How Much Moisture in a Worm Bin?

Earthworms need moisture taken from the surrounding soil to breathe, according to "The Worm Book." Moisture allows oxygen to dissolve and pass into the worm. The red wiggler worm, Eisenia fetida, which is popular for indoor compost bins, can tolerate a wider range of moisture fluctuation than many other species.

  1. Bin and Bedding

    • The function of a worm bin is to not only provide darkness for your compost worms but to conserve moisture. Bedding also serves the function of retaining moisture for the worms, while also allowing excess moisture to drain. Peat moss works especially well for retaining moisture in dry climates. 


      Indoor worm bins require a moisture level of between 70 and 80 percent. Outdoor worm bins need to be between 35 and 45 percent. Wooden compost bins "breathe," or allow the release of excess moisture, better than plastic ones. Use a moisture meter or probe, available at nurseries or online (see Resources), to measure the moisture level of either type. As a rough gauge, pick up a handful of bedding and squeeze it. Around four to five drops of water should drip out, not more or less.
      If the worm bin is too dry, spritz the bedding with a water bottle, using water that has sat overnight in an open-mouthed container to remove its chlorine. Or add moist food scraps such as tomato tops, lettuce leaves and mangoes or melon.
      If the bedding is too wet, add more cardboard or shredded junk mail or torn newspaper, and let the new bedding absorb some of the moisture. Some worm hobbyists recommend putting peat moss in a pantyhose to make a tube and placing it in the bin bottom to absorb dampness.

    Customizing

    • Monitor the eating and activity levels of your worms at various moisture levels to determine their preferences.
      "In summer months, consider lacing a piece of plastic over the top of the bedding to help keep moisture in," say Loren Nancarrow and Janet Hogan Taylor in "The Worm Book." They suggest making air holes in the side of your bin to maintain ventilation.
      You may have to discover by trial and error the best moisture level for your bin, given the humidity of your climate, the level to which you heat your house in winter and your worm bin's location.

    Troubleshooting

    • If your worms are dying, it's time for immediate troubleshooting of the moisture level in your bin. If the bin is too wet, sponge out as much water as possible and add rolls of newspaper. If the bin is too dry, make sure its drain holes are open and add dechlorinated water.