Precomposting

Precomposting, as the name implies, is allowing organic matter to compost on its own for a short time before adding it to a worm bin.

It is usually done in a way that allows the material to reach high temperatures to help promote more rapid and aggressive decomposition than red wigglers could survive.

If decomposition would be a demolition job, precomposting would be the first hits with the wrecking ball before the clean up crew of composting worms comes to clean up the wreckage.

Though I wouldn't say precomposting is ever absoloutely necessary, there are situations where its benefits are tremendously useful.

1. When you are using highly volatile feedstocks.

2. When you are using feedstocks that are difficult to decompose.

Reducing Volatility

There are plenty of feedstocks that work great for worms, but when added raw can start with, or go through, some pretty wild swings in temperature, pH, ammonia, and moisture. 

Raw manure, spent brewery grains, and even food waste in large amounts can have lots of ammonia, heat, and release a lot of moisture respectively when they start breaking down.

It is much safer to let these initial big changes happen where they can do so without risking the death of any worms.

If done correctly, after just a week or two, precomposted materials will be through their "temper tantrum" phase of decomposition, and be a lot more friendly to your worms once you've introduced them. 

Initial Breakdown of Tough Materials

Another benefit is that any materials that normally take a while for the worms to eat can have a lot of that lag time shortened when precomposted.

Like a wrecking ball, the rapid decomposition and high heat of precomposting does a lot of structural damage to these tough materials (woody materials, avacado shells, banana stems, etc) that lets the worms get to the finer details sooner.

Remember, we're not trying to start a complete composting operation in addition to the worms - we're just doing a tiny bit of the initial work.

Step 1: Getting to Temperature

There's 2 ingredients to hot composting. Nitrogen and Thermal Mass.

Odds are you're looking at precomposting because your feedstock has the ability to heat up on its own already. If that's the case, it's just matter of figuring out how much volume you need.

The more of the material there is together, the easier it can heat up together. On a small scale, I would start with at least 5 gallons of it. Follow standard worm composting procedures with moisture and airflow - which means you'll almost certainly need to add some shredded cardboard or otherwise to allow for airflow. Leaving the precompost somewhere where it doesn't have to fight cold air temperatures will help it succeed.

If you're going to hit good temperatures you should see some heat within just a few days. The goal is to get over 131F, but not to go over 165F. At those temperatures the clock of decomposition is gaining time tremendously.

If you're not reaching those temperatures, try doing a larger volume. If you can't do a larger volume, or it becomes apparent that volume isn't the issue, its time to add extra fuel to the fire.

Alfalfa pellets are my go to for helping heat up precompost, but you can use any high nitrogen fuel source, grains for animal feed being the most easily accessible.

Adding 5-10% alfalfa pellets by volume is a good starting place. Either add more alfalfa, or more volume as needed, until your precompost starts hitting satisfactory temperatures.

*Note that if your goal is just to have the feedstock not heat up when fed to the worms, I would still suggest adding enough volume to see it thermal compost and get the heat out, but I would not suggest adding additional nitrogen to help it get hot.

Step 2: Maintain Aerobic Conditions

Of course our goal is to make the feed more palatable for the worms, allowing the precompost to get to temperatures and then not re-introducing oxygen will make it begin to ferment, which will make it a bit less favorable for the worms.

You can introduce oxygen by "turning" the pile, which on a small scale, would just be fluffing it up or mixing it up.

If your goal is just to get the heat out, you can turn it as often as daily, or if your goal is maximum decomposition you can let it sit a bit longer, like every 2-3 days. 

Step 3: Deciding When It's Ready

If your goal is just to get the heat risk out of the way before adding it to the worm bin, once it stops heating up too much then you're ready to add it!

It is useful to consider how much volume is needed to reach temperatures. If the precompost is in the 100s, but you're only adding a half gallon or so to a worm bin, odds are it will cool down once its separated and isn't surrounded by other hot precompost. It'll be a bit of experimenting!

If your goal is to get things more fully decomposed to give your worms a head start, its really up to you on when you decide its decomposed enough, but generally once its all looking uniformly brown a lot of the initial decomposition work has been done.

Step 4: Adding it to the Worm Bin

Once you decide its ready, its time to get the worm's opinion! Follow proper feeding procedures for any sort of unfamiliar food if you're not sure how the precompost will behave in the bin by placing it just on one side of the worm bin and letting worms move into it as they please.