Building a worm pit

I favour the raised bed method, placed over 100 to 300mm deep pit for added drought resistance.

Construct a worm bed to suit the space and food availability - at least 200cm x 60cm for a small family, to 200cm x 80cm for a larger family.

The pHotos on these pages show a solid working model, three sleepers high, which is on display at Creek Road Community Gardens in Hobart.

Step 1

Nail together two of three rectangular wooden frames of identical size.

Choose a shaded area close to the house; dig a pit of the same dimension and drop the first frame inside it. This is to thwart burrowing mice and to stop soil from the edges subsiding.

Use heavy timber for this initial frame - the lower ground floor! For added protection against mice, place bird wire or concrete pavers on the floor of the pit. Add a second and possibly third frame to complete the enclosure.

Finally, construct a hinged lid to keep on predators.

Step 2

Collect some low nitrogen bedding material to cover the floor to a depth of 30cm, then moisten it. Choose from decaying grass, shredded paper, aged weeds, aged sawdust, fallen leaves, mushroom compost, hay, straw or sea grass. Use a combination.

Animal manure is what worms like best of all, which is why large amounts are not recommended for bedding - worms must be encouraged to eat food scraps first, before putrefaction sets in. Occasionally add some crumbled horse manure, or similar, to encourage breeding. In the absence of animal/poultry manures, other suitable high protein additives are rice/wheat pollard, blood and bone meal, or pelletised animal feedstock.

Step 3

Broadcast some gritty rock dust or powder on top of this. Choose from dolomite, limestone, basalt, wood ash, bluestone, granite or gypsum. Any of these help to sweeten the ingredients against increasing acidity.

Wood ash is highly alkaline, so use sparingly. Dry ash sprinkled on the moistened skin of worms can kill.

Use gypsum (calcium sulpHate) exclusively if you want to use the finished vermicast for acid loving plants.

Add a dusting of sandy loam blended with equal parts of rock powder to make the worms feel at home. This is the quickest and best way possible to condition sand or sandy loam which has no crumb structure.

Step 4

Add 1000 or more compost worms and start burying your food scraps inside the bedding. For every bucketful of scraps added, sprinkle a handful of rock powder. Worms eat about their own body weight in food each day and should multiply to become 15,000 to 20,000 in one year.

For optimum worm happiness:

• Don't add too much food at once, especially bulk fruit wastes, which may produce toxic alcohol or stink with putrefaction
• Strongly flavoured foods such as onion, garlic, citrus, vinegar or salted foods should be added sparingly. Also avoid excessive bread, fat and dairy products
• If your domestic waste tends to be sloppy - including lots of left-over meals, tea leaves and fruit peelings - then don't add water. The moisture content of the pit should not exceed 50 percent
• Adding too much green waste - weeds, lawn mowings and so on - may produce excessive ammonia gas, which can be toxic to worms
• All skins and stems need to be softened by microbial decay before worms can digest them. Chopping and shredding food into smaller pieces speeds up the conversion process
• Add more bedding and rock dust from time to time to maintain a 50/50 balance of wet, sloppy food. This will improve the friable quality of the castings

Step 5

Place black plastic on top of the food layer to keep out all light. A second, heavier cover of old carpet or clothing will minimize fly infestation. This also encourages worms to come up to the surface to feed directly beneath the cover.

Step 6

Once your system has been operating effectively, you will need to separate worms from the mature compost. Temporarily remove any undigested food in the top layer and place into buckets. Break up the remaining compost ready for use. This disturbance, with exposure to sunlight, forces any remaining livestock down to the bottom of the enclosure. After 20 to 30 minutes, remove all but the bottom 10cm of the compost. Then start again with new bedding, adding the food and worms in the bucket.

If you have the space, a two chambered pit is an option, which can make the removal process easier. When one side is full of compost, start filling the other with food scraps. Your worms will move across once the first chamber is digested.