Since I was a toddler, I have been making compost. My mother used to send me down to the compost heap to put kitchen scraps onto the pile. My grandfather and father were avid composters and religiously added compost to the soil each year. As far as I can remember, they more or less built up a pile of waste vegetation and, apart from adding a little lime and soil, did little else to the heap until they dug it in.
In my teens I read a little about composting and made a little better attempt, aided by my father’s admirable wooden compost bin constructions.
|How I used to make compost|
In Victorian times the gardeners had no access to commercial seed and potting composts, so they had to make their own. They did this by carefully composting grass turfs, leaves and garden wastes separately. After these materials had broken down and turned into compost they blended them carefully into their own potting and seed composts.
I think that a lot of misconceptions are being perpetuated by people that see composting more as a crusade rather than just a method of recycling. There are some reasons why good well made compost was valued in the past. It was used in various compost mixes to make various types of growing medium. Fine grained well rotted material was needed to produce this. Woody material was avoided because it did not rot down very quickly and tended to reduce the level of nutrient in the compost. The main decomposers of woody material are the fungi and they send out foraging hyphae that glean nitrogen and other nutrients to make the fungi’s structure. However, the extent of nitrogen depletion in soils with buried woody material seems to be questionable and the results of experiments are not very clear.
We must also not loose sight of the fact that invertibrates also have an important role in the decomposition of woody material and these organisms would add to nitrogen to the soil when they excrete or die.
The formula that I liked to use was:
Fibrous loam 4 parts
Well rotted sieved leaf mould 2 parts
Coarse Sand 1 part
Well rotted manure or sieved compost 1 part.
Coarse Sand 1 part
Well rotted manure or sieved compost 1 part.
As I buy sterilized general purpose peat free compost, I do not have to be as particular about how the compost is made. It is only going to be added to the vegetable plots and not used in seed trays or plant pots.
However, if you want the compost to be made quickly or more friably, then consideration of how it is made might help.
I maintain that, regardless of their chemical makeup, all things that were once alive will eventually decay and be recycled. This means that you can compost old woolen jumpers, cotton shirts, leather handbags, as well as the usual paper and card.
The problem is that a lot of these things, like woolen carpets, have been treated so that they will not rot quickly. Given time, you will find that even these things decompose after a few years. Having said that and buried a lot of carpets in my time, I would not bury carpets any more due to chemicals that are used in their production.
Most carpets contain a cocktail of chemical additives which are impregnated during manufacture or are applied externally on the finished carpet. The chemicals are added to protect people against dust mites, bacteria, moulds and fungi. So what effect does this have on the soil? I doubt that anyone has investigated this to find out. I would not advise anyone to cover their compost heap with an old carpet.
It is said that there are two types of decomposition in compost. One is called aerobic and requires the presence of oxygen. The other is called anaerobic and this can only occur in the absence of oxygen. There is some suggestion in the literature that aerobic composting is better than anaerobic composting. I would suggest that they are more intertwined than the literature suggests. Anaerobic conditions can occur much more easily even in a compost heap that has been comprehensively turned.
In aerobic decomposition carbon from the rotting plant material is taken in and used to make energy by converting its carbon to carbon dioxide by reacting it with oxygen the process is catalyzed by microorganisms and the formula is:
Of course the process of decay is a lot more complicated than this and the contribution of archaea is not fully understood at the moment.
This is called catabolism. The energy that is given out during this reaction is used to convert some of the carbon and other nutrients into the bodies of the decomposer organisms. Nitrogen is important because it is used with carbon to build proteins that make up many of the components of microorganisms’ cells. It seems that the theory is that more carbon is needed because it is used to make energy and is also used to make the body of the microorganisms. If there is a shortage of nitrogen then sometimes decomposition slows down. When the decomposing organisms die then their carbon and nitrogen can be recycled but more of the carbon is used to make energy and is lost as carbon dioxide into the air. Slowly the amount of carbon in the compost pile is reduced and the level of the compost goes down.
However, the final product of composting, everything else considered, is carbon dioxide and water. A proportion of the carbon will be diverted for a time into the bodies of bacteria, fungi, plants and animals but eventually, lastly and finally we all end up as carbon dioxide and water – with a relatively small amount of iron, calcium, phosphorus etc.
This is exactly the opposite of photosynthesis where carbon dioxide and water are combined with the help of sunlight energy to produce carbohydrates.
A lot of heat is generated when carbon is oxidized to carbon dioxide and this is why the compost heap heats up. There is some evidence that there is a succession of organisms that take over the decomposition of dead organic matter and when the temperature of the compost heap rises above the optimum for the early mesophilic decomposers (ones that grow best at temperatures between 20 to 40 degrees Celsius , different heat tolerant or thermophillic microorganisms take over.
Now it is said that this heat will kill pathogens and weed seeds and this may be true of well managed commercial compost production. However, I would be very surprised if this applies to the heaps of rotting vegetation found on most allotment sites. I would not add any diseased material to the compost heap unless you can guarantee a high temperature decomposition process.
I cannot believe that we can separate aerobic and anaerobic decomposition of dead organic matter. Anaerobic conditions will form in any compost heap due to rapid respiration and production of carbon dioxide. Compaction and excess water will also lead to anaerobic conditions even in the best of compost heaps and there are not too many of those.
In this anaerobic process carbon is converted to methane CH4 or other small molecule carbon compounds.There are other pathways for the carbon to go depending on the microorganisms that it encounters. If anoxic conditions occur – and they very often do, then methanogenic bacteria will use organic molecules to produce methane. They are strictly obligate anaerobes, which are poisoned by the presence of oxygen levels as low as 0.18 mg/L of molecular oxygen.
Composting, it is suggested, when done well constitutes an aerobic environment where methanogenic bacteria will not be able to live. Let me tell you a secret, methanogenic bacteria are ubiquitous. They are everywhere and what is more; anoxic environments are more common than you expect. Obligate anaerobic bacteria can happily live in the plaque on your teeth. Micro environments in compost heaps will be anoxic. Methane is produced.
Methane itself is a molecule that is used by organisms to give them a source of carbon for both catabolism and anabolism leading eventually to carbon dioxide the final gas of respiration.
So why are we not overwhelmed by methane? Why is there so little in the atmosphere? Possibly because there are another set of bacteria called methanotrophs or methanophiles that are able to use methane as a source of both carbon and energy. What is more they can grow aerobically like Methylococcus capsulatus or anaerobically, which means that regardless of where the methane is formed (in the compost heap or deep in the ground after I have buried logs and brushwood) these bacteria can metabolize methane by either incorporating the carbon into their bodies or producing carbon dioxide and water in energy production. Some like Methylomirabilis oxyfera reduce nitrate to nitrogen with the help of other microbes and so contribute to nitrogen loss from the soil. An archaeon is implicated in the breakdown of methane by sulphate reducing bacteria, and leads to the characteristic smell of hydrogen sulphide or rotten eggs that is sometimes associated with the breakdown of organic matter. .
Unfortunately some of these processes involve the production of hydrogen sulphide and other sulphur compounds, which have very characteristic smells. Methane is an odourless gas - in other words it does not smell.
What would happen if we just buried the organic matter. (I regularly make Hugelkultur trenches and bury logs and brushwood.)
The difference between the relatively aerobic conditions of a compost heap (I still maintain that even the best of compost heaps are mostly anoxic) and the relatively anoxic conditions of buried vegetation is the speed at which methane, if produced, can be metabolized. As methane use by methanotrophs is slower in anoxic environments there could be a buildup of methane but my contention is that I bury comparatively small amounts of organic matter and that the soil methanotrophs can deal with it fairly easily. In land fill sites there is a vast amount more organic matter compared to my allotment. This has been demonstrated to overwhelm the methanotrophs and the production of methane is very evident.
So I argue that burying vegetation will produce no more methane than a compost heap. If some of the carbon buried in whatever form is prevented from quickly decaying, possibly it could become a carbon sink. There is some evidence of carbon staying in the soil for considerable amounts of time.
Anaerobic composting does not produce very much heat and cannot contribute greatly to the destruction of pathogens. There is some evidence that there is some production of antibiotic compounds and this will lead to a reduction in the numbers of pathogens. However, I do not compost diseased material.
Undoubtedly, shredding plant material aids in making compost. I do not shred mainly because I don’t have a shredder. I think that I would shred most of the plant material that I compost if I did. Shredding increases the surface area and allows bacteria and fungi more of a surface to begin decomposition. It is particularly good when composting woody material. Indubitably, the compost heats up and decomposes far quicker if material is shredded.
If we read some literature, the ratio between carbon and nitrogen is paramount in composting. While I can see a valid point being made about commercially produced compost, I cannot for the life of me generate any enthusiasm for working this out for simple allotment compost heaps. Frankly, allotmenteers are going to leave the heap until it all breaks down and produces friable compost. This may take as much as a year but we are not in any hurry.
According to the books we need more carbon than nitrogen and this seems logical. The problem is how you calculate the available carbon and nitrogen. That is the carbon and nitrogen that is not locked up in hard to decompose molecules. If it can be reliably calculated, the optimum ratio for speedy decomposition is 25-30:1 carbon to nitrogen. I must admit that I just go by eye and if I am adding paper, sawdust, wood or straw to the compost heap then I make sure that they are layered with lawn mowings, manure, weeds, comfrey and other soft, green, plant material.
So adding carbon reduces the amount of available nitrogen; adding nitrogen reduces the amount of available carbon; adding air reduces both however, adding compost increases both. Simple...
Over the decomposition process the ratio of carbon to nitrogen will decrease because carbon will be lost due to conversion into the gases carbon dioxide and methane. This is why the compost level goes down when active decomposition is taking place. So maintaining the most favorable ratios seems to be a futile occupation and not one I would recommend to the ordinary allotmenteer.
|Mega compost heap|
A mate of mine on the allotment said that I could have some of his compost heap. This heap is about 6 feet tall and 15 feet long. It is a monster of a compost heap. It is obviously a compost heap that was neglected and forlorn because , as soon as I started to dig into it, I found various buried plastic trays, tubs, pots and other miscellaneous gardening paraphernalia. It is reportedly about thirteen years old at the bottom.
The top and sides were covered in a mat of couch grass and bindweed rhizomes and they had to be removed before the friable, clean compost could be reached. Now I can tell you that this compost has never been turned, layered, or otherwise mollycoddled, yet it was as good, if not better than, carefully crafted compost.
|Compost from Fred's mega compost heap|
Theory would have it that this compost, which continually grows through addition of extra material, should be a putrefying mess of foul smelling goo. Compaction and water logging should have produced an anaerobic compost heap. It is not foul smelling, slimy or putrefying.
As I have said, it would be very difficult to produce compost that does not contain at least some pockets of anaerobic respiration; it would also be difficult to make compost that does not have any oxygen at all. The one noticeable characteristic of this 6 foot mega compost heap is the number and variety of invertibrate animals that inhabit all parts of it. I can testify to this because I have been up close and seriously eye ball to eye ball with them on a large cliff face of compost.
It is unnecessary to list all the creatures that make compost their habitat; however worms could be found throughout the heap. The role of these invertebrates in keeping a supply of oxygen throughout the compost and allowing aerobic decomposition to take place cannot be overstated. They cannot be ignored when considering composting on allotments and in heaps that are more mounds of rotting material rather than pristine compost bins.
Burning just bypasses all of these processes and goes straight to the greenhouse gasses of water vapour and carbon dioxide without the opportunity for carbon capture within the bodies of heterotrophs, sequestration as organic carbon or as humic complexes with clay.