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Is your HOTBIN compost good, bad or fabulous?

It could be any of these!

Even if your compost looks brown, at one extreme compost can be harmful to plants, at the other, it is nature’s best growing medium. Not all ‘compost’ is the same and you need to know what you have before spreading it on your garden. It is hard for a home composter to do detailed testing.

We have outlined some simple guides that should help you ensure your compost is good. We have also given you a summary of answers we might get from a range of experts.
What is bad compost? Compost that contains toxic or potential toxic elements (chemicals) – it would be rare for domestic (home) compost to be polluted and there are no home tests you can do. The best option is to ensure no waste laden with toxic chemicals are added. This is often why wood containing preservatives or coloured pigments (inks) are not added – but use of toxic chemicals like arsenic, chromium and lead have been banned for many years, so the advice can get out dated.  Industrial compost should be made and tested to the PAS 100 standard (a BSI pre-standard) and this includes testing for toxic items.

Anything else? You do not want your compost to ‘rob’ nitrogen from the soil. Immature compost can do this. You want to avoid compost that is phytotoxic (ie dangerous to plants). Various organic acids created during both aerobic and anaerobic composting can be phytotoxic – ensure your compost is mature. You can do a Solvita C/N test at home. You can also smell the compost – an earthy musty smell is good. Do not spread if you have a fruity or putrid like smell – leave it to breathe/aerate and compost longer.

What is good compost? There is general gardening and academic agreement that adding organic matter (compost) to soil is beneficial. The HOTBIN team has spent considerable time ‘digging’ into the subject of soil fertility. The science indicates that whet really matters is not the total soil organic matter (SOM) but rather the amount of humeric substances (a special group of compounds remaining at the end of composting). These compounds add the really powerful benefits to soil: water retention, nutrient hold and release, soil aggregation and tilth.

Do soils vary in the amount of humeric substances they contain? Absolutely yes, from 0-8%. Many soil fertility issues are directly traced to lower or a decline in humeric content.

Do composts vary in the amount of humeric substances they contain? We have some personnel evidence this might be so. We have found academic papers that evidence humeric content can vary in compost from 2% to 50% of the dry weight. Most soil scientists will state that quantity of humerics in soil is determined by the soil environmental conditions.

Can we determine what sort of compost we have? Not easily. Here is what the experts would say:

Ask a gardening expert to describe good compost and you may well get a list like this:

  • It smells earthy (not putrid, acrid or drain like?)
  • It is fine particles with no sign of original waste or big bits
  • It is a dark brown material

Some might add a footnote that peat is not compost, farm manure is not compost and that the term ‘humus’ is better reserved to describe ‘very well matured compost’.

Ask a worm composter (vermicomposter) what good compost is and they will probably say:

  • Worm cast / vermicompost is the best compost
  • It smells earthy
  • It is always fine particles with no sign of original waste or big bits
  • It is dark brown
  • It is ‘extra good’ as the worms leave a sticky mucus (from digestion tract) in the compost that promotes mycorrhizal root zone activity

Ask an industrial compost maker and you should get the following:

  • It meets the PAS 100 standard
  • The C / N ratio has been tested and is within spec for “stable and mature” compost
  • It all passes through a given mesh sieve (ie below 8mm particles)
  • All potentially toxic elements (e.g. mercury, heavy metals) are below the guideline levels
  • NKP will be present, but compost is a soil amendment not an NKP fertiliser, so we only measure them in some compost formulations.

Ask a soil scientist to describe compost and they will struggle!

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  1. A fair bit longer. More imtropantly, the quality won’t be as good unshredded browns lead to sloppy, slimy piles that are hard to get air circulating through. Do the best you can with shredding all your materials for compost and you’ll see much better results. A little shredding goes a long way!

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