Vermicompost

What vermicompost is and what it is for

Vermicompost is compost made by worms. Compost worms eat kitchen scraps, damp bedding and other soft organic matter, and what passes through their gut comes out as fine, dark castings mixed with half-digested material. The article on compost introduces the method in a few lines; this page goes into the practice.

Unlike a hot heap, a worm bin has no hot phase. The worms and the microbes in their gut break material down at moderate temperatures, steadily and in small portions. That suits places where only a little material arrives each day: a kitchen, a school, a balcony, a small garden. In permaculture terms it is a tool for the principle produce no waste. What would leave the house as rubbish stays in the nutrient cycle, and the bin belongs close to the kitchen, in zone 1 or 2.

The worms

Not every earthworm will do. Species that burrow deep into mineral soil, such as the common earthworm Lumbricus terrestris, do not thrive in a box. Compost worms live naturally in leaf litter and manure, near the surface, where food is rich and decays fast. By far the most widely used is the red wiggler, Eisenia fetida, often together with its close relative Eisenia andrei. Rhonda Sherman of North Carolina State University calls it the one species most people use worldwide; Cornell also names Lumbricus rubellus. In warm regions growers also keep tropical species such as Perionyx excavatus or Eudrilus eugeniae, which like heat but cope badly with cold. They come from a worm grower or a working bin, not from the garden.

Setting up and keeping a bin, step by step

1. Container. A shallow box of wood or plastic, with holes in the bottom for drainage, a few air holes near the top and a lid against light. Shallow beats deep, since compost worms live in the upper layer. Stacked trays, in which the worms move upward towards fresh food, make harvesting easier.
2. Bedding. Fill the box about a third with moist bedding: shredded cardboard or plain newspaper, dry leaves, coconut fibre, a little old straw. Bedding holds moisture and air, and the worms eat it too. Glossy paper stays out.
3. Moisture. The bedding should feel like a wrung-out sponge. Sherman's guide says only two or three drops should come out when you squeeze a handful; it names about 80 percent moisture as the optimum, within limits of 60 to 90 percent. Moisten the bedding before it goes in rather than pouring water into a running bin.
4. Worms. Sherman's guide recommends starting with at least a pound of worms, roughly 1,000 animals, per square foot of surface, which comes to about five kilograms per square metre. With fewer worms, feed less at first; the population grows into the space.
5. Feeding. Raw fruit and vegetable scraps, coffee grounds, crushed eggshells, all chopped small. Bury each portion under bedding, in a different corner each time. According to Sherman, a pound of red wigglers eats up to two pounds of scraps a week; it is wiser to start below that and watch how fast food disappears. Meat, fish, bones, dairy, fat and salty food stay out, and so do larger amounts of citrus, onion and garlic.
6. Temperature. NC State gives 15 to 25 degrees Celsius as the range to keep in the bin; according to Clive Edwards, compost worms reach their limit at around 35 degrees. Direct sun can kill a bin within an afternoon.
7. Harvest. After three to five months, according to Cornell, the box is full of castings and little bedding remains. Stop feeding for a week or two, push the contents to one side and put fresh bedding and food on the other. Within two or three weeks most worms have moved across. Alternatively, tip the contents into small cones under a bright light and take off the outer layer every few minutes as the worms retreat downward.

Using the harvest, and caution with worm tea

Vermicompost is fine and even, and it goes a long way: mixed into potting soil and seed compost, worked into planting holes, or spread thinly under mulch so it does not dry out. How much to mix in depends on the batch, because vermicompost reflects what the worms were fed. In a trial in Valencia, Rosa Belda and colleagues tested two vermicomposts made from tomato crop waste: they were poor in nutrients, held back plant growth when used alone and did better mixed with peat. A small test pot shows what a batch can do better than any fixed recipe.

Worm tea needs care, because two different liquids go by the name. The dark liquid that drains from the bottom of a bin is leachate. It comes partly from food that rotted without air and is not a reliable fertiliser. Tea in the proper sense is made by steeping finished vermicompost in clean water: NC State suggests about two tablespoons per litre, a day of steeping, and use within 24 hours, and advises against putting it on edible plant parts unless you are sure it is free of pathogens. Research by the US Department of Agriculture shows why: when additives such as soluble kelp or fish hydrolysates were stirred into compost tea, a few E. coli and Salmonella bacteria multiplied strongly (Ingram and Millner 2007). Plain tea without additives, poured on the soil rather than over the salad, is the cautious way.

Typical mistakes

• Too much food. The most common cause of smell, flies and dying worms. Food that lies uneaten starts to rot.
• Too wet. Water collects at the bottom, air is pushed out, the bin smells. Add dry bedding and feed less.
• Too hot. A bin in the sun, or in a closed shed in summer. In hot climates deep shade, a damp sack over the lid and a box partly sunk into the ground help.
• Fresh manure. Fresh manure can heat up and release ammonia. Worms do better on manure that has aged for some weeks.
• Expecting it to sanitise. Without a hot phase, a worm bin does not reliably kill weed seeds or plant diseases. Diseased plants belong in a hot heap or not in compost at all.

Further in the TerraWiki

• Compost: the hot heap for large amounts, for seeding weeds and diseased plants.
• Soil food web and humus: what vermicompost feeds once it is in the ground.
• Nutrient cycle and material cycle: where kitchen scraps go when they stay on the land.
• Produce no waste and zones in permaculture: the principle behind the bin and the right place for it.

Vermicompost at Sa Medada

Sa Medada lies on the east side of Sardinia, with mild winters and a dry season from roughly May to September. For compost worms this turns the northern calendar around. The difficult season is not winter but summer, when shade and moisture decide whether a bin survives. Project Eden begins with pioneer plants, ground cover and holding water. A laboratory analysis of the soil does not exist yet, so this page makes no claims about what vermicompost would change in it.