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Compost Activators, Accelerators, Inoculators and Humification Agents - What are they, do they Work?

Tony Callaghan 10/11/2016

Compost Activators, Accelerators, Inoculators and Humification Agents - What are they, do they Work?

In gardening, the term compost activator is used to cover many products with names such as compost maker, accelerator, inoculator, catalyst, and humification agent also being used.  In this article, we define the terms more precisely and use these to look at what they contain (the ingredients) and how well they work. The blog was updated in December 2022.

What are compost activators?

To activate means to start, to get a reaction going. A compost activator should therefore help get things started. There are several types of activators and we go through each below.

What are compost accelerators? 

To accelerate means to increase the speed (rate), so they should increase the speed of composting (ie reduce the time it takes to make good quality compost. We can define a compost accelerator as a product that increases the speed (rate) of decomposition. We do not believe commercial compost accelerators accelerate the rate of garden composting to any meaningful degree.  (We explain why and the detail below). The only meaningful way to accelerate (increase the rate) of decomposition is to increase the heap temperature.

What are compost inoculators?

Inoculation is now mainly used when talking about vaccines, but it comes from the term to add an effective agent into a medium. We define a compost inoculator as a substance that adds microbes (bacteria/fungi) or microbial-enzyme-based substances to the waste to ‘kick start’ (inoculate) the composting mix. Based on our composting work, except in extremely rare situations, paying for inoculators is not required.  We know most food and green waste decompose even when left on a flat surface  - because bacterial spores are everywhere. If you really want to 'kick start' the compost heap/bin, the best method is to add a handful of compost or soil into the new waste - as these both contain billions of bacteria that will inoculate for free. 

What is a Biochar compost additive?

Biochar can be mixed with finished compost as a means of activating the biochar. Biochar can also be added to the compost mix to co-compost biochar and waste. This creates activated compost-biochar such as SF40 and SF60. 

What is a Compost Humification Agent?

This is a new term introduced by SoilFixer. During composting, compounds known as humus (or colloidal humus) are formed. These compounds are the real active items in compost (read here). Typically compost contains less than 1-5% of these valuable compounds. A humification agent increases the amount of colloidal humus in your final compost typically to 15-30%.

Avid readers of our blog will note that the compost Humification is 80% biochar combined with trace minerals.

What is the best Compost Activator?

It all depends on what you are trying to achieve or the problem you are trying to solve.

Some common trade names would include: Westland ‘Make your own compost’ (powder), J Arthur’s Garotta (powder), Vitax Compost Maker (powder), Envil, Seaweed and Seaweed extracts (liquids and powder), Bio-tal (liquid), Neudorff (powder), Sachets of enzymes, microbes or herbs (e.g. QR), our own SoilFixer Compost Humification Agent (powder/granule).  (SoilFixer recognizes the above names and trademarks. Use of a trademark name does not imply any association with, or endorsement by, the trademark owner of any SoilFixer product. The views expressed on these products are those solely of the author).

Do I need to use/add Compost Activators?

It all depends on what you are trying to achieve or the problem you are trying to solve. However, in almost all cases, we can advise the purchase of activators is not needed as there are alternatives readily available in your garden. 

The one exception is when you are trying to create a biochar super compost - which requires the addition of a biochar and or compost humification agent (e.g. SoilFixer's CHA).

Will Compost Activators help my compost breakdown more quickly?

Technically yes... but practically, the answer is a huge NO  because the temperature (heat) has massively more impact. Let us assume as an example that the activator gives a 10% increase in performance:

Compost at 10C Hot compost at 60C
Days to complete compost

540 days

(18 months)

16 days
Increase in rate from "activator" 10%-54 days-1.6 days
Days with activator486days

14.4 day

Unless you are a commercial composter, it is unlikely you going not going to notice the 10% reduction in days. However, the difference between the time at 10C and 60C is huge - 14 vs 486!

For every 10C increase in bin temperature, the speed doubles, ie takes half the time to decompose. Heat will reduce the time by a factor of X8, X16, or x32 times faster.

We are often asked why is my compost not breaking down?  It has been 18 months and I still don't have any compost - how do I get my compost faster?

Garden composters often read articles on how Activators A or B will speed up composting.  If you are looking for any compost activator to speed up composting in a meaningful or noticeable way, I'm afraid you are going to be disappointed in your purchase. It really does not matter which one you opt for: none can change the fundamental laws of nature dictating how fast microbial reactions take place (see Q10, Arrhenius equation). This does not mean activators do not work, nor that it is impossible to get fast compost - it just means there are some facts to know about what works and why. 

First, let's look at the rate (speed) at which composting takes place:

What will make my compost break down faster?

The following all can impact the rate of microbial reaction: type of bacteria (meso, thermophilic etc), surface area (large bits or shredded bits), nature of the waste (is it easy to digest (e.g. grass, cooked food, proteins, and carbohydrates like cellulose) or hard (e.g. wood, lignin).

There are then a number of what we call limiting factors that can limit or totally prevent decomposition. For example: if you have no nitrogen (or not enough) the reaction cannot proceed. If you have no micronutrients in your waste, some of the bacterial enzymes cannot be made/proceed. If you have no oxygen or water aerobic composting cannot proceed or can be limited.

You will find many websites advising adding nitrogen as 'the' solution to increasing the rate of decomposition.  Indeed almost all the compost activators are "ground up nitrogen". But and we stress again, nitrogen does not increase the rate of decomposition - it merely ensures it can happen. Once it can happen, the rate at which it happens is determined by the temperature (i.e. Arrhenius Q10 equation)

The Q10 law of natural science (named after Arrhenius) states: for every 10ºC increase in (composting) temperature, the speed of reaction (rate of decay) doubles. Hot composting at 60ºC is x32 times faster than cold composting at ambient 10C. Hot composting can (and does) deliver compost in 30 days, cold composting delivers compost in 12-18 months). 

You can use this 'rough rule thumb': take the length of time your composting takes at ambient temperature in months (e.g. 36, 18, 12, six months) and convert to days (so 36, 18, 12, six days) for how long it will take for hot composting.  If your current compost takes 18 months (cold system) it will take approx 18 days at 60C (hot).  

The only other activity on the list that gets close to helping increase the speed of composting is increasing the surface area of the waste to be composted. To reduce the particle size - i.e. shred, shred, and more shredding.

Tip: keep your compost warm! (see how at this page).   [If you want to know more about hot composting, please visit blogs I authored on HOTBIN composting]. 

If heat (temperature) is the secret to fast composting, what benefit do all the composting activators on sale really have?

What are compost activators made from -  part 1, nitrogen-based activators 

After a lot of searching of labels and material safety data sheets, it became clear the major ingredient in almost all commercial activators is easily digestible nitrogen. Many (indeed most) of the compost activators on sale are ground-up substances that contain a fair amount of nitrogen. These will usually be made from cheap resources such as chicken poo, blood and bone meal, fish waste. 

But here is the catch - unless your waste has no nitrogen (or not enough) then adding more will not change things.  Most compost waste simply does not need extra nitrogen.   

From personal testing, I became dubious about the benefit of adding accelerators. I looked at the science and ingredients a bit further. They all state 'concentrated, readily available.  Ok... but if we're generous and say they are 100% concentrated nitrogen, then if you added 10g (typical scoop as recommended); that is 10g of 'nitrogen' bacterial food. Now look at the waste you're adding: typically you add 5-10 Kg of waste in a caddy/bucket and 25-50 Kg in a wheelbarrow. This will have around 5% nitrogen on a weight basis, so each addition of green waste/food waste adds 250g of nitrogen. In other words, you add x25 the quantity of nitrogen than that added from the activator packet. Is the nitrogen in the compost waste readily available? It really depends - in any soft food (kitchen waste) it is just as easy for bacteria to access (eat) as in the activators. Items like grass are high in nitrogen (c10%) and are readily decomposed - days at 60C.

If you're an advocate of one of the activator brands (i.e. they work for you), then maybe have a look at the contents label and just check you are happy this is a cost-effective way of adding the contents.

Natural-nitrogen activators

There are plenty of naturally occurring nitrogen "compost activators" such as:

  • Soft greens (comfrey, nettles, grass clippings, alfalfa, clover)
  • Manures (cow, pig, chicken, horse etc)
  • Urine (pee - from various sources!)

All these materials are 'soft' and 'easy' for microbes (bacteria) to digest. They have high nitrogen-to-carbon ratio (towards 10:1) and are hence good C/N balancers where you have a lot of carbon-rich materials like straw, woody stalks or wood.

If you do not have access to soft items like grass clippings, our advice would be to seek out the lowest cost "nitrogen". Rather than opt for a branded compost activator,  seek out the cheapest form of organic nitrogen fertilizer. For example, a 10Kg tub of chicken pellets (or blood meal) is about half the price Kg/Kg than a small pack of "compost accelerators" ie,  £1.50/kg versus £3.00/kg.

You will often see urine (pee) listed as a compost activator. We are highly dubious of the real value of adding pee to compost. Urine has a tiny amount of urea, which in turn has tiny amounts of nitrogen. If you are hot composting - the addition of +99% water (which takes heat to remove it as steam) is negative. In a cold composting system, if the heap is very dry urine is OK. If the heap is damp, it compounds aeration issues. Our expertise says those seeing really positive impact probably have compost bins that were too dry and the water is improving the process. 

Tip: If your waste is general food and garden items and not particularly woody first ensure it is well shredded, second insulate your compost bin to keep the bin contents hot (see these tips). These are much more likely causes for slow composting than lack of nitrogen. 

What are Compost Humification Agents

This is a new term introduced by SoilFixer. During composting, compounds known as humus (or colloidal humus) are formed. These compounds are the real active items in compost (read here). Typically compost contains less than 1-5% of these valuable compounds. A humification agent increases the amount of colloidal humus in your final compost typically to 15-30%.

The SoilFixer humification agent is a mixture of biochar, micronized oxides (tiny bits of trace minerals), and composting microbes (from SF60/compost).  

  • Trace minerals (e.g. calcium, iron) are key to humification
  • Composting microbes are needed - adding a small amount helps guarantee a rapid start
  • Biochar is known to act as a catalyst/activator in the composting process.

The science behind how and why biochar works is constantly developing (e.g. new information on DIET (direct interspecies electron transfer) is speeding the process by 0-30%).

Using the above definitions of 'activator' and 'inoculator', the SoilFixer Compost Humification Agent (CHA) is both an activator and inoculator.  We are not aware of any other compost additive that acts to increase humification.  It also helps to reduce compost odours, lower nitrous oxide and methane (potent climate change gases).

If you want to learn more about the humification agent please visit our product information blog,  or you can buy it now in our shop - just click here.

Biochar - the double win

Used correctly biochar can have a tremendous positive impact on soil health and plant growth. In recent years, adding biochar to compost has become the recommended way to "activate" biochar and help ensure success. Think of raw biochar as a dry sponge.  To activate it, (i.e. charge the biochar with water and plant nutrients), it needs to soak in the compost.  When you add the SoilFixer humification agent, not only do you increase colloidal humus, and improve composting,  as biochar is 80% of the CHA (SoilFixer humification agent) mix, you are activating and adding biochar to your soil. Using CHA you make a DIY version of SF60 - our Biochar Super Compost

NOTE: the above is written primarily looking at compost activators from the home gardeners' perspective. A typical 'cold' (ambient) garden compost heap takes 12-18 months to fully process into rich humus-based compost. Adding an activator that gains say 10-30% increase in rate is not significant when compared to a hot temperature that can make the process X32 times faster. However, things change when you look at municipal-scale composting. Large windrow systems are built to achieve 60C, i.e. fast composting. Additives that give a 10% increase in rate over huge tonnage can have an enormous impact on the economics of the process. We have a similar situation in large-scale Anaerobic Digesters. Using biochar (carbon) for DIET to gain a 20% increase in gas output is a huge benefit!