Type Compost Activator into a search engine will produce details of numerous products and it may well also give details of other products described as Compost Accelerators,  Compost Inoculators and Compost makers. The terms appear to be interchangeable.

 It is generally accepted that if a heap or bin does not  start decomposing, or is slow in doing so, it is likely to be due to a  poor C:N  balance with the plant material containing insufficient  nitrogen. Nitrogen is used by the bacteria to build proteins and reproduce while carbon provides  a source of  food.

 Hot Composting.

 If the compost bin or heap is of a good size (at least cubic metre if a New Zealand bin or heap), has been filled in a single batch, layered so that it contains the correct balance of Greens and Browns, turned regularly in the first month  and has the correct moisture level it should not be necessary to adjust the green brown ratio by using additional activators. Some layering systems include one or more  layers of manure or comfrey as activators to speed the composting process.

Cool Composting

 In cool composting i.e. adding waste  as it becomes available, on occasions  the necessary green/brown balance and moisture level  may not be achieved requiring remedial action to be taken.  Normally  additional Greens would be added as an organic activator to provide additional nitrogen to correct  the C:N ratio and  water or liquid weed/compost feeds added to increase the moisture level. However, if greens are not available blood meal, manure or one of the other activators listed below. Similarly, Greens or other natural activators can be added in the spring to kick start the composting process as the temperature starts to rise.

 Most compost activators  stimulate  the composting process to help  start or  speed up decomposition by  providing additional nitrogen. Compost accelerators should increase the rate of decomposition in a cool compost bin usually by providing more nitrogen, so it seems to be another name for a Compost Activator. This may be required by the composting  microbes when a significant quantity of browns, carbon based materials, are being composted, e.g. straw, hay,  dry leaves, wood chip or related woody materials, making  the addition of  nitrogen necessary to achieve a good  carbon nitrogen balance. The extra nitrogen  should result in increased microbial activity and a rise in temperature. To have a significant effect this temperature will need to be maintained so the bin will need to be insulated  to hold the temperature and addition foodstock will need to be added at regular intervals. If the temperature can be increased by 10^oC there should be a doubling of microbial activity.  Aerated by turning will also help.

The use of natural activators  in the form of  more greens can be added to give the correct C:N ratio shown below, are free and available in the garden.

Suitable greens will have a  high nitrogen value and be 'easy' for the composting microbes to breakdown.

     The “natural” activators include:

• Green Plants,  e.g. comfrey,  clover, grass clippings,  nettles, or alfalfa. 

• Matured manures (fresh or dry) e.g.  chicken, cow, rabbit, pig or less common farmed animals such as  alpaca. The manures may be used fresh but it  preferable rested for a season before use because fresh manure can burn plant roots and leaves.  Horse manure can be used but it may contain pesticides and undigested seeds. Manure can be added as a layer in cool composting, but it should be covered with browns. Some manures are available  dry or in the form of pellets which can be scattered onto the brown layers of a layered system or when adding material to a cool pile.

• Powdered products such as are blood and  bone meal,  have also been traditionally scattered  on kitchen  scraps and garden waste each time these are added to the bin in the UK while in the US, alfalfa, cottonseed, soybean meal are also used. 

• Coffee grounds  are relatively rich in nitrogen (up to  2 percent by volume) while coffee chaff from the manufacturing process contains more. . The grounds can be added to cold composting systems as they become available or they can be mixed with equal parts of grass to make a green layer when preparing a hot compost bin or smaller amounts can be added to each  green layer. The Rodale Guide to Composting suggests mixing coffee grounds with lime.

• Human Urine. Many gardening and composting sites recommend adding urine to the compost heap to help speed decomposition. Containing a relatively low level of nitrogen it consists of  about 98% water which makes urine be readily available to the microbes.  Urine  is probably most effective on dry compost bins.    It is difficult to prevent the compost  drying out if using an uninsulated plastic bin in hot weather rainwater,  diluted urine or a liquid feed made from soaked weeds can increase the moisture  level and give the bin a boost. With cold composting this provides the bacteria with a source of food which can be digested quickly producing heat rapidly. It can also be added toa hot composting material in New Zealand or pallet bins but is not recommend for composting using a Hotbin

The value of adding activators where the Green/Brown ratio of the composting material  is correct is open to discussion. But one of the problems found in vegetable  garden and allotment bins is an excess of browns during parts of the year making the use of additional greens or a commercial activator beneficial. In home composting where kitchen waste is available throughout the year this is less likely to be a problem.

“ Which” tests (2008)  on activators  showed that the products included  in the test did not speed up the process if the balance of greens & browns was right and  research by Razvi and Kramer showed that that grass clippings can be composted as efficiently with naturally available materials such as topsoil or mature compost, as with commercially available compost activators, or an unactivated control. This was an interesting choice of test material as grass  as  a natural  Green being high in easily accessible nitrogen.

However,  in my talks I always make the point  that  if someone is routinely using  a commercially available activator  and producing good compost  it is probably better to continue using it on the basis of “if it’s not broke don`t mend it”. 

There are  a wide range of compost activators (or compost makers)  available online and from garden centres including organic products. I have listed three below as examples of the products and how they are used. This information is taken from the manufacturer’s web sites to give an idea as to what is available.

DOFF® Organic Compost Activator (https://www.doff.co.uk/product/organic-compost-activator/)    for composting kitchen and garden waster. The specially prepared nutrient mix contains a mixture of organic ingredients. It feeds the naturally occurring microbes to accelerate the composting process.

How to Use:

On a new compost heap evenly sprinkle 150-200g of DOFF® Organic Compost Activator over 8-10 inches (20-25cm) of composting material and fork in.

Re-apply the activator each time 8-10 inches (20-25cm) of composting material is added to the heap. Forking the area over each time to keep the air in. Fork the heap monthly, even if no new material is added.

Garotta ( https://www.gardenhealth.com/products?brand=80)  is made from materials selected to help improve the structure of home-produced garden compost, maintain a neutral pH and speed up the process of decomposition. It provides food to promote bacterial growth which in turn encourages the temperature in the compost heap to rise, kill off weed seeds and hasten the conversion of garden and kitchen waste into a rich compost ideal for enriching garden soil.

Sprinkle 2-3 tablespoons per sq.m. over each 8cm (3") layer of new material in your compost bin and water in. Repeat each time a new layer of material is added to the bin until full

 Vitax Compost Maker ( https://www.gardenworld.co.uk/our-products/soil-and-compost-additives/)   is an economical compost accelerator containing organic nutrients that speed up the breakdown of garden and household materials.
It can be used to recycle all kind of organic garden and household waste, including grass cuttings, weeds, leaves, vegetable matter, egg shells, shredded woody materials etc., to create your own rich, dark compost.

On compost heaps, Vitax Compost Maker should be layered alternately with the compost materials.
Keep the top of the heap warm and moist by covering with a polythene sheet.
In compost bins, Vitax Compost Maker should be mixed with the waste materials as the bin is filled.

 A second type of activator, sometimes described  as an inoculant,  provides additional microorganisms to boost the decomposition process or  are enzyme activators that when mixed in water are sprinkled on the organic material to  “activate” the composting  microbes. Both methods  are said  to speed the decomposition process. The theory is that the addition of an inoculant containing the  bacteria and fungi  at the stage of decomposition that they are needed will speed   the process of    decomposition and lead to the all-important increase the temperature of the composting material.

 Such a kick start would also be useful  when starting a cool bin as the contents are likely to be less well balanced than in a layered hot compost bin or if it dries out during a hot spell.

Fungi and bacteria are certainly necessary to start and continue the composting process but the necessary organisms are readily available on the organic material being added to the bin and in the soil or the compost from another bin.

However, there the use of inoculants  is controversial .There are those who argue that the application of additional microbes to the compost heap,  by means of an activator, makes little difference as the organisms are already in the organic material and quickly multiply to the limit that the bin conditions allows. Tests on bacterial inoculates recorded in Rodale`s Complete Book of Composting and the Guide to Composting published in the nineteen seventies gave completely negative results  when composting city refuse  the additives  failing to make the composting any quicker than composting without the inoculant allowing the indigenous organisms to work unaided. .

 The addition of “indigenous” microbes to a new heap or bin  by seeding compost from another bin at a more advanced stage of decomposition is now a tried and tested method When mixed thoroughly this will provide the “kick start” necessary and reduce the time lag before the bin reaches the thermophilic operating temperature. However, the time saved when seeding a new hot composting bin is relatively small as a well-made  layered bin should reach 400C in five days.  This type of kick-started compost is sometimes referred to as accelerated composting

 There have been a number of papers published on the use of inoculants when composting  but relatively few when using home composting. The results  are variable.

Karnchanawong and  Nissaikla   looked at the effects of microbial inoculation with two commercial inoculants and mature compost on the composting of household organic wastes (food scraps and dry leaves)   using passive aeration compost bins. The study showed that mature compost can be used as a seed starter to improve composting and that  while it was not necessary to add commercial inoculants  when composting inoculation with MC had a positive effective even though the improvement was not very high.

Nair and Okamitsu (2010) used Effective Microorganism (EM) and Trichoderma sp. in the composting of kitchen waste. They found while inoculants are not essential to accelerating composting EM inoculation enhanced the reproductive rate of earthworms.

A study by Abdullah et al. (2013) using a solution of mixed bacteria as starter culture when composting kitchen-waste found no apparent differences when  composting  with an added starter culture.

Two papers by Van Fan, et.al. published in 2017 and 2018 gave very interesting results. the first  looked at 14 studies evaluating the effect of microbial inoculants on the organic fraction of solid wastes ( mostly food wastes) and 11 studies of the effect on lignocellulosic wastes (mostly yard wastes) while the second looked at home composting

In the first study 38% of the cases  using microbial inoculants showed a positive impact of inoculation on the composting of the organic fraction of the solid wastes. In the case of  lignocellulosic wastes 80% percent showed a positive impact .They noted that the greater positive impact of inoculants on lignocellulosic wastes composting was due to the presence of enzymes breaking down the more decay-resistant lignin compounds.

 An evaluation of the use of Effective Microorganisms in home scale organic waste composting by Van Fan et al in the Journal of Environmental Management looked at their use during home scale composting of food waste, rice bran and dried leaves. The results were compared with and without the addition of EM. It was found that composting with EM achieved a slightly higher temperature at the early stage and that less odour was produced. The humification process was enhanced.

Composting and Microbilal Inoculants BioCycle.net   https://www.biocycle.net/2019/02/01/composting-microbial-inoculants/

 Somjai Karnchanawong & Siriwan Nissaikla ( 2010 )  Effects of microbial inoculation on composting of household organic waste using passive aeration bin  International Journal of Recycling of Organic Waste in Agriculture 

Nair J, Okamitsu K (2010) Microbial inoculants for small scale composting of putrescible kitchen wastes. Waste Manag 30:977–982

Abdullah N, Chin NL, Mokhtar MN, Taip FS (2013) Effects of bulking agents, load size or starter cultures in kitchen-waste composting. Int J Recycl Org Waste Agric 2:3. doi:10.1186/2251-7715-2-3

Van Fan, et.al. published in 2017 and 2018  

Van Fan, Y., et.al., “Efficiency of microbial inoculation for a cleaner composting technology”, Clean Technical Environmental Policy, 20:517-527, 2018.

Commercial Compost activators that  contain  microbes, rather than  nutrients, and  may be sold as compost inoculators or activators. These products may include  blends of beneficial bacteria, fungi and enzyme. It is said that these strains boost the composting process and improve the quality of the finished compost by the addition of bacteria or fungi to boost the activity of those already on the organic material.

The inoculant is usually mixed  with water and sprayed onto the compost as the heap  is made or turned. Specialist compost inoculants are available for specific purposes e.g. lignin degrading fungi can be used on  woody material to reduce significantly the time taken to  produce a rich compost more suited to the growth of annual plants or a more fungal dominant compost for perennial plants.

Examples of these products are given below.  The wording is taken from their websites. I have not used any of these products

Dr Earth Compost Starter  ( https://www.groworganic.com/products/dr-earth-compost-starter-3-lb)   produces remarkable results because it significantly speeds up the decomposition of raw organic materials to create true organic humus. ProBiotic® ensures organic nutrients are thoroughly broken down then released in the compost pile for preparation of soil applications.  Infused with ProBiotic®—consisting of “Seven Champion Strains” of beneficial soil microbes to ensure that all materials are digested to the fullest., Ingredients: alfalfa meal, seaweed extract and beneficial soil microbes.   Use 1/2 to 1 cup on top of the green layer in the compost pile.

Microbz Compost Activator

 https://microbz.co.uk/wp-content/uploads/2019/09/gardens_compressed.pdf

Designed to break down organic matter quickly on compost piles and reduce unpleasant smells. The resulting compost is intended to give an effective nutrient rich and natural soil fertiliser. Can also improve the breakdown of grass clippings and support the delivery of nutrients to the soil

 For use on compost heap. Dilution Rate: 1:4 1L Compost Activator to 4L of water. Coverage rate: 4L of diluted solution per cubic meter.

Agralan Compost Accelerator

Available from https://www.twowests.co.uk/compost-accelerator?gclid=EAIaIQobChMIi6G8tbvO6AIVmK3tCh26zgpXEAQYAyABEgLS6PD_BwE

Speed up the composting process in traditional compost heaps, bins or tumblers with this biological Compost Accelerator. A mix of specially selected microbes and enzymes this chemical free powder can be used wet or dry.

For best results each layer should be treated with Compost Accelerator as the pile is  built the. It can be applied either wet or dry:

To use wet simply add one sachet to a full watering can. Stir briefly and then sprinkle on to fresh waste in your compost heap.

To use dry, mix the contents of one sachet with sand or dry soil and sprinkle over fresh waste.

Pfeiffer™ Compost Starter

https://jpibiodynamics.org/pages/biodynamic-preparation-instructions

This Compost Starter helps to ensure proper introduction of the Biodynamic preparations with minimal knowledge of biodynamic agriculture. It is unequalled in its composting effect, yielding a rapid breakdown of material. Under proper conditions of moisture, aeration, and organic matter (as given in the Carbon/Nitrogen ratio below), the BD Compost Starter will:

Quickly decompose raw organic matter, breaking it down into simpler compounds and re-assemble these simple compounds into complex long-lasting humus, fix nitrogen from the air and otherwise make it available for plant nutrition, increase the availability of minerals, transforming them into their available form, improve soil structure

The special humus producing character of the BD Compost Starter yields high quality compost full of beneficial soil organisms and colloidal humus.

The special humus producing character of the BD Compost Starter yields high quality compost full of beneficial soil organisms and colloidal humus.

 Home-made  aerated compost tea (ACT) can be added as a free source of moisture, fungi and bacteria. The microbes in the tea will be those found in the mature compost used and  that live and multiply at ambient temperature.  Depending on the climate these  are most  likely to be mesophilic organisms which  predominate at the beginning and end of the composting process in the temperature range 10-40°C so as the bin with exceed that temperature in two or three days they will only have a very short time to make their contribution. As once the compost heats up above 40°C the mesophiles are replaced by  thermophilic bacteria.

These  microbes can also be found in topsoil but probably not in the same proportions. The  soil will not provide moisture to  the organic material achieved by  spraying the inoculate but rainwater is free. In practise it is best to get the right C:N ratio when preparing the food stock and use  the  compost tea as a foliar feed or directly on the soil. If perennial weeds are killed by drowning the remaining soup can be added to the bin.

In cool composting where the materials may not be as balanced or turned as effectively as in a New Zealand or pallet  bin  the temperature will not be as high or evenly distributed as in system involving regular aeration  with both mesophiles and  thermophiles being found in different parts of the bin. (There might also be pockets of anaerobic bacteria where the contents have become matted and are not aerated effectively).