Which Potting Mix is Best? 12


When people try to grow plants in containers, one of the most common pitfalls that I see, is the use of poor quality soil, or potting mix. Last summer we conducted a small experiment to demonstrate the importance of soil in growing great veggies. We produced some very interesting results from the experiment, and generated quite a deal of interest from our gardening community.

This summer, we expanded the experiment to cover a wider range of potting mixes. We also investigated a range of off-the-shelf fertilizers (mostly certified organic ones). This was to see what combinations might work well for our container gardening supplies, for those of us limited to major hardware chains and local nurseries. We wanted to see their effectiveness at improving the plants’ ability to grow in poor quality soils. See our blog post on our Fertilizer experiment for more information on this.

Potting Mix Experiment Round Two

Which Potting Mix is Best? Melbourne

Ten of the different potting mixes we trialed in our experiment

In the first round of our soil and potting mix experiment we looked at only four different types of soil and how effectively they could support plant growth. This year we compared 11 different types of potting mix and soils. Most of the varieties were purchased from “The Big Green Box” and covered a range of potting mixes, including premium potting mixes and certified organic potting mixes. We also purchased a bulk potting mix, and a veggie mix from Fulton’s, one of our local garden supplies centres. A summary of the potting mixes and soils are tabulated below.

Potting MixPurchased FromCostBag SizeOrganically Certified?Meets Australian Standard AS 3743?
Fulton's Five Way Soil MixFulton's$80Per Cubic MetreNoNo
Natures Soil Potting MixNature's Soil, Silvan$625 LitresNoNo
Scott's Pure Organic Premium Potting MixBunnings Warehouse$13.9925 LitresYesYes
Earthwise Growing Essentials Multipurpose Potting MixBunnings Warehouse$2.4525 LitresNoNo
Brunnings Tomato and Vegetable Growing MixBunnings Warehouse$3.9925 LitresNoNo
Fine Farms Blended Cow ManureBunnings Warehouse$3.9525 LitresNoNo
Debco Organic Potting MixBunnings Warehouse$13.9825 LitresYesNo
Fulton's Bulk Potting MixFulton's$240Per Cubic MetreNoNo
Osmocote Professional Premium Plus Potting MixBunnings Warehouse$10.9725 LitresNoYes
Osmocote Plus Organics Vegetable & Herb MixBunnings Warehouse$9.9825 LitresNoYes
Yates Premium Fruit & Citrus Potting MixBunnings Warehouse$8.9830 LitresNoYes

All potting mixes, except for the Scott’s Premium Organic Potting Mix were purchased no more than the week prior to the experiemtn being set up. The Scotts Premium organic mix was a bag left over from our first experiment from last year. The bag had been sitting, opened, but under cover for 12 months.

We had only intended to use 10 potting mixes and soils. However, when purchasing the products for our fertilzer experiment, I came across the Fine Farms Blended Cow Manure. The word “blended” piqued my interest and upon reading the product description, it sounded more like a growing medium, than a soil conditioner. So we decided to use this as an eleventh “potting mix” in our test.

Of the 11 mixes tested, four of them were the same products tested in our first experiment from last year:

What does potting mix that complies with the Australian Standard (AS 3743) mean?

Which Potting Mix is Best? Melbourne

Potting Mixes with the AS 3747 Redtick will support plant growth for at least three months

Potting mixes can be certified to Australian Standard AS 3743. Redtick, or premium grade potting mix include a minimum supply of nutrients from fertilizers such as a controlled release fertilizer. They may also have and other additives such as water crystals and wetting agents. To ensure that the mixes conform to the Australian Standards, a number of different tests are completed on raw materials and throughout the manufacturing process. To use the word “premium” on the packaging, the potting mix must comply to this standard. Beware of companies labeling their product with synonyms of “premium” (such as “professional” or “superior”) as these usually aren’t compliant with the Australian standard and can be misleading to the consumer. Premium potting mixes tend to be more expensive due to the accreditation process involved.

What does “organic” potting mix or soil mean?

When it comes to soil, there can be two uses of the word “organic”. Organic can simply refer to the carbon based, organic matter in the soil. This may have come from manures, leaf litter, woody debris etc. Most soils have a component of organic matter. Soils high in organic matter are great for growing veggies.

The second use of the term “organic” refers to “organically certified soils”. These soils are certified to comply with one of the several organic certification standards. These soils are free from inorganic inputs and are suitable for growing organically certified produce. Organically certified potting mixes tend to be more expensive and often don’t meet the Australian Standard for premium potting mixes.

You’ll often find garden supply yards labeling their bulk soils as an “organic soil mix”. This refers to the fact that the soil is high in organic matter, not that it is organically certified.

Setting up the potting mix experiment

Which Potting Mix is Best? Melbourne

We purchased 20 punnets of seedlings that all appeared to be of equal size and equal condition.

We purchased 20 punnets of capsicum seedlings whilst at Bunnings purchasing our various potting mixes for the experiment. These were all relatively the same size, in the same condition and appeared to have been sown at the same time. Using each of the different potting mixes and soils, we potted up five of each of the seedlings. We tried to make sure that each soil had a seedling from five different punnets to reduce the chance of experimental error.

The potted-up seedlings were then placed in our greenhouse. The pots were placed out in a pattern to ensure that the five pots of each of the soils were spread across the shelf. Each pot appeared to have equal access to light and water. The automated irrigation system watered every morning with a fine mist for 5 minutes. After six weeks, we assessed the plants and took photos of the results.

Results of the Potting Mix Experiment

We assessed each of the plants and recorded the colour of the leaves as well as the number of plants in each group that had set fruit. We also recorded the total number of flowers and fruit set on the 5 replicate plants combined. See the results tabulated below.

Potting MixColour of LeavesNumber of Plants With FruitTotal Number of Fruit across the five plantsOther Comments
Fulton's Five Way Soil MixLight Green1/51Weeds growing in 3 of 5 pots
Natures Soil Potting MixDark Green5/516N/A
Scott's Pure Organic Premium Potting MixYellow0/50N/A
Earthwise Growing Essentials Multipurpose Potting MixYellow0/50N/A
Brunnings Tomato and Vegetable Growing Mix2 x Dark Green
3 x Light Green
4/56N/A
Fine Farms Blended Cow ManureDark Green3/54N/A
Debco Organic Potting MixLight Green0/50Flowers present, but no fruit. Tall and spindly plants.
Fulton's Bulk Potting MixLight Green0/50One flower present, but no fruit. One plant showing evidence of disease.
Osmocote Professional Premium Plus Potting Mix4 x Dark Green
1 x Light Green
1/55N/A
Osmocote Plus Organics Vegetable & Herb MixLight Green1/51N/A
Yates Premium Fruit & Citrus Potting Mix3 x Dark Green
2 x Light Green
2/55N/A

We also took photos of the plants. Below is an image of the best plant out of the five replicates for each soil type. As you can see there is a huge variation in the performance of each of the potting mixes.

Which Potting Mix is Best? Melbourne

The results from our potting mix experiment

So what does it all mean?

Never underestimate the importance of good quality soil in growing your fruit and veggies. Interestingly, price isn’t always a good indicator of quality. We found that some of the best results came from cheaper potting mixes such as Brunnings Tomato and Vegetable Growing Mix. As a general rule, the certified organic potting mixes performed fairly poorly. Which is understandable given they are not conditioned with inorganic fertilizers.

Keep in mind, that the premium potting mixes are only confirmed to support plant growth for the first three months. If you’re growing veggies, and especially fruit trees, in pots, over an extended period of time, it’s likely that you’ll need to top up the soil nutrients. You may be interested in the second part of our experiment, which looks at a range of off-the-shelf fertilizers that you may like to try.

On 03/04/17 we contacted the following manufacturers or suppliers of each of the products investigated:

We invite them to leave feedback on our experiment in the comments section below.

Update 30/04/17: We have finalised discussions with Scott’s Australia regarding the performance of the Scott’s and Debco products in our experiment. See below for details.

Disclaimer: All of the products tested and materials used in this experiment were purchased by Leaf, Root & Fruit at full retail price. No incentives or sponsorship are received from any of the companies mentioned in this blog post.

Response from Scott’s Australia regarding the performance of the Scott’s and Debco products in our potting mix experiment.

After releasing the results of our potting mix experiment, Scott’s responded with a series of phone calls and emails. We provided Scott’s with information relating to some of the batch numbers of potting mix used and further clarified how the experiment was run. They provided this written response. We have a few comments regarding their response:

Scott’s cannot ensure that the product used were appropriate for trialling based on the ages found. The data described is not a true reflection of Scott’s and Debco quality and provides a biased view we believe based on bag age alone. Therefore the integrity of this trial has been placed in jeopardy when the data is stacking 1 year old mixes against other unknown aged products.

The trial run the year previous (Dec 2015-Feb 2016) was with the Scott’s Premium Organic, which was purchased the day the experiment was set up. It had the same results as this year. So I’m not sure that the shelf life did have that much to do with the poor performance of that product. This is unless it had been sitting in Bunnings for considerable time before we purchased it. However, Scott’s response indicates that this is unlikely to be the case, due to high turnover of their product. It would be ideal to test all potting mixes fresh off the production line, and compare apples with apples, rather than apples with oranges. However, this experiment reflects the actual use of this product by the public. If a true, premium graded product, performs as the consumer expects it to, then this shouldn’t affect the results.

The Australian Standard recommends a best before use date. This is a process Scott’s has considered but also knows from 6 years of sales data that no growing media is available for sale in stores longer than 4 months from delivery date.

Other than the Scott’s Premium Organic, ALL potting mixes used in the experiment were purchased in the week prior to the experiment being set up back in December 2016. We have receipts available to substantiate this. The Scott’s Pure Organic Potting mix manufactured on 19th February 2016 was 10 months old when we purchased it in December from the Hawthorn Bunnings Store. If potting mixes have such a short shelf life, then we really can’t comprehend why they are not applied to the bags. Otherwise, manufacturers are setting unknown consumers up for failure. In many cases, consumers will not be using the potting mix the day they purchase it. It may sit in the garden shed for months or even years prior to use. Leaf, Root & Fruit are trying to support backyard growers to be more successful when trying to grow their own produce. A shelf life applied to the potting mix could further assist our work.

Aspects of our Pure Organic Premium Potting and Planting and Debco Organic were labelled as high priced and ineffective in growth. When comparing organic lines against non-certified you are making large comparisons that cannot be measured equally.

This experiment looked solely at the performance of capsicum seedling growth in a range of potting mixes. There are plenty of other ways to assess performance. But we feel that this (admittedly simple experiment) is a great way to do this on a tight budget. Readers of our blog post are welcome to make their own decisions about which potting mix to use based on their preference for organic vs non-certified vs budget vs performance.

 

Leaf, Root & Fruit would like to thank Scott’s Australia for their response. It has certainly been an eye opening experience and helped us to understand the industry better. It has also raised a few concerns for us regarding the consistency of potting mix quality and performance that each product may afford the consumer. We’ll look to repeat this experiment again in the future and would appreciate feedback from the public on how it may be improved to obtain even more meaningful results for our readers and gardening community.


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12 thoughts on “Which Potting Mix is Best?

  • Polly

    Thank you for this experiment. Some of your results mirror my experiences with vegetable growing, and what I was already suspecting eg. higher price does not always ensure a better outcome of fruiting.

    • Leaf, Root & Fruit Post author

      Hi Polly,
      Thanks for your feedback. Hopefully you’ve managed to find a reliable product that suits your needs.
      Good Luck and Happy Gardening!
      Duncan

  • scotty 2 hotty

    why is it so bloody hard to get a bag of compost thats actually compost these days, i know what you mean about the cow shit i noticed a bag of that yesterday all finely broken down and i was scared to smell it cause it didnt say composted but omg im adicted to the smell it smellt like rainforest soil or something such a nice smell, i think ive had enough of buying animal by products tho as a vegan.. might just go back to using dirt out of the ground untill my own compost is fully broken down, f u scotts whys it so hard to care about the people and test your product before you sell it, perhaps they thrive off wasting lives, they do have a long time partnership with monsanto maybe they just want us to fail so we have to buy food or something, conspricies lol im just upset sorry, glad to caonfirm with your research thankyou for sharing

    • Leaf, Root & Fruit Post author

      Thanks for your thoughts Scotty. Yes, it’s a good idea to make your own compost if you have the sapce and facilities to do so.

      Good Luck and Happy Gardening!

  • scotty 2 hotty

    it is very possible that scotts doesnt know that certain barks are alleopathic, or that woodchips rob nitrogen and alter the ph of the soil which is non existent (lol), so no matter how much nitrogen u feed it it wont come out of the funk it just seems to make the dying happen faster

    • Leaf, Root & Fruit Post author

      Hi Scotty, There are many reasons why their potting mix may not have performed well. Your theory on woodchips robbing nitrogen etc seem plausible to me. I suspect that the quality of the input may have had an impact, but as I said, so could many other factors.

      Thanks for your thoughts.

      Duncan

  • Annie

    I am checking out soils and potting mixes because I am experiencing an epic fail in my new garden. I have a long and successful experience with gardening and put in a new garden December last year using Richgro soil and Richgro organic mix. Results are awful , in the first 3 months soil levels as expected were down a third (I had overfilled the garden beds)and the soil had either separated into sawdust or around the bigger plants impenetrable blocks of mouldy looking stuff. Nothing much was growing even with extensive fertilising. Couldn’t keep it wet either. Ive filled the garden beds up but the levels just keep sinking and I’m faced with pulling everything out and starting again. How do they get away with this and why wasn’t Hortico used in the test?

    • Leaf, Root & Fruit Post author

      Hi Annie,

      Sorry to hear that you haven’t had the best results. We conducted this test on a limited budget (as it was it cost us several hundred dollars in materials to run the experiment). So we didn’t have the budget or space to test every single potting mix product out there. As it is, I suspect that each batch of potting mix is going to give you different results anyway. The experiment is more to highlight the importance of using good quality (not necessarily expensive) soil or potting mix in getting good results in the garden.

      Are you using potting mix to fill a raised garden bed? I would recommend that in future you use a bulk, premium blended soil to fill raised garden beds rather than potting mix. Especially if you have clay soil underneath. The negative charge of the clay soil may be why your potting mix is becoming very hydrophobic.

      Thanks for sharing your experiences with us.

      Best of luck with any future attempts that you make!

      Duncan

  • Nic

    Nice experiment. I just planted out 8 tomatoe plant seedlings in Bunnings tomatoe and vegetable growing mix and one in a potting mix from a local nursery in Hobart (Chandlers) which costs 3 times as much as the Bunnings. Two observations after two weeks:
    1. All tomatoe seedlings in Bunnings soil show yellowing leaves especially at base and some show a discolouration and stunting of upper leaves on large plants. The seedlings in Chandlers soil looks good and just like the ones in adjacent garden.
    2. The miniature capsicum plant in Bunnings soil looks fine as well.
    I suspect that the Bunnings mix is not mature enough and as someone above suggests is robbing nitrogen from the soil. I am adding a liquid fish waste mix in an effort to correct this. It also seems that tomatoe plants which are very fast growers may have a higher nutrient demand than other vegetables which makes the choice of potting mix even more critical than in your experiment.

    • Leaf, Root & Fruit Post author

      Hi Nic,

      Thanks for sharing the results from your own trials. Interesting observations you’ve made regarding the tomatoes versus capsicum seedlings. It might be worth us investigating that further.

      Please keep us updated on the progress of your trials.

      Good Luck and Happy Gardening!

      Duncan