How Can I Avoid Buying Plastic Bagged Potting Mixes?

Question
Hi Jerry,

I don’t know if you remember me…I’ve seen you at lots of Save the Mary things and on Gardening Australia, which I love!

I have a vege and herb garden in pots and need to find a soil solution. Presently I buy bags of potting mix at Bunnings, choosing one that’s free of fertiliser etc. I mix in worm castings from my worm farm and a measure of Mcleods soil improver. But the end result doesn’t retain moisture all that well and my plants don’t thrive…I want to find a more sustainable way of doing it, and to end up with a mix that’s better than what I’m currently use too. I’m really conscious of the plastic consumption from buying small-ish bags, the morality of buying from a chain store, the transport miles, etc. And I want to nurture my plants with perfect soil too.

Warmest regards
Tilly x

Answer
Hi Tilly,

1 A bag of certified organic potting mix can become a reusable ‘grow bag’. Pierce drainage holes in one flat side, then slice out the upper side out and you’ve got a growing container;

2 Being fairly durable plastic, the empty bags can be reused many times. I reuse them to separate viciously spined plants from ones I can handle without gloves when tidying the garden;

3 Only certified organic potting mix won’t contain industrial chemicals, such as wetting agents and ‘slow release’ (sic) fertilisers. Expect potting mixes made to Australian standards to also contain industrial chemicals;

4 Old-fashioned gardeners and nurseries used to use sieved soil gathered from under lantana thickets as it’s of excellent quality, and has a high level of organic matter and nutrients, but it’s not pathogen free and so should not be used for germinating seed, seedling raising or striking cuttings;

5 Premium quality home garden soil can also be used for potting established plants, but pathogens will limit its use in the same way as for sieved lantana soil. To make a really good growing medium, suitable for growing dwarf fruit trees in containers, mix equal parts of garden soil, washed river sand and premium potting mix;

6 If you can get a back order copy of ‘The Organic Gardener’ magazine you can read my article on ‘straw bale gardening’, which describes how you can generate various grades of mulch and also create your own potting mix from decomposed bales. They decompose using natural summer rainfall. That method requires space and time to do. The picture above illustrates some fully decomposed sugarcane which takes five to seven months to form in my Brisbane garden. As the section of bale in contact with the soil decomposes it is heavily fed on by earthworms, which fill that section with worm castings. Set out in August, around March this lower layer can be carefully gathered, sieved and stored for use as potting mix;

7 You can bulk order loose, unbagged potting mix from landscape suppliers and store it in a compost bay for future use;

Purchased potting mixes are primarily based on bark, so they never retain moisture for long. Certified organic potting mixes will be the most fully decomposed and therefore retain the most moisture. But you’ll still find that when bark-based potting mixes become very dry they are difficult to re-wet. Plunging pots in water for a few hours works well.

You can improve moisture retention of potting mixes by adding some pulverised cow manure. And you’ll find that mulching potted plants using chopped sugarcane, or chopped teatree mulch, will extend the period between watering.

Jerry Coleby-Williams
4th June 2010