book entitled ‘Organic Gardener’s Composting’, which contains a compendium of constituent materials (C:N’s), a chapter on vermi-composting and one on making superior ‘Indore’ compost – If you’re interested in the history and origins of the organic movement and the theories and practices behind it, then Steve’s Agricultural Library is a must, with many classic books available online in pdf (Adobe Acrobat) format. Effectively it is a trust and Steve is now looking at how he might ensure this library and its artefacts are perpetuated when he eventually succumbs to rapid decomposition!

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