POTTING MIX 101

There are a lot of different potting mixes on the market. Which one you choose DOES potentially make a big difference.

Container planting is gaining in popularity, the available choice of potting mixes only seems to be increasing and the price differences between some are seriously surprising.

This can make it tricky to choose the right one.

Especially as not all potting mixes are created equal in terms of performance!

If you are either already experienced in what you require for your growing media or are willing to follow sound advice you may well be putting your own potting mixes together, but if you are not in either category and rely on store bought product please read on....

 

MANUFACTURED TO A STANDARD, NOT A PRICE

There are two Australian Standards for potting mixes.

AS3743 for Premium Grade potting mixes (red tick label on bag) and AS3743 for Regular Grade potting mixes (black tick label on bag)
 

Think carefully about buying any potting mix for any purpose if it does not at least meet AS3743 Regular potting mixes.

If you do, you might get lucky and pick a good one, but there is a risk in doing this unless you are experienced and are able to assess the product thoroughly. Caveat Emptor.

(Inferior mixes don’t just have the potential to limit your plants growth and vitality, in a worst case scenario they can actually be a significant contributor to plant mortality)
 

AS3743 Regular potting mixes will often require a controlled release fertiliser* to be added at the time of potting up (if you don’t wish to use water soluble fertilisers on an ongoing basis during the growing season).

Mixes meeting the Premium Australian standard should have enough controlled release fertiliser for a month or two at minimum but check the packaging as some Premium mixes contain long-term release fertilizers of six months or more^.

Always check the product prior to purchase to determine the fertiliser content.

 

*You will often find AS3743 Regular potting mixes contain trace elements but not the main nutrients required for plant growth (nitrogen, phosphorous and potassium - NPK)

^Be careful if your potting mix is designed for plants with low fertiliser requirements such as cacti & succulents. IF the potting mix contains fertiliser avoid any supplementary applications until your are sure the potting mix fertiliser has been used up.

 

OTHER POINTS TO CONSIDER

 

• The wettability of a Premium Grade mix should be better than Regular grade. This means less chance of the mix becoming water repellant over time.
• General purpose potting mixes are exactly that! There may be times when something a little more specific is required.

For instance, Cacti and succulents require really free draining mixes and this might not be achieved with a general purpose potting mix without adding an ingredient that will have the effect of enhancing drainage such as perlite.

• Potting mix pH is an important consideration. For most situations a pH of around 6.5 is ideal. Refer to our blog on pH here.
• The price difference between potting mixes is seriously surprising!

Typically expect to pay anywhere from 30 cents/litre of product through to $1/litre (smaller bags tend to cost more per litre of product).

But we found what I would describe as a ‘boutique’ mix selling for $3.44/litre and then we found another selling for $19/litre!

• Potting mixes are not intended to last forever

Have you noticed how potting mixes seem to shrink in the pot over time? Organic matter content breaks down over time leading to shrinkage, or as it is more correctly termed, slumpage.

The general idea is that the potting mix lasts long enough for the plant to outgrow it's pot, at which point the plant is potted up into a bigger pot with fresh potting mix. 

 

FOR THE TECHNICALLY MINDED 

In 2012 Kevin Handreck, Horticultural Consultant and former CSIRO soil scientist, purchased 30 bags of general purpose potting mixes from retail garden centres in NSW, Victoria and SA. Ten per state, with a range of potting mixes that met AS Standard and some non- AS Standard mixes.

He thoroughly tested them to see how they stacked up, both against each other and the Australian Standard.

The report is readily available online (you can find it through Burke’s Backyard) and makes for excellent reading (if you're into potting mixes!). I just want to pick up on a few things his research highlighted in this blog:

• The potting mixes that didn’t meet AS standards were described as “cheap and nasty” and the majority would have  "....given very disappointing growth or would have killed your plants".

pH

• In his results most had ok values, one was slightly high and three were low. Of the brands that were low, two had pH values of 5.2 and one had a pH value of 3.7! When you leave out these three low pH mixes the average pH of the others was just on 6.5 with a range from 5.5 to 7.7

Wettability

• As microbes break down the potting mix ‘organics’ the mix can become water repellent. Because of this manufacturers of mixes meeting the AS Standard incorporate wetting agents in the mix.

• In AS Standard mixes the wettability is measured by how long 10ml of water takes to soak into a sample of dried potting mix.

• For Regular grade potting mixes that should be <300 seconds and for Premium grade potting mixes <120 seconds.

• Across all the potting mixes in this trial, the wettability ranged from 5 to 810 seconds! with the worst being non-AS Standard mixes, although some AS Standard mixes did not perform well here either. As Handreck’s report highlights, “…. if these (bad) mixes had dried out in pots, it would have been necessary to dunk the pots in water for hours to re-wet the mix”.

Air-filled porosity:

• The potting mix is first saturated with water then drained after which the percentage of it’s total volume that is air space is immediately measured, giving us the mixes air-filled porosity.

• The minimum porosity the AS Standard dictates is 13%.

• A mix with porosity that doesn’t meet the standard is at risk of not having enough oxygen for root growth when watered frequently. A mix too high in porosity just won’t hang onto water. Most mixes in the trial met the AS Standard, but six were lower. The air-filled porosity of the mixes that didn’t meet the AS standard ranged from 7.9 to 12.8%. The average of the others was just over 16.5% with a range from 13.2 to 38.9%

 

The bottom line

• Don't treat your potting mix purchase as an afterthought and skimp on what you pay for it.
• Buy a potting mix that is AS Standard approved (preferably red tick label/Premium Grade).
• Buy a potting mix that is appropriate for what you will be growing