Hot Composting #2

In the previous blog post, I discussed what hot composting is and why you might want to hot compost in an urban garden. A functioning hot composter maintains consistently high temperatures - typically 50 to 60 ℃ in the top 20 cm layer. This photo was taken with a FLIR One Pro camera attached to a Moto G6 phone...

Thermal infra-red photo of one of my HotBins during active composting

Firstly a reminder of what hot composting is:

The biological decomposition and stabilization of solid organic substrates under aerobic conditions, that allows for the development of thermophilic temperatures as a result of biologically-produced heat, to yield a final product that is stable, free of pathogens and plant seeds, and can be beneficially applied to soil.

Solid organic substrates - general waste from the garden (e.g. grass clippings, hedge/tree cuttings, weeds, vegetable & flower trimmings), kitchen (vegetable/fruit peelings, cooked food leftovers, coffee ground and tea leaves) and office (photocopier paper and corrugated cardboard).

Aerobic conditions - maintaining sufficient airflow through the compost pile at all times to encourage the fast decomposers (aerobic bacteria, fungi, invertebrates) and prevent anaerobic fermentation which leads to smelly slimy compost heaps.

Thermophilic temperatures - achieving temperatures of 55-65 ℃ for a minimum period to kill pathogens, weeds and plant seeds.

Stable - immature or unfinished compost can be phytotoxic, especially for seedlings, due to the presence of intermediate composting products such as acids and alcohols. Partially composted material may utilise nitrogen already in the soil to continue its composting process and reduce the availability of this vital nutrient to your plants and vegetables.

Some of the benefits of composting are:

  • turns a waste stream into a valuable product (value-added)
  • reduces landfill and associated issues (e.g. uncontrolled methane generation, leaching of phytotoxic materials into rivers, attracting vermin)
  • saves money by reducing need to transport waste to the local collection facility and less visits to the garden centre to pick up bags of compost
  • improves all soils (clay, sandy, loam) by improving drainage, adding slow-release nutrients, improving water retention especially during drought-periods, attracts earthworms, etc
  • can be used as a mulch to retain soil moisture
  • increases the carbon content of the soil (climate change)

And hot composting just does it faster enabling greater quantities of organic waste matter to be converted in rich humus in much smaller spaces.

I highly recommend this guide as a thorough, yet easy to read, guide to the ins and outs of composting. By understanding the basic principles, you will have fewer failed heaps and will know how to rescue an underperforming compost pile. Of course, in real life as an urban gardener, you will never have the perfect mix of compostable material for guaranteed 100% success everytime. Hopefully, in the following blog posts you will learn some 'tricks', and even develop some of your own, as you experience the pleasure of turning waste into a valuable, nay essential, product. What they used to call adding value (they may still call it that).

As part of my composting procedure, I include a final stage where cool/warm material from the hot composting process is allowed to mature at ambient temperatures. This is enables the worms, mini-beasts and fungi - all excellent biodegraders in their own right - to continue the composting process. Some pics of fungi growing (and still composting) in the maturation bins...

Fungi growing on woody substrate in compost bin

Younger version of the same fungi?

Identifying fungi is definitely not a skill I possess. So if anyone can help that would be great.

I just let the fungi get on with it as they are particularly adroit at breaking down lignin (woody stuff). When the time comes, they are mixed into the compost and used to enrich the garden soil.



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