Author: Mariel Vilella

At home we have two cats, a tortoise… and a thousand worms provided with full room and board in a box! They eat part of our organic waste and give us top quality plant fertiliser in return. Some visitors find them disgusting, but that’s because they haven’t got to know them.

Useful sites

On worm-composting:
Manual Lombrices trabajando [Working worms]:
Blog Picarona y su huerto [Picarona's garden]:
In English:

Buying worm composters and worms: > Vida ecológica,,,,


What can you do with the 200 grams a day (70 kilos a year) of organic waste that the average home produces?[2] If you have plants and a spare square metre of space where you can put a container of the type described below, you can choose not to rely entirely on the public system of incinerators and recycling plants to dispose of your organic waste, and instead convert part of it into fertilizer using worm composting. Waste thus becomes a resource, providing an opportunity to close the materials cycle and to get to know some very special housemates: red earthworms.

These earthworms are hardworking, efficient creatures whose lives are spent peacefully swallowing and excreting food waste. This digestive process produces a humus of far higher quality than that produced by composting without worms, partly because worm casts are enriched with microorganisms from the worms’ own bacterial flora.

The worms are the stars of the process, but they do not work alone. A whole gang of friendly bacteria, protozoa and fungi live in organic residues and help break down the material to speed up the worms’ task. Given the right materials and environmental conditions, the worms will work much better (see next page).

Surprising as it may seem, worm-composting is a very clean process. The worms speed up the mineralisation of the organic material, and move around in the material, aerating it and preventing the bad smells that rotting normally produces.

The resulting product is vermicompost, an earth-like substrate that is rich in nitrogenated substances, and therefore an ideal plant fertiliser. Producing and using vermicompost on your plants closes the cycle, enabling you to participate in an in situ waste recycling process, to help alleviate some of the problems caused by waste and to see for yourself the natural cycles that maintain life.

Did you know…?

There are municipalities that give you a discount on the taxes you pay for waste collection if you compost your waste at home. This can be up to 90% in the case of Sabadell (Barcelona). Consult your town council.

What do you need?

The vermicomposter This is best made of plastic, which is durable and easy to clean. Any container will do (a box with ventilation holes). Or there are special containers with a horizontal separating grill, such as the Vermicasa model from, and multi-layer systems like the Can-O-Worms from We will look at this last system. You can buy or build any of the systems yourself. Instructions on how to build a multi-layer composter can be found at

The worms The most voracious and adaptable species, and the one that reproduces fastest, is the redworm (Eisenia fetida). Although vermicomposting can be done using ordinary garden earthworms or fishing worms, this is the recommended species.

Proportions for container/worms/organic matter: 500 to 1000 worms can live in a 50-litre container with a surface area of 50 x 50 cm and can compost up to 2 kilos of waste a week.

How does it work?

  • Bottom collector tray This is where the leachate liquid that seeps from the food waste and is produced by the worms is collected. This liquid has great fertilising properties. It should be diluted ten times before being used to water plants.
  • First layer Place torn up newspaper (preferably without coloured pigments, i.e. black and white) or coconut fibre (available from garden centres) as a substrate. This must be thoroughly dampened but not soaked so that it goes mushy (75% wetting). Mix in some food waste and the worms which you have bought (see Useful sites) or been given. From now on, the food is added to this layer.
  • Second layer When the first layer is full of food waste, fit a new tray on top so that the base is touching the food in the lower layer, and prepare a substrate as described for the first layer. The worms will work their way up through the holes looking for more food, leaving the first layer behind. When the second layer is full, switch the layers around.
  • Lid, fitted on the upper layer.
  • Removing the humus The vermicompost is ready when it looks like coffee grounds: dark, spongy organic material with the same consistency throughout. Use a trowel to add the compost directly to the earth around your plants, every 6-8 weeks. If the humus in the first layer is not ready when the second layer is full, you can add a third layer.

Where and how do the worms thrive?

Place Indoors or in an outdoor location protected from direct sun and rain. Locate the composter away from vibrations (e.g. washing machine, music equipment, areas with a lot of people passing through, etc.).

Temperature 15° to 25°C.

Darkness Ensure the trays and lid are firmly closed.

Ventilation Check the air holes are not blocked.


  • How much? You can add up to two kilos of food waste a week, either daily or whenever you prefer. It helps the worms if it is well chopped up. After a four-week settling in period, the population numbers will adapt to the amount of food being provided. The worms can go up to two weeks without food, and if you leave a couple of trays filled you can leave them for up to a month.
  • Do give raw fruit and vegetables (except acidic items such as citrus fruits, tomatoes and onion skin), coffee grounds, tea leaves and teabags (remove any staples), crushed eggshells, vacuum cleaner dust and hair clippings (provided no chemical products have been used on the hair).
  • Don’t give garden waste, meat and fish, pet faeces, bones, shells, seeds, pickles or salty foods.

Not recommended: very fibrous foods (e.g. banana skin, artichokes), dairy products, hard-boiled egg, cereals, cooked food, fats, leftovers.

[1] The title of a classic text on worm-composting by Mary Arlene Appelhof, better known as the ‘Worm woman’, who dedicated over 30 years to the practice, research and advocacy of worm-composting in the US.

[2] Fundación Tierra: Lombrices trabajando [Working worms]. Perspectiva Ambiental no. 41, November 2007.