Tuesday, 22 February 2011


Really, I cannot say that piling a heap of waste plant material into mound and leaving it for six months or more is any worse a way of producing compost than any other. 

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. 

These sandals had been buried for at least ten years but they still had not rotted away. 

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.  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. 

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.  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. 

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.  

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. 

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.

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.  

No comments:

Post a Comment