How good is your compost?

Wednesday, 5 February 2014  |  HOTBIN Composting

Compost Quality - Good, Bad or Fabulous?

Compost is defined as a brown crumbly mass in which no original waste plant or animal material is distinguishable (i.e. you cannot tell which bit was the banana skin).

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 and printed paper are not added. The use of toxic chemicals like arsenic, chromium and lead was been banned many years ago, so it is not always a case of not adding these items. Industrial compost is made and sold conforming to PAS 100 (a BSI pre-standard) - this includes a test to ensure PTEs are below set limits.

Anything else?

You do not want your compost to 'rob' nitrogen from the soil. Immature compost can do this, Your compost should not be phytotoxic (ie dangerous to plants). Various organic acids created during both aerobic and anaerobic composting can be phytotoxic - so again you want to ensure you compost is mature. You can so a Solvita C/N test at home. You can also smell the compost - look for an earthy musty smell - do not spread if you have a fruity or putrid like smell - leave it to aerate and compost further.

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 humic 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 humic substances they contain?

Absolutely yes, from 0-8%. Many soil fertility issues are directly traced to lower or a decline in humic content.

Do composts vary in the amount of humric substances they contain?

We have had some personnel evidence this might be so. We have found academic papers that evidence that humic content can vary in compost from 2% to 50% of the dry weight. Most soil scientists will state that quantity of humic in soil is determined by the soil environmental conditions.

Can we determine at home if what sort of compost we have?

Not easily. Here is what the experts would say

Ask a gardening expert to describe good compost:

  • 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:

  • 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! It has no scientific definition - they will refer to “Soil Organic Matter” (SOM). This is the total sum of all dead plant and animal matter in the soil - it excludes roots, living plants, living worms and bugs). They measure the labile part (compostable to you and me) fraction and the non-labile fraction (the bit that resists further decay and is known as the humric substances fraction. They will then offer you +10 soil types, each with a different ratio of sand, clay and SOM. The ‘fertile soils’ (what gardeners want) typically will have:

  • SOM of 2-10% (UK norm 2-4%)
  • Within this, humic substance content of 1-5% (i.e. 50% total SOM)
  • A fertile soil is a balanced mix of SOM, sand (silica) and clays

What if you Googled ‘quality compost’, ‘best compost’ or ‘the world’s best compost?

  • There are claims and methods that profess to offer “colloidal compost”.
  • Methods such as Luebke, CMS, biodynamic and QR that all purport to make fabulous compost

Ask the HOTBIN expert (Tony Callaghan, the HOTBIN's inventor) to describe good HOTBIN compost!

  • It smells earthy (not putrid or drain like)
  • It is fine particles with no sign of original waste or big bits
  • It is a dark brown material
  • It is very wet and sticky - so much so it appears 'large and lumpy' - but looks can be deceptive - for more information see the FAQ on sieving compost.
  • When dried, about 80% will pass through an 8mm sieve, 5% will be oversize (too large) and 15% will be 0.5-1cm pieces of wood chip (bulking agent) coated in humus.
  • It will be pliable i.e. show signs of high levels of colloidal humic substances
  • The C/N ratio will be in the “stable and mature” zone (when tested using the Solvita compost test kit)

We have found no definition or standard that can be laboratory tested for a good, bad or fabulous compost. Only tests for ''stable and mature compost'. There is no system that routinely tracks 'cause and effect' for any difference in compost quality directly back to a composting method.

The question remains: can one compost method deliver better compost than another?

  • There is ample scientific evidence that nature’s composting (i.e. on/in the soils) does produce different levels of residual humic substances – varying from 0 and 3% depending on a range of conditions (soil structure, temp, water, oxygen).>
  • We have found some science papers that have measured different humic content in different composts made by different methods and using different waste materials.

Does the HOTBIN deliver a better, richer compost than any other compost bin?

We do not know the answer – only detailed testing will tell us. We do know the HOTBIN offers a degree of control over the composting conditions. We have evidence that some batches appear to be high in humic content (based on simple tests). We will not be able to substantiate this until numerous samples have been analysed under laboratory conditions and a theory has been produced and peer reviewed for why. Is it due to temperature (unlikely), quantity of aeration (possibly), the mixture of wastes (possibly).

For now we know this - HOTBIN compost often appears much wetter and more colloidal than other composts.

Why not join the debate and help us sort this out - you never know, one day we might see an annual award for the best compost and best composting method!.