Compost Architecture

Compost Architecture

When building a compost heap it is not only the construction and size of the container that one should be mindful of. For it is as if not more important to ensure that the material itself has the right internal architecture to encourage and maintained the microbial communities whilst they perform the relevant task.

In practice this means having access to the feedstock substrate, water and oxygen that they need to proliferate. Feedstock is provided by the material used in the construction, water is usually added during construction and the oxygen is maintained by the integrity of the internal construction.

Thus it is the manner of construction, the internal compost architecture of the heap that above all else is the most important factor in maintaining the correct environment for the microbial communities to manufacture high quality compost.

Most material when first composted has a reasonable amount of structural integrity, a variable amount of water and limited access to the majority of the nitrogen needed by most microbial communities.

In order to liberate this Nitrogen the initial structural integrity of the material must be degraded. This is achieved thorough encouragement of the thermophillic bacteria which require a moist, oxygen rich environment within an organic substrate. Thus a heap should be constructed to be open, porous and adequately moist to achieve such an environment.

Constructing a heap in 6”-8” (15-20 cm) layers, with each layer consisting of either different or different mix rates of material, aids in developing both a reservoir and a good infiltration of air.

keeping a heap open

When using a single or homogeneous material the heap should still be built in successive layers. In this way it is kept “open”. Care should be taken not to compress or create any impermeable barriers.

Each layer should be built from the outside working in. Starting from the centre compacts the heap and does not result in the necessary physical structure.

Furthermore varying the quantities or types of material in each layer creates variations and gradients in the Carbon/Nitrogen/water ratio’s which provide “niche-planes” that help to encourage rapid growth of microbial communities.

creating C:N niche planes


Microbial growth can further be encouraged by sprinkling healthy soil or fresh well-made compost evenly between layers to inoculate and introduce micro-organisms.

As each layer is constructed it should also be watered. The amount of water needed however depends on the moisture state and type of material used and whilst it is impossible to give a precise figure, layers of 6”-8” (15-20 cm) are unlikely to need more than 3 litres per square metre. Material that is particularly dry may not absorb sufficient water or may benefit from pre-soaking.


The Three Stages of Compost Manufacture

The Thermophillic stage (hot)

The thermophillic stage is the most intense generating temperatures as high as 80 degrees C. This intense stage commences at temperatures above 45 degrees C and performs several crucial functions; attacking the structural integrity, so making it easier for the material to be consumed by other organisms in the succession.

microbial population changes-during-composting

Although the stage is intense with extremely high numbers of bacteria, few organisms can tolerate such high temperatures and as a consequence the thermophillic stage is characterised by relative few genera and species (Sylvia, Fuhrmann et al. 1999).

With the high heat generated, the material is partially or “selectively” sterilised with the temperature exceeding the 60 degrees C needed to kill pathogens, including human, and weed seeds.

Necessity of Turning

Turning a heap, an operation that occurs 2-4 weeks after first construction is an important part the composting process. The structural integrity of the feed stock will have been attacked and begun to collapse, thus reducing oxygen infiltration and impacting on microbial growth. Variations in feed stock condition though will mean some material has not been attacked and towards the outside of the heap the higher rate of heat and water loss has prevented the material similarly reaching it’s optimum. As a consequence the heap will have begun to cool and the decomposition process will have slowed down but is far from complete.


temperature profiles of a turned heap

Turning the heap at this point allows for the opportunity to address some of these issues. The manner in which the turning is done though is critical. It is not simple a case of turning a heap over but more a case of turning it inside out.


For the object is to reconstruct the heap so that the material originally located on the sides becomes the centre and the material from the centre becomes the sides of the new heap. As with the original construction the heap should be built in layers and if dry wetted up. It may also be prudent to add fresh layers of easily digestible material such as seaweed (algue) or fresh grass clippings.

After the second rise and fall of temperature the compost enters the mesophillic stage. Although it is possible to encourage a third thermophillic stage by turning again this is not necessary and even undesirable. If the first two thermophillic stages have progressed correctly then the material is ready for the mesophillic stage.

The Mesophillic stage (Warm)


The second or mesophillic stage cccurs at temperatures below 40 degrees C but in practice compost fluctuates between the meso and thermo stages as changes in localised conditions increase and decrease the supply of oxygen.

However once the heap temperature falls below 25 degrees C (approx 4 weeks after turning) the heap is fully into the mesophile stage.

After a further 4 weeks or once the compost has cooled to below 10 degrees C it has entered the final stage of curing and is ready for use. The micro-flora will again change and larger fauna particular brandling or tiger worms (E. Fotida) will begin to migrate and inhabit the compost. The whole process having taken as little as 12 weeks.



Compost produced according to the principles above should be ready for use as a surface mulch or for production of Aqueous Compost Extracts following the second thermophillic stage and throughout the mesophillic stage. Once the compost is approximately two months old it will have completed the thermophillic and another month later the mesophillic. Thus after just three months composting the material should be ready for use as a soil amendment.

Further processing

Mature compost whilst structurally very different will still have much of the appearance of its constituent ingredients and whilst perfectly acceptable for use as a general soil amendment for use in potting composts it needs further maturation and digestion by soil animals. One way to accellerate this process is to use the thermophillic compost as a feed substrate for brandling worms which can digest and convert the compost into a highly valuable compost suitable for the manufacture of potting media.

In Depth: Carbon and Nitrogen demystified  and Adding Mineral supplements

Next : Utilising Compost


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