My early experiences with bagged potting mixes were not happy ones. With a sea of choices, clueless salespeople and confusing labels, I made more than one bad choice.
I got so seriously cheesed off wasting time and money with ‘dried arrangements‘ as a result, I decided to make my own mix. It had to be better than going through all that again!
Now, I try to be self-reliant and budget conscious where possible by making my own supplies. If you don’t already, give home made potting mix a go – it’s easy and saves you a whole lot of headaches!
The ‘Dirt’ on Commercial and Soilless Potting Mixes
- Surprisingly, the old adage ‘You get what you pay for’ doesn’t always apply. You often take ‘pot’ luck! Gardening Australia ran some potting mix tests that proved this is the case – even their seasoned experts were surprised by the results.
- Quality varies tremendously from certified organic products with strict standards to unlabelled contents of questionable origin and quality.
- Poor labelling leaves consumers in the dark.
- Peat and bark (commonly used ingredients) tend to become hydrophobic (water-repellent) as their moisture content drops to below 30%.
- Often include chemical polymers in wetting agents to compensate for the ingredients that are often hard to wet.
On the up side, commercial mixes are sterile, disease free and very convenient as you just open the bag!
So Why Make Your Own Potting Mix?
- SAVE MONEY. Potting mix bags range in price but you can ALWAYS make your own premium quality mix cheaper!
- SAFE INGREDIENTS. Many non-certified organic commercial mixes contain water crystals or soil wetters made from chemical polymers and after researching the dangers of these, I’ve decided not to use or recommend such products. By making your own mix, you know exactly what’s in it and can control the outcome you want.
- SELF-RELIANCE. Making your own supplies is incredibly satisfying and you can share these skills with others.
- LONGER LASTING. By choosing the right ingredients, you will get more mileage out of your mix than a bagged mix based on bark which quickly decomposes.
The Role of Potting Mix Ingredients
A ideal general potting mix should be light, airy, long-lasting (doesn’t break down or become compacted), moisture-retentive and contain some nutrient value.
Similarly, in your potting mix, you need ingredients that provide:
- Drainage – to help hold the soil structure open so water moves through and it doesn’t become anaerobic.
- Aeration – a good mix will be light and fluffy, allowing air pockets to form in the soil structure so your plant roots and micro organisms have the oxygen they need to thrive.
- Water retention – moisture holding capacity is essential or you will have a water repellent mix and waste money on unnecessary watering.
- Nutrient retention – ingredients that bind or hold onto the minerals means less leaching of nutrients; improves plant health and saves you money.
- Plant Food – vital nutrients for plant growth – the amount depends on how long you want the mix to feed your plants for.
- Support - the soil crumbs need to be small and fine so the plant roots (especially young seedlings) can take hold and easily expand through the mix.
- Microbes – play a vital role in plant health and growth and I include them in my mix although many mixes are devoid of soil life.
- Thermal Insulation* – whilst this is an optional function, given that most container plants experience extreme weather conditions at times (hot or cold climates), this can be a beneficial characteristic to include.
“Some ingredients perform multiple roles and I’ve chosen mine carefully to minimise cost and maximise the benefits. This mix is suitable for use in pots, hanging baskets and gardens.”
Basic Potting Mix Recipe
You’ll need a container for measuring, a large bucket for mixing in, access to water (kettle and hose/watering can), sieve; a small fork and trowel, a container for pre-soaking the coir peat and your ingredients.
- 1 part pre-soaked Coir Peat – Coir peat is a cheap but long lasting renewable resource so is a good environmental choice (a waste by-product from coconut-processing industry). The finer product left behind after the husk fibre is processed is called coconut coir or coir peat – not to be confused with peat moss!
- 1 part Vermiculite* (Grade 3 is a good size) – Vermiculite is the silvery grey colour you often see in potting mixes. It is natural volcanic mineral that has been expanded with heat to increase its water holding capacity and can come from a variety of sources.
- Vermiculite has a moderate CEC (cation exchange capacity) so can hold/make available minerals to the plants. [* If unavailable, use coarse sand - see Tips]
- 2 parts sieved Compost - (preferably home made but a commercial certified organic mix is an alternative if you haven’t got your own).
- 1/2 to 1 cup* Worm Castings or Vermicast (humus) – ideally you will have your own worm farm to add this perfect humus to your mix. Note: * this is an approximate quantity based on making 36 litres (4 x 9 litre buckets) of potting mix using a 9 litre brick of coir peat. Feel free to add more if you have it! [If you can't access vermicast, you can buy worm castings or use some humus from the bottom of your compost pile that is most decomposed or use good quality compost]
- A “part” can be whatever quantity you need: a small scoop or icecream tub; a 9 litre bucket or even a wheelbarrow depending on how much potting mix you require. I make 60 litres at a time in a large flexible bucket and store the rest till needed.
STEP 1: Pre-soak coir peat in warm water in a large plastic container. Tip: To rehydrate a 9L block requires 4.5L of water so you need a container bigger than a 9L bucket to work in (minimum 14L size).
STEP 2: Mix equal quantities of pre-soaked coir peat and vermiculite (or coarse sand if using) together well in a large separate container.
STEP 3: Next, add the sieved compost and worm castings and combine thoroughly with (optional) nutrients.
STEP 4: Check the pH with a meter. Most plants require a pH of between 6.0 and 7.0 but if you are growing vegies, from my experience, they grow best in the range of 6.2 – 6.8 pH.
To raise the pH of potting mix by about one unit (make it more alkaline), add 1 – 1.5 grams of dolomite (lime)/litre of mix. To lower the pH by about one unit (make it more acidic), add 0.3 grams of sulphur/litre of potting mix. Keep the mix moist and recheck the pH again a few days later.
Add Nutrients (optional but recommended!!)
I used to add nutrients to the pot AFTER I’d added the plant, but opening up packets and containers every time was repetitive and time consuming. I have limited time in the garden so have developed systems that make it quick and easy to get my tasks done. While making a batch of potting mix, I now also add minerals and slow release organic fertilisers. I blend these additional ‘ingredients’ into the mix all at ONCE so then all I have to do then is plant and water! The plants have everything they need to start growing.
Here’s what I add:
- Rock Minerals – are essential. About 1 cup of NatraMin, a balanced dry mineral mix (or use crusher dust).
- Slow Release Fertilisers/Soil Conditioners – I’m thrifty wherever I can be in the garden but there are some key resources I invest in and that really provide good value in return. I use products such as Organic Xtra, Organic Link, Searles Kickalong Organic Plant Food and McLeod’s Organic Soil Conditioner as these are all balanced organic fertilisers that provide great results without taking out a second mortgage!
- Seaweed & Fish – These provide essential trace elements that boost root growth, plant health, disease resistance, transplant shock and many other benefits. Good value products I use are certified organic Eco C-weed (convenient powdered seaweed concentrate which is easy to add to the potting mix); Searles organic range of Kelp & Fish liquid products and NatraKelp.
Finally, My Potting Mix Tips:
- I make my potting mix in a large flexible bucket with handles. It makes it easy to carry to the potting area or garden.
- If you want fast results, soak coir peat block in hot water to speed up hydration.
- Coarse washed river sand (salt removed) or builder’s sand can be substituted for vermiculite as an alternative ingredient for drainage – or to minimise cost, use a combination of both. “Coarse” is the key word – the rough shape and size of the individual grains of sand allow space for water to pass though. If the grains are too fine, smooth and round (like you find on the beach), water will cling to them and they’ll compact, drowning your plants.
- Use sand if you need to weigh your container down e.g. for a windy balcony so it is less likely to blow over. Add more sand for a faster-draining succulent mix.
- Once you have potted up your plants, avoid letting the mix dry out – coir peat holds moisture well but if it really dries out over time, it can take time to re-wet thoroughly. Mulch really well.
- Compost breaks down as the nutrients are used up by the plants so the volume of mix in your pot will gradually drop. You will need to top up with additional fresh mix around your plants over time.
- Some potting mix recipes suggest using perlite instead of vermiculite however I don’t recommend this due to the risk of Silicosis (overexposure to dust containing microscopic silica can cause scar tissue to form in the lungs, reducing the ability to extract oxygen from the air).
- When buying a commercial potting mix, look for the Australian Standards Mark (AS 3743) on the packaging. A black tick indicates a basic potting mix and a red tick has added fertiliser which means it will feed your plant for a period of weeks. If I buy a bag of commercial potting mix, I choose Searles. Alternatively, look for an equivalent quality guarantee in other countries.
So, that’s my take on potting mix! … What recipes do YOU use if you make your own potting mix? What’s worked? What hasn’t? I’d love you to share your thoughts here.
© Copyright Anne Gibson, The Micro Gardener 2010-2013 – http://www.themicrogardener.com. All rights reserved.