Compost heap and worm farm

What are the benefits of having a compost heap in your food garden? And how do these benefits compare with having a worm farm? So why do some food gardeners have both?

In this blog post I look at worm farming and backyard composting - not the composting of large heaps of organic material at farms, nurseries and municipal tips.

How do the two processes compare?

Backyard composting

In a compost-heap microbes break down organic matter into nutrient-rich humus in an aerobic decomposition process. 

Worm farming

In a worm-farm 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.

At first glance the two processes sound similar. However, the temperature in a successful backyard compost heap may be around 55-60 degrees Celsius at the peak of composting, whereas worms operate best at an even 20-25 degrees Celsius. The microbes involved are also totally different.

A comprehensive research program on vermicomposting by researchers of the Dept. of Horticulture and Crop Sciences of Ohio State University (see reference at the end of this blog 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 on your property.

A compost heap will need to be at least 1 metre x 1 metres in size. Food Garden Group blog post Making Compost – part 2 shows many examples of compost heap structures. Some are really easy to make. 

Plastic compost bins have a smaller foot print than compost bays, but 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. Often, however, the quality of the compost coming out of compost bins is not great.

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 inside 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 you like, 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 sounds 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, 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 farm sounds ideal.

So why do some food-gardeners have both a compost heap and a worm farm?

Here is where another finding of the research project by the Dept. of Horticulture and Crop Sciences of Ohio State University comes into play:

• 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 %) 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 very 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.

Thank you!

Food Garden Group member Rosalie GL has had many compost heaps and worm farms. She was happy to read a draft of this blog post and provide feedback. Thank you, Rosalie! A thank you too to others who contributed.

The research findings quoted in this blog post came from 

Effects of vermicomposts and composts on plant growth in horticultural container media by researchers from the Soil Ecology Laboratory and the Dept. of Horticulture and Crop Sciences, Ohio State University. 

You can download this article by clicking on

https://www.researchgate.net/post/Vermi-compost-vs-thermo-compost-which-one-is-better-as-a-soil-amendment
Then move down to the Most recent answer section
In that section move down to the second answer by Ricki Allardice on 23Oct14.
And click on the third pdf attached to that answer: the article will be downloaded to your computer.

I found that other articles and papers dealing with the science behind worm farms come to similar conclusions.

For more info about making compost

Making Compost - part 1 covers:

Why make your own compost?

In order to make compost you need ..

Setting up a compost heap

Hot composting compared to cold composting

Do not add the following things to a backyard compost heap

How to avoid mice, rats and native wildlife