There are a few important parameters for keeping healthy, productive worms. Unfortunately many bins' marketing makes it sound like a worm bin is a vending machine: put food scraps in and take out gorgeous castings. Living systems are never that simple! Like any organism, caring for a bin requires a desire and willingness to meet the organism's needs so it can in turn meet yours. With attention to a few important cultural parameters the rest of the details like feeding and harvesting all start to fit together and make sense. Like most living systems, your bin is most healthy and productive when it strikes a balance along several continuums that we will cover here. Basically, we are working to meet the needs of the two main partners in your bin: the worms themselves, and the microbes that actually break down the waste and are in turn consumed by the worms.
Moisture and oxygen is our first balancing act. At the risk of oversimplifying, too much moisture limits oxygen and too much aeration reduces moisture (especially in our semi-arid climate). Worms don’t have lungs and must “breathe” through their skin, and their skin must be moist for this gas exchange to occur. Both the worms and the microbes they depend on require oxygen to thrive and be healthy. Sour and smelly bins are a sign of a low-oxygen or “anaerobic” environment: a healthy bin should have an earthy smell like good compost.
So what’s the perfect balance? You’ll get a feel for it quickly, honestly the best way to assess is by diving in and handling the material in the bin. As a reference point, something along the lines of a wrung-out sponge or cooked rice is in the ballpark. Kind of like a good potting soil, if one were to pick up a handful and squeeze, you should be able to squeeze out just a couple drops to at most a very brief trickle.
On a more detailed level, worms actually like more moisture than most worm farmers want, as long as there is still oxygen. Worms can actually live underwater, as long as it’s well-oxygenated water! This is why we stress having bedding that has some fluff and texture to it: those tiny air pockets (such as in corrugated cardboard) make sure that the habitat doesn’t develop anaerobic areas of compacted food or bedding. Super-moist bedding that’s still aerated is GREAT for worms, but can feel muddy and hard to manage for the worm farmer. You’ll find your own balance point on this continuum as you observe your worms and bin health.
Your choice of container has a lot to do with moisture management. Plastic tote-style bins are great entry-level worm bins, but one of the things in managing them is that they can tend to get wet and at risk for anaerobicity (not having enough air flow) on the bottom and in the corners. On the other end, bag-style flow-through bins (VermBin, VermiBag et. al.) can be so well-aerated that they require occasional watering. You just need to monitor and adapt to your bin’s needs. Balancing food with bedding is, in part, about balancing moisture (foods) with oxygen (from the drier and fluffy bedding).
This part’s pretty easy for indoor vermicomposters: most composting worms work best at the same temperatures that humans find comfortable. Below around 60F and they’ll start to slow down, and will start to die below about 40F. Above 85F or so and they will start to get uncomfortable, and above about 95F and they’ll start to die. Outdoor vermicomposting is absolutely possible in our environment but this guide will focus on indoor/basement vermicomposting.
Biological heating is covered in the separate section on Feeding and Bedding but is worth mentioning again: having too little bedding (carbon) or too much nitrogen (feed) in your bin can cause heating and serious harm to your bin!
The balance between feed (food scraps) and bedding (like shredded cardboard, finished compost, paper shreds etc.) is also important. Refer to the Feeding and Bedding section for more detail, but basically we want to make sure we’re adding plenty of bedding/dry carbon materials to balance out the nitrogen and moisture that’s coming in with food scraps. You really can’t add too much bedding, but too much food scraps and the balance can tip very quickly into heating and ammonia release which both are emergency situations. If you see worms trying to climb out of your bin, check immediately for both heat and anaerobic (smelly/mucky) spots in the bin. Both conditions should be remedied with adding Living Materials and/or dry bedding and mixing it in thoroughly to aerate and dry out the affected area.
An important concept in bin management is having enough area of stable, processed material and/or moist bedding to provide a safe refuge for the worms. Most bins will occasionally experience a little heating, or a pH swing from a food addition, or other issues will crop up from time to time. If a recent feeding heats up or otherwise has a problem, having a bin large enough that the worms can flee to another part of the bin can save your whole operation. This is one reason starting with a larger bin and/or multiple bins can provide valuable insurance while you’re learning the worm’s needs and habits.
The last element to consider is the pH of the bin, which we usually maintain via addition of something called buffering grit. Buffering and grit are actually separate processes but both needs can be met easily with one addition. The biological process in the bin where microbes break down organic waste will gradually reduce (acidify) the pH of the bin overall. This is a slow and natural process and it really doesn’t need to keep you awake: pH usually will keep itself in a good range just from adding occasional buffer grit to the bin. What the heck is buffer grit?? Let’s back up a sec. Worms do not have teeth. They have tiny little soft mouths that slurp up the materials once microbes have broken them down sufficiently. They do however need some abrasive material to grind up what they ingest. So we provide them with what we call “grit”, like sandpaper they use in their stomach (in a worm the grinding stomach called a “gizzard”, just like birds). It doesn’t take much, so when we add grit we usually do it in the form of something that will also gently raise the pH at the same time.
What most worm farmers do is save up their eggshells and then grind them to a powder. Adding a couple shakes of this to every few feedings will provide both grit for the worms’ gizzards and calcium carbonate to buffer pH. Alternatives could be using simple powdered calcium carbonate (also called shell grit), garden lime, or dolomite lime. One to avoid however is builder’s lime (like for concrete): be sure to get lime meant for use in the garden. Worms actually have a special gland called the “calciferous gland” for storing up calcium just for the purpose of balancing pH, so you don’t need to frantically add buffer all the time. A good dusting very few feedings should be plenty, you don't need to get anxious about adding grit.
Believe it or not, this does matter! Composting worms are sensitive to light and to vibrations. Pretend that you are the favorite snack of birds, skunks, moles and the like. Light means that you are no longer protected by the matter you are trying to hide in, you're exposed and visible! Vibrations mean something is digging after you and your friends. So except when we're working the bin it's important to offer the worms darkness like an opaque bin that doesn't admit light. And for the "quiet" part, have them in a sturdy location where they're not moved around a lot (like on a clothes dryer) or subjected to lots of vibration. Not to say that you have to Craigslist the Metallica vinyl, just be considerate of where you place the bin and what conditions it will be subject to.