With thanks to our friends at Garden Organic for supplying their expertise on potting mixes. By Dr Anton Rosenfeld, Knowledge Officer of Garden Organic
Why Make Your Own?
There are a number of good reasons for making your own potting mixes.
- You are avoiding the use of peat which is still found in many bags of potting compost. It is now well-known that peat is a non-sustainable resource. Extracting it destroys wildlife habitats and results in the release of locked up carbon as CO2.
- 'Compost miles’ and all the energy associated with transporting bulky substances over long distances.
- In many cases you are ‘closing the loop’ and turning materials found within your own garden into a product that you would otherwise have to buy.
But perhaps most importantly, you know what has gone into something that you have made yourself. A bag of commercial potting compost may contain added artificial chemical fertilisers or animal by-products such as blood, hoof and horn. What can be better than saving money and having the satisfaction of creating your very own mix?
What You Need From a Potting Mix!
Although there are plenty of good reasons for making your own potting mixes, it can take a bit of tinkering around to get it right. Commercial potting composts are the result of a considerable amount of research, development and refinement. A plant growing in a pot has restricted access to water, nutrients and biological life so is quite fussy about it's living conditions and what you feed it.
An ideal potting mix should:
- Provide the correct nutrients
- Retain moisture, but drain well
- Retain air, yet hold plant roots firmly
- Not slump, so that all the air spaces collapse
- Be free of pests, diseases and weed seeds
What Materials Can Be Used In a Potting Mix?
A potting mix consists of bulky material to provide support and materials to feed the plants. Here are some suggestions of things that you can use:
Bulky Materials:
- Loam (soil) provides good structure, and is good at moderating nutrient and pH imbalances. Ordinary garden soil can be used, including excess from mole heaps. It should ideally be sieved before use. You can also make loam by turning over turf onto a pile and covering it for 6 – 12 months until all the grass has died.
- Leafmould is a dark crumbly substance that provides a useful structure, adds organic matter and often adds useful microbes to the soil. It’s easy to make, by collecting damp fallen leaves and leaving them in a reusable bin bag or wire frame for 12 - 24 months. For more information visit www.gardenorganic.org.uk/leafmould.
Green waste compost (sometimes called ‘soil improver’) is available from some household recycling centres, councils, and online. It provides structure and aeration to the mix, as well as slow release nutrients. It is best to obtain a PAS 100 certified compost with a smaller screen size (<10mm).
Sources of Nutrients:
- Home made garden compost provides a good balance of slow-release nutrients. For information on how to make compost visit www.gardenorganic.org.uk/compost.
- Animal manures need to be well rotted down before use. Chicken manure is the highest in nutrients and should be used sparingly, whereas horse manure is often high in organic matter but low in nutrients.
- Worm compost is rich in nutrients and makes an ideal addition to homemade potting mixes. You can set up your own small wormery, see how at tinyurl.com/DIY-wormery.
Some Potting Mix Recipes:
Seed-Sowing Mix:
The mix should be fine in texture, and quite low in nutrients, as a seed already has all the nutrients it needs to get going. It is more difficult to make good seed sowing compost than potting compost. A suitable mix is equal parts sieved leafmould and loam.
Potting On:
Established seedlings need a slightly higher nutrient content in the potting mix to sustain them. A suitable mix is equal parts leafmould, loam and garden compost.
Longer Term Potting Mix:
Mature plants need an even higher nutrient mix to sustain them for longer periods. A suitable mix is 3 parts loam and 1 part leafmould for the bulk with 1 part manure, homemade compost or worm compost to provide nutrients.
Tips
- Avoid putting diseased material and weed seeds into your home compost to reduce the risk of it contaminating your potting mixes.
- If using loam in seed compost, it should be sterilised in an oven preheated to 85°C /180°F for 30 minutes to prevent damping off and weed seeds germinating. Leafmould and garden compost should not be sterilised as this would destroy the structure and any biological life.
- If you need to improve the drainage of your mix add some horticultural sand up to ¼ of the mix.
- Most longer term potting mixes will run out of nutrients after about 4 weeks, so will need feeding. Use comfrey liquid, nettle liquid, urine (all diluted at around 1:10) or an organic plant feed.
This advice is provided by Garden Organic, the UK’s charity for organic growing. Garden Organic shares practical growing advice to help people adopt sustainable methods in any growing space.
To find out more, including how to join the charity, visit www.gardenorganic.org.uk.