What are the benefits of having a compost heap in your food garden? And how do these benefits compare with having a worm farm? Why do some food gardeners have both? Tasmanian Food Garden Group coordinator Max Bahrfeldt looks at worm farming and backyard composting.
To start, let’s compare the two processes.
Backyard composting: Microbes break down organic matter into nutrient-rich humus in an aerobic decomposition process.
Worm farming: Worms eat organic material. Microbes in the gut of the worms then break down the organic matter into nutrient-rich worm castings, also called vermicompost.
The two processes sound similar. However, the temperature in a successful backyard compost heap may be 55 to 60 degrees Celsius at the peak of composting, whereas worms operate best at an even 20 to 25 degrees Celsius. The microbes involved are also totally different.
A comprehensive research program on vermicomposting by researchers of the Department of Horticulture and Crop Sciences of Ohio State University (see reference at the end of this post) concluded about the resulting composts: “Overall, there is a considerable overlap between both the nutrient content and the form of nutrients in vermicomposts and composts. Vermicomposts have a much finer structure than composts and contain nutrients in forms that are more readily available for plant uptake. Vermicomposts are finely divided peat-like materials with porosity, aeration, drainage, and water-holding capacity that are better than conventional compost.”
How much space will I need and where?
Backyard composting
A compost heap needs to be outside, ideally in a shady spot. It will need to be at least 1 metre x 1 metre in size. Plastic compost bins have a smaller foot print than compost bays, and the compost coming out of these bins is often not of great quality because the content of the bin may be too wet or too dry or too devoid of air. Remixing the content of a plastic compost bin can be done with a special aerator.
Worm farming
A worm farm should be kept in a spot that is in the shade, not windy, not too warm, and not frosty. That can be outside or on a balcony, on a veranda, or in a shed. Worms prefer to be kept in the dark, so you can even have a worm-farm box in a well-ventilated cupboard.
If kept outside, the worms will be much slower to produce worm castings in winter. This is why some people move their worms inside in winter.
What materials can I use?
Backyard composting
Good composting is achieved when nitrogen-rich green materials are mixed with carbon-rich brown materials, and all materials are moist, and have access to air.
To a compost heap you can add any organic material, including kitchen scraps, garden prunings, lawn clippings, dairy, citrus, bread, fresh and partly composted manures of all kinds, seeds, paper, weeds, leaves, straw, hay, sugar cane mulch, egg shells, coffee grounds, tea bags, saw dust, meat and fish scraps.
A compost heap is a great way to soften the harsh influence that undiluted and raw manures and fertilisers can have on your soil microbes. If you add them to your compost heap, they will be broken down and made ready for easy uptake by the microbes in your soil.
For anyone who has a garden that produces a lot of prunings, grass clippings, weeds and vegetable waste, a compost heap is ideal.
Worm farming
A worm farm is the healthiest if it is fed a healthy balance of green and brown materials. These materials should be cut into small bits to give the worms ready access.
To a worm farm you can add: aged manures, shredded paper, finely chopped fresh kitchen scraps, leaves, tea bags, coffee grounds and non-woody mulches.
Do not add to a worm farm: citrus, onions, spicy foods, fat, very acid or salty substances, foods that have gone off, egg shells, fresh manures, large amounts of grass clippings, bokashi, seaweed, fresh saw dust, organic fertilisers, dairy, meat, fish scraps.
For anyone with a small garden or living in a flat with a balcony garden (and therefore little garden waste) a worm is ideal.
Why do some food-gardeners have both a compost heap and a worm farm?
Here is where another finding of the Ohio University research: “A consistent trend in all these trials has been that the best plant growth responses, with all needed nutrients supplied, occurred when vermicomposts constituted a relatively small proportion (10 to 20 per cent) of the total volume of the container medium mixture, with greater proportions of vermicomposts in the plant growth medium not always improving plant growth.
In other words, they found that more is not necessarily better.
The best results in the food garden are achieved when small amounts of vermicompost are added to conventional compost. And that fits nicely with the fact that most worm farms produce a lot less vermicompost than most compost heaps.
My conclusion
Ideally, all food gardeners have both a compost heap and a worm farm.
Some food gardeners who have a healthy backyard compost heap will claim that they already have a combination of the two. Worms don't survive in compost heaps where temperatures during the composting process go up to 55 degrees Celsius or higher, but if that does not happen in your compost heap, chances are you will have worms assisting microbes in their decomposition task. There will therefore be worm castings in your compost.
The beauty of having both a compost heap and worm farm is that you can make sure that for every job where you use compost, be it potting up of plants, or adding compost to a garden bed, you always add a mix of both.
. . .
The research findings quoted in this blog post came from the report, “Effects of vermicomposts and composts on plant growth in horticultural container media”, by researchers from the Soil Ecology Laboratory and the Department of Horticulture and Crop Sciences at Ohio State University. You can download this article by going to this web page, scrolling down to the second of the two responses
Ricki Allardice, and clicking on the third PDF attached to that answer.
I found that other articles and papers dealing with the science behind worm farms come to similar conclusions.
Max Bahrfeldt started The Food Garden Group a decade ago after retiring from many years of full-time teaching, managing and designing courses for adult education. The aim of the group is to create an active community of Tasmanian food gardeners, who freely share their knowledge, surplus produce, seeds and plants. Meeting other food gardeners face to face is an important part of the group, and through this many food gardeners have found new friends with like-minded interests. Members include beginners, experienced food gardeners, and some horticultural professionals. This article first appeared on The Food Garden Group blog (foodgardengroup.blogspot.com).