A possible challenge for a beginning vermicomposter to get a handle on is keeping the moisture levels in the bin at a reasonable level. Thankfully, composting worms are very tolerant of a wide range of dampness, between 50-90%. But “wet it and forget it” is decidedly not a winning plan. A sopping wet worm bin can cause a slowdown in worm activity and reproduction and worse yet, stinky, anaerobic conditions which may spoil your entire bin. The following tips assume you have an indoor plastic bin (the most common, but wettest bin set up) and should help you keep clear of turning your bin into your own personal Swampland (or Sahara) in a box.
1. Measure Moisture in Several Places
Compared with commercial systems, home vermicompost containers are not uniform in their content. Home bins will feature shredded cardboard and paper, peat moss, coffee grounds, corn cobs, apple cores, banana peels and whatever food waste that household is producing at the time. I've even seen a fellow worm nut toss old t-shirts in his worm beds, presumably for insulation.
If you use some sort of a probe to measure moisture, this presents the challenge of inconsistent readings. Sinking the probe into an area full of watermelon chunks is sure to give you a higher-than-expected result. Likewise, insufficiently watered peat moss may wrongly give you the impression that it's time to water the bin. Take the average of numerous readings to get a more accurate result.
2. Be Aware of the Moisture Content of the Food
BREAKING: Watermelons have a lot of water! While you're picking yourself off the floor, let me take the time to admit I'm often guilty of feeding my worms according to the “a little here and a little there” without much regard to what exactly I'm feeding them. Vermicomposting is not cosmic stuff, and a consistently-fed indoor bin is likely to maintain appropriate moisture levels with very little effort on your part. But it's always advisable to (research water content of various organic foods) to consider your worm food choices and how they might affect worm bin moisture.
3. Pay Attention to the Weather
This is a no-brainier for outside bins exposed to the elements, but for indoor systems, ambient humidity still plays a large part in maintaining proper water levels. (Ask me how I killed a bin of worms in a very dry winter recently). If you're like me, and your spouse or roommates demand you keep your buddies out of sight, you'll probably store your bin in the basement if you have one which, during periods of rainfall, will be pretty damp. This is a good thing! But if you have a dehumidifier working around the clock, keep an eye on how the desired humidity levels are affecting your worm bin.
4. Harvest Castings and Add Bedding
By now you're aware of the value of vermicast or worm castings. The nutrient level and availability of those nutrients far surpasses that of conventional compost. A lesser known characteristic, however, is how well worm poop retains water, able to hold 2 to 3 times it's own weight in moisture. This is wonderful in your garden or flower pots, not so much in a worm bin. It is very easy in a plastic bin to find moisture levels that can turn your bin from a healthy, pleasant smelling aerobic environment into a malodorous nightmare for your house and your worms.
You may also choose to add more bedding (peat moss, shredded paper, coco coir) instead of – or in addition to – your worm castings harvest.
5. Use Dry Cardboard or Newsprint to Regulate Moisture
If you open the top of your worm bin and find condensation on the bottom side of the lid, you may be at or approaching the top end of your humidity level. Since I don't care to leave the bin uncovered, I will often layer newsprint or some dry cardboard on top of the bedding, which wicks some of the moisture out of my bedding. It's an inexact technique, but it may help buffer your moisture levels.
Maintaining appropriate moisture is not that difficult. But letting it get out of control can spell doom for your worms. A little vigilance alongside following these guidelines should steer you clear of calamity. If you have anything to add, let me know in the comments!