Which potting mix should you choose?
ONE of the myriad of interesting things we horticulture teachers really enjoy (everything about horticulture is enjoyable!!) is to help students learn about growing plants in media other than soil.
And one of the more interesting facets of learning about growing media is our potting mix comparison trials.
This usually involves growing similar plants in similar containers, in similar conditions, but in different potting mixes.
The results? Well, interesting to say the least.
Let’s begin with the cornucopia of potting mix varieties from which to choose.
There seems to be one for nearly every purpose and type of plant you can grow.........well, almost.
There’s General Purpose, Pot and Peat, Terracotta and Tub, Citrus, Azalea and Camellia, Native, Rose and Gardenia, Premium, Professional, Vegetable and Herb, Bulb, African Violet and Indoor, and Tomato and Vegie…...phew!
Most come in 25 litre bags, though some are in 30 litre and others in 50 litre bags.
Prices are hugely variable, and range from around 12 cents per litre up to nearly 70 cents per litre.
And to top it all off, there are mixes made to Australian standards, and others made to fairly nebulous recipes of all sorts of materials.
Potting mixes made to Australian standards have to conform to strict performance requirements such as air-filled porosity, pH, wettability, and nutrient availability.
There are two standards, signified by a set of Black Ticks (Regular) or Red Ticks (Premium). Regular mixes are made to the same physical standards as the Premium varieties, but have no added nutrients.
Premium mixes are required to provide nutrition (fertiliser) for a minimum of one month if used within two months from the date it is manufactured, and usually have superior water-holding and wettability characteristics. Non-regular mixes are usually cheapest, Regular mixes are usually dearer, with Premium mixes nearly always the most expensive.
Our potting mix trials usually involved purchasing bags of mix from nurseries and retailers, then potting up flowering plants such as Petunias, and edible plants, such as lettuce, to compare results. Generally, a minimum of 5 pots of the same mix were trialled, all in the full sun section of our nursery.
Plants potted in non-standard and regular mixes had a set amount of slow-release fertiliser added to the pots, as recommended on the bags. This usually amounted to around 3 to 6 grams of fertiliser per litre of mix in the pot. Premium mixes had no extra fertiliser added during the trials. What were the results? In a nutshell, quite variable. If there was a trend to report, it would be that in most (not all!!) cases the mixes made to Australian Standards performed better than the non-standards.
What we have discovered is that the resultant growth and health of the plants does not necessarily relate to the cost of the mix, if the instructions on the bag are followed.
So begs the question. What can possibly be causing this variability in results?
After much deliberation, and many potting mix trials, we’ve concluded that TIME is one of the main reasons for these differences. Yes, time.
You see, a bag of potting mix is at its peak condition the second the bag is sealed at the end of the production line. All materials are “fresh”, that is, physically and chemically stabilised and ready to support plant growth.
In an ideal world, the bag will then be whisked out to a retailer and be purchased and used by you within a week or so of manufacture.
Realistically, it can sit on a pallet at the warehouse for a week or two, be transported to a retailer who then stores it on the pallet, inside or out, for a week or three until the previous batch sells, then go out on sale, maybe in the hot sun, for another week or four.
What happens in the meantime? The organic components of the mix (peat, coir, pine bark, wood chips, composted grass clippings, sawdust and manures) all interact and micro organisms begin the decomposition process. Moisture and heat accelerate the process, with a myriad of fungi and bacteria producing who-knows-what in the way of plant pathogens and chemicals within the mix.
Heat and moisture also hasten the release and breakdown of slow-release fertilisers in the Premium mixes as well. In the end, it can mean that what you get to use in your pots can be quite different, physically and chemically, to what first came off the production line.
This manifested itself in one of our trials, where two Premium mixes were rated with the worst performers. Subsequent investigations and emails to and from the manufacturers showed that the bags were quite old when purchased, one of them nearly 6 months from the date of manufacture. The mix had physically deteriorated, the fertiliser gone, and the pH markedly different from the standard.
What can be recommended from these trials?
Buy potting mix only when you will be using it straight away - don’t store it in the garden shed for a few months (or years) before use. If possible, check the bag for a date of production (recommended to the manufacturers from our trials). Don’t choose the faded bags, or from a pallet you know has been out in the hot sun for a while.
Finally, you’ll probably get best results, financially and growth-wise, from Regular (Black Ticks) mixes with a quality slow-release fertiliser added at the time of potting.