Potting Compost or Potting Mixes?

Apparently there is an Australian Standard for potting compost regular or premium. But, like all stds the public aren’t encouraged to read them, you have to pay for the privilege!
A few months ago I bought some 25 L bags of Potting Mix from Woolworths, two brands Brunnings and The Green Gardener, both 'regular with 5 tick implying the std.
Everything I have used them for has been a dismay failure. Sowing seeds like parsley and basil, although they germinated the result was pathetic. Potting on larger plants into bigger pots, equally disappointing.
I realised with the Regular that they would need fertiliser and soil wetting agents, which they got.
I think the issue is most of the ‘mix’ is actually bark or wood chip, with virtually no soil/and/mineral type component.
I also spoke to ABC TV Gardening Australia, and they too seem somewhat confused by the terms Potting Compost and Potting Mix. They presume that the former should contain more organic, ie, compostable material, the latter the basic ingredient needed to put in pots, to include finer things that enable roots to grow, like soil.
Is it the std that is wrong, maybe useless, or is it the manufacturers doing the wrong thing?


Unfortunately with highly variable products like potting mixes, the quality even within the one brand can also be variable.

The Australian Standard (AS) recognises this and the test parameters are based on physical and chemical properties and not the individual components persay.

I have also noticed that many potting mixes based in pine bark or other pine vegetative wastes have a strong pine scent when the bags are opened. Such smells usually indicate that the pine materials are not fully composted. An example of such was a bag of the Brunnings brand I purchased which I expect also manufacture the Green Gardener Woolworths brand. The bag when opened had a distinct pine scent. I ended up spereading it on the garden rather than using in a pot.

When shopping at other retail outlets I have also noticed distinct pine scents coming from potting mixes being sold cheaply…these have included Bunnings, Coles, Aldi, Kmart and Big W…so the problem with the potting mix you have bought is not isolated.

One of fhe problems with pine materials if they are not fully composted out (whereby the compost would have an earthy damp odour) is the materials can be allelopathic. This means the raw or uncomposted materials contain natural chemicals which suppress plant growth…if you have ever been to a pine forest one will see that it is close to being a monoculture as the natural allelopathic chemicals in leaf litter suppresses other vegetation which would otherwise compete with the trees.

Now, back to the AS, any potting mix must not be toxic to plant growth. I would expect that allelopathic materials would be considered toxic…and as a result would not meet the AS. Testing cor toxicity is done by germination of radish seeds in the mix.

It is likely that the manufacturer of the potting mix has tried to push the composting processes so that it occurs faster or is completed sooner than samples provided for initial and ongoing AS certification. Tgis is done to increase production quantities and reduce costs.

I would speak to the folks at the customer service desk at Woolies (take the remaining potting mix or bag with you) as you should be entitled to a refund for the potting mix purchased as it won’t meet the AS (you are also welcome to take this post if you think it helps and your potting mix had a distinct pine scent). Advise of the problems you have experienced with the potting mix.

Woollies needs to know that its supplier if delivering products which don’t meet the claims on the packaging. They are more likely to have clout to get the quality rectified than you do…having message on the Choice Community forum may also stimulate their interest as well.

If you are a keen gardener and do a lot or potting…even make your own potting mixes, Growing Media by Kevin Handreck is an excellent text for reference purposes. It is written in a way that it is easy to understand by most people, even n if you don’t have a science background (it has been written for horticulturists which come in all shapes and sizes)

Let us know how you get on.

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In relation to the lack of fine material in the potting mix, this can be a real problem if one is trying to germinate seed or establish seedlings (subject to plantout shock) in such mixes.

The problem with germinating seeds in such mixes is they tend to dry out and are unable to fully surround seed to allow the seed to imbibe water…to trigger the germination process. The more rapid drying out of mix can kill the seed if it has imbibed water and germinatiin has commenced.

A good seed germination mix should contain significant fine materials to allow good contact with the seed. There are purpose made mixes of seed raising. They can also be made by modifying a standard potting mix through the inclusion of fines such as a good quality loam and/or sieved compost. I generally make my own and use soil from the garden (sandy loam) with some potting mix and compost.

There is a balance with germinating most seeds for the garden…they can’t dry out as well as can’t get too wet…the seed also needs oxygen to germinate otherwise it will rot/die.

Therefore a good seed raising mix must retain water, be releatively fine sized, but must also be free draining.

The AS has tests to try and measure these such as air filled porosity, hydraulic conductivity/infiltration, wettability and water holding capacity.

It is also worth noting that some plants such as orchids can get fungal diseases if their roots are kept wet…and there are specific potting mixes for such plants. These potting mixes usually contain coarser particles of stable composted materials.

A standard potting mix (if it meets the AS) is usually only good for potting up plants or topping up pots. Potting mix bags should also state what the mix is suitable for.

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Are you aware if Potting Mix vs Potting Compost have different AS standards; or are the terms considered to mean the same, ie, they are both meant to serve the same purpose, and if so what does the AS standard define that purpose to be?

There are two standards,

  1. AS 3743 Potting mixes; and
  2. AS 4454 Composts, soil conditioners and mulches

There isn’t a AS for ‘potting composts’. Such would fall under AS4454.

The other standard relating to lanscaping materials is AS 4419 Soils for landscaping and garden use.

It is also worth noting that a product with AS 4454 certification is not designed for to be used as potting mixes/plant growing medium by itself.

If you click on the above links it will take you to the SAI Global website. Click on the AS link and it will provide a summary of the standard’s scope.


With regard to the amount of wood chips or shavings in so called ‘potting mixes’, I have found the same to be true with manures.
I purchased a couple of bags of ‘cow manure’ the other day, and I reckon they were about 40% wood chips or shavings.
If it truly was ‘cow manure’, the cows who had to pass that rubbish have my deepest sympathy!
Potting mix should be just that - potting mix! Not partially composted wood shavings.
Problem is, nobody is doing anything about it - it is such a ‘nothing’ problem in the scheme of things. So they get away with it.


For commercial supplies, sawdust/wood shavings would be present in chicken manure as many farmers use these as flooring in sheds…it is scraped up and replaced periodically.

Cow manure is different as most comes from feedlot scrapings. Some feedlots use sawdust as a flooring, but it isn’t as common as for poultry.

I expect the sawdust/shavings are added for a number of possible reasons, including

  • adding it as a carbon source if the manure is composted before bagging,
  • to reduce the moisture content and to free up the manure (it will pack and become a mass in a bag making it hard to removeor use)
  • to bulk out the manure and make it lighter…or potentially to reduce costs.

The main concern would be if the sawdust/shavings were not composted as they could affect the availability of nitrogen when used. The other concern would be spontaneous combustion where the moisture content and component overhead causing a fire.

If manures were dried and/or pelleted, there would be no need for any sawdust/shavings to be added…as tge dried material would be reasonably stable. Drying/pelleting does significantly increase cost of production and bagging.

There is no Australian Standard for manures and one has to take what the seller states on bags/supporting information.

Manures can be highly variable, even from the same source location. It can also may contain contaminates such as heavy metals, endocrine disrupters and pharmaceuticals (depending on what the animals are fed), chemicals in concentrations or forms that burn roots/plants if applied in high amounts, plant and/or human pathogens and other undesirable things such as salts or high sodium (which can accumulate in soils). Because it seems to be a natural product, does not necessarily mean it is a safer option and thought needs to be used when applying.

Note: pharmaceuticals can usually be avoided by using an organic certified product.

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Thanks for the info.

I tried the link you posted, but it didn’t work.

The following link leads to the $74.95 book: