Soil quality is the most important factor in any garden and even more so when you are planning to eat the resulting produce. To enrich my soil I have been composting my own garden and food waste for years but, like many gardeners, I find that it is hard to produce enough to cover the whole area. This year I have been transforming my front garden into a network of raised beds to be built up with high-quality compost. I wasn’t content to go for the cheapest stuff I could find, so a bit of research was in order. This culminated in a visit to a composting site, to find out just what goes into compost and how organic it really is...
Compost is the best treatment for almost any garden. Of course, gardeners often use manure, seaweed extracts or manufactured products to enrich soil but they can have their disadvantages (see my article on The Problem with Manure from last year). In contrast, compost is:
- Cheap and easy to get hold of
- Good for enriching all soil types: it helps break up heavy soils and improves the nutrient and water retention of sandy soils
- A great way to recycle waste and reduce the amount that gets unnecessarily sent to landfill
Traditionally commercial composts have used non-renewable resources such as peat but increasingly other options are available and most of those contain recycled plant waste.
To find out more I visited The Compost Shop in the North of England, a large recycled compost producer. They recycle about 15,000 tonnes of green waste from local council collections every year and turn it into high-quality compost and topsoil products. Their manager, Ben Jackson, gave me a tour of the site and explained the process:
- Green waste arrives at the site and is initially sorted to remove undesirable content. This is a laborious process as some people still throw plastic bags or plant pots in with the waste, which must be removed by hand.
- A huge industrial shredder is used to chop and crush the material (pictured above) before being fed into a screener which removes material larger than 30mm diameter.
- The waste is then mixed in huge piles, producing the ideal mixture of moisture, air and green/brown material. These quickly heat up through the microbial activity which is the basis of hot composting. Temperatures reach up to 170°F (75 °C) which kills all weed seeds and ‘burns off’ any undesirable elements such as weedkillers.
- This mixture is turned 8 times during the 12 weeks it takes to complete the composting process. Temperatures are monitored daily for the first half of this period, ensuring that the mix remains above 140°F (60°C) for at least 5 weeks. In fact, you can see clouds of steam rising from the piles as they are turned – particularly striking on cold winter days.
- Finally the matured compost is screened down to 10mm size and samples are sent for testing. The result is a high-quality sterile compost that won’t contain active weed seeds and is packed full of the nutrients plants need to grow.
Ben was quick to point out that this isn’t the same as the potting soil you would use to raise plants in pots. Compost from this recycled process will always contain a sizable proportion of material which has yet to break down. In essence, it is not as ‘even’ and free draining as the products used for germinating seeds. However, this is a distinct advantage when adding it to garden soil or using it as a mulch. The mixture of material gives it a very fibrous content which conditions soil well and the proportion of partially broken-down material means that it will release nutrients over a long time as these continue to break down. Potting soil, with its intensive nutrient balance and very even texture, is just what delicate seedlings need. However, it isn’t long before the nutrients are depleted and, as most gardeners know, after a few weeks you have to add fresh compost to the pots or supplement with liquid feeds. Recycled compost, on the other hand, is a slow-release source of nutrients that will improve soil structure and feed plants for many years.
So, what should you look for when purchasing commercially produced compost? The following factors are important:
- The source of the material.
- The composting process being used
- The tests and monitoring that are done
- What happens if compost fails the tests
The Compost Shop take samples from every batch which are sent away for extensive analysis before they can be sold. They were happy to provide me with a complete list of the analysis done on each batch which includes tests for toxic materials such as heavy metals, pH, pathogens and even the growth of tomato seedlings, checking for abnormalities. They have occasionally had batches which have failed the stringent tests and which are then subjected to further composting and verification.
In the UK the process to look out for is British Standards Institution’s PAS100, on which the Soil Association organic standard is based (even though many companies don’t then pay the extra fee to have the certification logo). In the US look for the US Composting Council’s Seal of Testing Assurance (STA). These standards usually ensure that everything from the materials used to the laboratory testing are carefully monitored. In many other countries there are statutory regulations but you may need to check exactly what is covered if you are keen to stick to organic principles.
I have been making compost at home for many years. I like knowing exactly what has gone into it and how it was produced. But I have to admit that the quality of the commercial recycled compost is considerably higher – a much darker, richer texture and no chance of weeds surviving. So, this year I have ordered 3 tonnes of compost from The Compost Shop and will be using it to boost my garden soil into a rich paradise for vegetables! I wish I could produce enough compost to do this myself but in the real world I know that composting on a commercial scale yields a superior grade of soil conditioner that will allow me to reap benefits for years to come. What is more, I now feel much more confident, knowing just how it has been made and tested. If only shovelling all that compost were as easy as ordering it...!