Compost – Looks can be deceptive!

What comes out of your compost heap, compost bin and indeed your HOTBIN can vary an awful lot.

With many HOTBIN® composters coming up to their first autumn, there will be a lot of hot compost being taken out 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 look at several batches of compost alongside some HOTBIN composts. Suggest how they might be graded based on common expert visual assessments. Then we will take you on a journey beyond the first look to give you some 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 do a bit of mud pie play to demonstrate a property called ‘colloidal behaviour’’. Finally we will tie these properties back to humeric substances and suggest that what you really need to look for in fabulous compost is high humeric substance content.

This blog is a little more scientific in nature than some of our others – so if you want the headline without the science it is this:

What a compost looks like can be deceptive. HOTBIN compost is often very sticky and very moist/wet and looks lumpy and perhaps even needing further composting. Tests show rather than it being ‘poorly’ composted, quite the reverse – it appears to have a very high humeric substance content and this is good news for your soil and plants – humeric substance is known as ‘black gold’ for a reason!

If your compost looks like (Fig 4&5), or even (Fig 6) below it is OK to dig in to your soil and does not need ‘more composting’. If you only want ‘fine’ particles of compost e.g. looking like (Fig 1) or (Fig 2) – the solution is fairly simple – dry your compost and then sieve it – you will surprised by how much fine material there is in HOTBIN Compost.

Our analysis tour includes;

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 fabulous?

1. Visual inspection – Below are 6 samples of compost 

(Fig1) Vermicompost

(Fig1) Vermicompost

(Fig2) Garden centre compost laid out dry

(Fig2) Garden centre compost laid out dry
(Fig 3) Typical cold compost

(Fig 3) Typical cold compost

(Fig4) HotBin compost at 3 Months

(Fig4) HotBin compost at 3 Months
(Fig5) HOTBIN compost that is it too wet

(Fig5) HOTBIN compost that is it too wet

(Fig6) HOTBIN compost that has gone anerobic

(Fig6) HOTBIN compost that has gone

anaerobicNow in our earlier blog (good, bad or fabulous compost) we outlined details on how different experts defined good compost. If we asked this group of experts to visually judge the samples above here is what we would expect:

(Fig1) Vermicompost – looks fine and rich, super

(Fig2) Garden centre compost laid out dry – looks very fine and free flowing, uniform and has probably been sieved

(Fig 3) Typical cold compost – looks mature, maybe 2 years old and as expected,

(Fig4) HotBin compost at 3 Months – looks ok, dark brown and a little lumpy

(Fig5) HOTBIN compost that is it too wet – looks water logged, lumpy, immature and soggy

(Fig6) HOTBIN compost that has gone anaerobic – definitely looks anaerobic, it’s black sludge not compost

2. Carbon / nitrogen maturity test

Industrial producers of compost (i.e. compost sold in garden centres) 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 at home, although it gets 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 recently tested a range of compost sieves (see guide to compost sieves post). We know wet and sticky compost is a complete pain to sieve. When testing the Compostsifter it failed to sieve any of HOTBIN (Fig5). So we dried the compost and tried again. This reminded us of an old saying – ‘looks can be deceptive’. Below is the result of the sieving test.

HOTBIN compost sieved

HOTBIN compost sieved

The result was about 80% of all the compost went through the fine (8mm mesh) sieve. There is 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 – but that’s another blog!). I’d just like to say this sieve result is not a one off – we have seen it many times, same results.

If we visually compare the sieved HOTBIN compost with the sieved garden centre compost they now look very similar. So was the problem just that the HOTBIN sample has been stuck in the base and got wet? Is the difference just about one compost being very wet and another compost being dry? No!

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, when you grab a handful you can make a ball. It is ‘pliable’ just like a potter’s clay or children’s plasticine.

HOTBIN Vs Garden Centre on compost pliability

HOTBIN Vs Garden Centre on compost pliability

There are two common substances in soils and composts that create a pliable mix – clay and humeric compounds. Both these are ‘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 after drying

HOTBIN Vs Garden Centre – compost pliability after drying

The garden centre compost sample behaves more like peat – it sticks when wet, dries fast (i.e. looses water 2-4 times faster) and then the organic material falls apart when the dry ball is pressed. Whilst the HOTBIN sample is still damp and pliable, it forms a very hard solid outer layer material with soft inner. Even after 100% drying and re-wetting the pliability returns.

At this stage we should mention that HOTBIN composting (and certainly the batch of HotBin compost above ) had no soil added – hence the pliability is not believed to be due to soil clay.

5 Humus test

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

HOTBIN compost - Pliable humus

HOTBIN compost – Pliable humus

Can we test for humeric 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 humeric 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% humeric substances. It cannot be relied on for concentrations well above this.

6 Result to quality – is it fabulous?

We believe there is a real difference between compost and compost that contains very high amounts of humeric substances. Partly decomposed material (compost) will continue to decompose when added to the soil and eventually the carbon cycle completes and it is returned to carbon dioxide and a small amount of recalcitrant humus in the soil. Humeric 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 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 prove it yet!) this back to higher humeric 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!



Filed under Compost, How to get the best out of your HOTBIN

5 responses to “Compost – Looks can be deceptive!

  1. One thing you’ve not mentioned is the quanties and the types of materials being put into the Hotbin. I clicked on this blog post because I’ve just scraped out the bottom 20 cm from my very full Hotbin and decided they needed further maturation.
    My ‘problem’ if indeed it is one, is that I have access to vast quantities of fruit and veg, and, fortunately, sawdust and shredded hedge clippings. I could easily fill the Hotbin every couple of days… I therefore am not a ‘normal’ user.
    I believe the elevated temperatures, which cause (and are caused by) a faster bacterial activity, use up oxygen faster than it can diffuse into the pile; therefore I am getting some anaerobic pockets, which don’t break down into compost as fast as better oxygenated materials.
    So, I’ve taken out the bottom layer, on three occasions now, and put it into a nearby ‘dalek’ cold composter, to go aerobic again and to mature for a while.
    I am happy with my Hotbin and do recommend it to people who ask me as a York Rotter Master Composter. I think if used in a more average way, it should make compost pretty well. I don’t know of ANY ‘fast compost’ system which doesn’t benefit from letting the fast composted materials mature for a good length of time after the initial decomposition.

    • Hi John
      You have done more batches of compost than most of us put together. The following might help – if nothing else try a few tests to tweak your recipes to see if they help

      We have found to get oxygen to every bacteria feeding at a microscopic level around every particle of food waste requires huge surface area to transport oxygen. The only thing we have found suitable is a bulking agent, ie variable sizes of wood chip, from 0.5 to 1.5cm that creates a stable structure with massive surface area over which the air can diffuse. The HotBin will fill faster as wood chips it will not compost down as fast, but the waste will compost without turning or going anaerobic. (The volume of air/oxygen needed is vast, refer to Haug, composting Engineering Handbook p365 to reassure of the technical basis for airflow and bulking agent). With high volume compost systems, I would suggest sieving out and re-using the wood chip again and again.

      The other cause of anaerobic pockets especially in fruit waste stream is excess water. Fruit can be 90% water. As soon as plant cells start to compost the water is released. If there is more water than calories (energy / heat) to drive of water as steam, the water will build up and block airflow (even when bulking agent added). I’d suggest mixing in corrugated cardboard or shredded office paper (not high lignin cereal packets or newspaper). This is purely an easy to digest calorie energy source. You need around 30g per Kg of wet food waste, ie an awful lot for big systems like yours. (You might need to start collecting shredded office paper on your rounds!). My guess is excess water is most likely to be causing problems.

      Sawdust can be a problem. It is high surface area – so will compost faster compared to other woody materials. but adding it to a hot compost pile is akin to filling a chimney stack with damp sawdust – it will block upward airflow. I’d only use in small quantities, ensure well mixed in and ALWAYS still add wood chip for bulking purposes. (If you have too much sawdust – I think you can make the wood burner fire bricks/blocks from sawdust using a special block press).

      Yep totally on board with the fast + maturation. We know many materials are ‘compost’ in 7-10 days, but we always say 30-90 to allow for maturation. Undertaken numerous C/N stability checks after 30 and 90 days so comfortable will be mature. Temperature is also a great guide – if above ambient – then is still active and not mature.

  2. I am no expert but I am having problems about excess water content as I fill my hotbin with mainly grass cuttings, kitchen scraps and dead cut flowers. I put in two handfuls of woodchip for every caddy of grass clippings but this has not been enough for me. I also use shredded office paper. I have just switched to a ‘cross-cut’ shredder as this cuts the paper / cardboard into tiny rectangles rather than long strips. I found it very difficult to mix paper shredded into strips with the rest of the compost – it tends to just wrap about itself.

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