How Does HOTBIN Compost Look and Feel?

28 August 2013  |  HOTBIN Composting

Compost - Looks can be deceptive!

What comes out of your compost bin can vary an awful lot.

With many HOTBIN composters coming up to their first autumn, there will be a lot of hot compost being harvested and used in the garden. We thought it would be good idea to let you know what to expect as ‘looks can be deceptive’!

To do this we will compare several batches of compost alongside some HOTBIN compost and suggest how they might be graded based on a common visual assessment. We will also give you an insight into compost stability and maturity tests (we’ve done them for you!), discuss what happens when you dry and sieve HOTBIN compost (don’t worry no-one is suggesting you need to do this) and finally demonstrate a property called ‘colloidal behaviour’’. Finally we will tie these properties back to humic substances and suggest that what you really need to look for in great compost is high humic substance content.

As this post is quite scientific,  if you'd like the summary without all of the science, here it is:

HOTBIN compost is usually very sticky and moist, potentially quite lumpy and perhaps even looking as though it needs to be seived or composted further before it can be used. Tests show that rather than being ‘poorly’ composted in this state it's actually quite the reverse. HOTBIN compost appears to have a very high humic substance content which is great news for your soil and plants as it increases the water and nutrient retention of soil. 

If you only want ‘fine’ particles of compost e.g. looking like (Fig 1) or (Fig 2), the solution is simple. You will need to dry your compost before sieving it - you will be surprised by how much fine material there is in HOTBIN Compost.

For those wanting the science behind the look and feel of compost, our analysis will include:

  1. Visual inspection
  2. Carbon / nitrogen maturity test
  3. Visual after drying and sieving
  4. Pliability / colloidal behaviour
  5. Humus test
  6. Result to quality – is it great compost?

1. Visual Inspection - Below are 6 samples of compost

Photo 1 Photo 2 Photo 3
Vermicompost Garden Centre Compost Traditional (Typical) Cold Compost
Vermicompost Dried garden centre compost Typical cold compost
Looks fine and rich

Looks fine and free flowing, uniform and has probably been sieved

Looks mature, maybe 2 years old and as expected

Photo 4 Photo 5 Photo 6
HOTBIN Compost at 3 months HOTBIN Compost too wet Anaerobic compost
Hotbin Compost after 3 months Hotbin compost that is too wet Hotbin compost that has gone anaerobic

Looks great; dark, rich, sticky and crumbly

Looks water logged, lumpy, immature and soggy

Definitely looks anaerobic, it’s black sludge not compost

2. Carbon / nitrogen maturity test

Industrial producers of compost (i.e. those behind the compost being sold at your local garden centre) do a check to establish how active the bacteria are. Above a certain level and the compost is too active (i.e. not mature & stable) it needs to be left longer otherwise it can draw nitrogen from the soil as bacteria continue to use the carbon in the remaining compost.

The maturity and stability test can be undertaken via the Solvita compost test method. You can do these tests at home, although it can get expensive!

Garden centre compost (Fig2) would not have gone on sale without passing the C/N maturity and stability tests.

How did HOTBIN (Fig4&5) compare? Well they both fall in the stable and mature range. This will surprise a lot of experts as they look lumpy and lumpy normally means large pieces of non-composted material which is highly likely to result in an ‘active’ rather than stable result.

3. Visual after drying and sieving

We know wet and sticky compost is a complete pain to sieve. When testing the Compost sifter it failed to sieve any of HOTBIN (Fig5). So we dried the compost and tried again.

HOTBIN compost sieved

HOTBIN compost sievedWhen dried HOTBIN compost was sieved, approximately 80% of all the compost went through the fine 8mm mesh sieve.

What didn't go through the sieve was made up of about 15% of wood chip pieces (0.8-1.5mm) and 5% oversize non-composted items (notably pampus grass roots – these are possibly going to take the record for the hardest most difficult material to compost).

This sieve result is not a one off – we have seen it many times with the same results.

If we visually compare the sieved HOTBIN compost with the sieved garden centre compost they now look very similar.

4. Pliability / colloidal behaviour

When we sieve and handle HOTBIN compost we notice something else – it is very sticky. It rolls into balls in the sieves and when you grab a handful you can make a ball. It is ‘pliable’ just like a potter’s clay or children’s plasticine.

There are two common substances in soils and composts that create a pliable mix – clay and humic compounds. Both of these are examples of ‘colloidal’ materials and it is this property and the resultant way they hold water that leads to the pliability. 

We can take the pliability test a little further. If we make a ball of moist compost from Garden centre compost and HOTBIN compost (Fig5) and leave then to dry in the sun for 4 days. Now what happens when we try and squeeze each ball?

HOTBIN Vs Garden Centre
compost pliability  

HOTBIN Vs Garden Centre
compost pliability after drying

Compost Pliability Test Compost pliability after drying

The garden centre compost sample behaves more like peat – it sticks when wet, dries fast (i.e. it looses water 2-4 times faster) and then the organic material falls apart when the dry ball is pressed.

In contrast, the HOTBIN sample is still damp and pliable. It forms a very hard solid outer layer of material with a soft inner. Even after drying and re-wetting the compost sample, the same level of pliability returns.

Please note that the HOTBIN compost (and certainly the batch of HOTBIN compost above) had no soil added. As a result, the pliability of the above compost is not believed to be due to soil clay.

5. Humus test

We believe (and we mean ‘believe’ because we have no laboratory proof) that it is possible that HOTBIN compost has more humic substances than many other composts.

Compost pliable humusHOTBIN compost – Pliable humus

Can we test for humic substances?

The answer is yes but not easily outside a fully operational soil testing lab. There is a relatively simple humus soil test – targeted at measuring the concentration of humic substances in soils.

The HOTBIN samples are off the scale – but a note of caution – the test aims to give a reliable field test for soils with 0.5-6% humic substances. It cannot be relied on for concentrations well above this.

6 Result to quality – is it great compost?

We believe that there is a real difference between ordinary compost and compost that contains very high amounts of humic substances.

Partially decomposed material (compost) will continue to decompose once it has been added to the soil and when the carbon cycle eventually complete, the material is returned to carbon dioxide whilst a small residual amount of humus remains in the soil. Humic substances do not decompose in the soil (to any great extent) so adding a concentrated form will improve your soil faster.HOTBIN compost – drying out

HOTBIN compost – drying out

As a company we are not into making claims about our products which we can’t substantiate – to be clear – we believe we can explain the appearance and behaviour of HOTBIN compost.

We believe we can trace (but not yet prove!) this back to higher humic substance content in HOTBIN compost. One day we will have the evidence – until then we believe our logic and science has merit, you can do your own testing, contribute to the debate or ignore our findings as just marketing waffle!