Friday, March 4, 2011

How many composting worms do I need?

I need to figure out how many red wigglers composting worms I need to purchase for my new compost bin (yet to be built). I know that I could start these worms out in five gallon buckets, with holes in the bottoms for drainage, so I'm not worried about building the bin quite yet. Several sources confirm that a pound of red wigglers (about 1000 worms) will consume between 1/2 and 1 pound of food each day and turn that into castings. One pound of food scraps or worms are about the same in volume at about 2 cups. There are many sources that sell them in 1 lb increments.

Worms are going to be able to consume scraps that have been ground or broken up a lot easier than large chunks, but Anna at The Walden Effect has been experimenting with vermicomposting with food wastes from a local elementary school in her area and found that with nine pounds of worms, they were able to consume all 37 pounds of the whole food scraps (including meat) within five days. If she had equipment to grind up those scraps, it may have been even quicker. This equates to each pound of worms being able to consume 0.8 pounds of food, confirming the estimate of a pound of worms consuming 0.5 to 1 pound of food.

Food scraps are not the only thing to add to your worm bin. They need a supply of clean, moist bedding and grit or sand to help them grind their food (birds need grit too). I have tried using our paper shredder on newspaper and it works great. Non-glossy junk mail and cardboard would work well too. Mixing shredded paper in a bucket of water will shrink it much more than you might suspect, so having a large bag of shredded paper can save time from having to go back to get more. Don't forget to consider adding crushed leaves, grass clippings, and straw to the shredded paper. That type of bedding will take longer to break down, but the end result for your garden will be much better than just shredded paper which has a Carbon:Nitrogen ratio of 175:1.

What is not recommended to feed worms?
  • Animal manure from cows, horses, or other farm animals should be composted or aged before adding to a worm bin.This high nitrogen source may start composting in the worm bin, elevating the temperatures, and "cook" your worms. Once the "heat" is taken out of the compost, worms do an excellent job at finishing that compost to make it more available for plants.
  • The levels of bacteria and parasites present in cat, dog, and human manure manure would contaminate your garden produce and make its way into your food. It shouldn't be considered unless composted under high temperatures first. Expert composters don't want to lose this plentiful source of nitrogen, but personally, I have enough other sources of manure so I'm not going to risk it.
  • High salt content would have a similar effect to your worms that it does to slugs. They are both get their body moisture from their very porous skin, so salt can kill in large quantities. If you see food in your bin that the worms are avoiding, this may be why.
  • Large amounts of acidic food (citrus, coffee grounds) can throw off the pH of your bin and introduce mold and spoilage. Small amounts of mold are a natural part of decomposition and fine for worms but too much mold is an indication that the worms are not able to consume the food they are given (too much or wrong kinds).
  • Oils, milk products, and meat (especially uncooked) can go rancid and attract local vermin (skunks, opossums, raccoons, in particular). Worms can handle them in small quantities, but you will need to make sure that their bin is secure. These animals would love to find a good source of food scraps and will likely consume a lot of your worms at the same time.
  • Large, hard food scraps such as corn cobs, fruit pits, seeds, and thick fruit peelings are better off ground up or thrown to your traditional compost bin instead. Worms are not able to do anything with them until normal decomposition processes break them down into soft, small bits that they can deal with.

Anna has experimented with various levels of worm castings and found that although castings improve the fertility of soil significantly and increased levels of beneficial bacteria, you can have too much of a good thing. Soil made up of more than 20% castings doesn't have as much macronutrients as traditional compost but at this level, "plants germinate better, grow faster, and produce higher yields. ... Worm castings have much higher percentages of humus than either soil or compost, which helps the castings hold more water and stay aerated, while also providing binding sites for micronutrients that would otherwise wash out of soil during heavy rains.  The castings are also chock full of plant growth promoters like cytokinins and auxins, along with increased levels of micronutrients like calcium, magnesium, and sulfur.  Worm castings also host ten to twenty times as much microorganism activity as plain soil." (Thanks Anna!)

Something else to consider is how fast red wigglers reproduce. Several sources seem to confirm that this species of worms will reproduce quicker than your standard garden variety earthworm. Your population of red wigglers will double every three to four months in optimal conditions. Wow! So if you decide to purchase all that you need right now, you will need to think about selling off the extras (or feeding them to your chickens) in a few months. Since I have a lot of hungry chickens, I don't think that this will be a problem!

and watch our home food waste to determine how much food we can give to worms every day. Adding cooked foods, including meat, will make a difference in that calculation, but if included, will determine why type of bin needs to be purchased or constructed.

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