Pre-composting Feedstocks for Vermicomposting: Benefits and Recommendations

by Matt Ball 

Matt Ball is the Director for Industry Development at the US Composting Council in Raleigh, NC. Prior to that engagement, Matt operated the composting program at North Carolina State University. The photos below are of NC State's composting facility. Matt is also a certified Soil Food Web Laboratory Professional and operates Red Mountain Soil Ecology, where he analyzes and reports on the functional biology in soil, compost, worm castings, liquid extracts, and potting mediums.  If you’re interested in learning more about Matt’s lab services, you can visit his website, or reach out to


Vermicomposting is an excellent process to produce biologically-rich compost. However, using an unprocessed organic feedstock can be detrimental to the process, potentially introducing pathogens and generating higher temperatures as the feedstock breaks down, which can harm the worms. Thermophilically pre-composting the feedstock prior to feeding it to the worms can mitigate these issues and improve the speed, efficiency, and output volume of the vermicomposting process.


Benefits of pre-composting:

 Size Reduction - Throughout the composting process, there is substantial volume reduction of the compost pile.  This size reduction will help keep your feedstock supply contained and easier to manage.

  1. Reduces Odor Potential - When raw feedstock decomposes (nitrogenous feedstock in particular), volatile and odorous compounds will accumulate and be released into the atmosphere.  By composting your raw feedstock in an aerobic system, potential odors are diminished.
  2. pH Moderation -   A worm bin should maintain a fairly neutral pH (pH of 6-7).  Composting raw feedstock will help process the organic acids which can build up naturally through organics decomposition, typically rendering a pre-compost with a fairly neutral pH.
  3. Pathogen Reduction - Thermophilic compost will achieve temperatures of 131 degrees F or more for a minimum of several days.  This temperature is hot enough to kill any unwanted pathogens that are in your raw feedstock and keep them from contaminating your worm bin.  In addition, the beneficial biology present in an aerobic compost will out-compete human and plant pathogens that typically thrive in a low oxygen environment.
  4. Vector Attraction Reduction - Thermophilic compost will drive out vermin, fly larvae, and many other vectors that may be attracted to an unmanaged pile of decomposing organic material.
  5. Reduces Volatilized Ammonia -  Ammonia can kill your worms.  If raw nitrogenous feedstock (such as manure) is not balanced with a substantial carbon source, there will be excess nitrogen during decomposition - and this will be volatilized as non-organic ammonia.  Pre-composting with a good C:N ratio will allow the biology an opportunity to sequester the available N or have nitrifying bacteria convert it to plant-available and non-volatile NO3.
  6. Terminates Weed Seeds - The high temperatures achieved during thermophilic composting will terminate any seeds that enter your system via raw feedstock.
  7. Homogenized Feedstock - Pre composting raw feedstock will turn a heterogeneous mix of different solids into a homogeneous mix that will be equally available for your worms to consume.
  8. Lower Biological Demand - The amount of oxygen required for microbial respiration drops dramatically as thermophilic composting progresses.  Pre composting raw feedstock will insure that your worm bin will not go anaerobic due to the large amount of oxygen being consumed by microbes in a worm bin during early stage decomposition.


Composting Recommendations

The way I like to define composting is: Managed aerobic decomposition.  It’s a managed process in that there are a handful of variables (chemical and physical requirements) that need to be accounted for in order to compost successfully.  It’s an aerobic process because the biology present in a thermophilic compost pile requires aerobic conditions from start to finish. 


The chemical and physical requirements necessary to compost are:


  1. Carbon - C is the basic building block in cells and is required to energize the composting process
  2. Nitrogen - N fuels microbial population growth and metabolism
  3. Oxygen - Aerobic biology


  1. Aeration - Aeration provides a means for microbial respiration
  2. Moisture - The microbiology in a compost pile are active within the water surrounding compost particles


Making a Compost Pile

The carbon and nitrogen necessary for composting is provided by different feedstocks. Feedstocks are the raw materials that go into a compost pile and are typically categorized as a carbon source or a nitrogen source.  Some common carbon feedstocks are: Animal bedding, straw, dead leaves, wood chips, paper towels and pizza boxes.  Some common nitrogenous feedstocks are: food scraps, fresh grass clippings, manure, coffee grounds, and garden residues.  These carbon and nitrogen sources each have different C and N content/availability.  Generally, when making a compost pile, your mix will need a C:N ratio of around 30:1.  It’s good practice to try and source a variety of feedstocks that vary in particle size to try and encourage microbial diversity in your compost pile.


After the feedstocks themselves, moisture is perhaps the most important factor in decomposition of organic materials.  Water provides the medium for chemical reactions, dissolves and transports nutrients, and allows the microorganisms to move about.  The best time to wet a compost pile is while you’re building it.  One can wet a pile up by adding water when mixing feedstocks, soaking your feedstocks overnight, or by adding naturally wet feedstocks.  A compost pile should have a moisture content of 50%-60%.  If a compost pile is too wet (>65% moisture), the free air space in the pile will get taken up by water and will likely go anaerobic.   If a pile is too dry (<45% moisture), composting will come to a halt.  An easy and effective way to test moisture content is to do a squeeze test.  This works by grabbing a handful out of your mix and squeezing it as hard as you can.  If you can wring out only a couple of drops then you’re probably at 50%.


Most composting techniques at some point rely on passive aeration to keep the compost pile aerobic.  This means that a compost pile should have plenty of outward facing surface area and lots of interior pore space.   Building good structure into a compost pile will help ensure there is enough pore space available for oxygen to fully diffuse within the pile, keeping it aerobic.  The best way to achieve this is by mixing a larger particle size carbon (things like wood chips) into your pile. 


If your compost pile has the proper C:N ratio, moisture content, and structure, the beneficial microbiology in the pile will be active and metabolizing, which ultimately generates heat in a thermophilic compost pile.  Making sure the pile reaches 131 degrees F for several consecutive days will kill any potential pathogens and terminate any weed seeds in your mix.  It’s advisable to turn your compost pile while in the thermophilic stage to ensure that the outside portion of the pile is moved into the hot center.


Once your compost has gone through the thermophilic process and has cooled back down to ambient temperatures, you can feed this compost to your worms!


Some sources that I like to recommend to new and established composters are:


The Worm Farmers Handbook  Rhonda Sherman

Rhonda Sherman – NCSU

On Farm Composting Handbook  Robert Rynk

The Composting Handbook  Robert Rynk

Cornell Composting Science

US Composting Council

Compost Research and Education Foundation


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