Which Potting Mix is Best?

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. Stay tuned for 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
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
Osmocote Plus Organics Vegetable & Herb MixLight Green1/51N/A
Yates Premium Fruit & Citrus Potting Mix3 x Dark Green
2 x Light Green

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. Stay tuned for 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 07/04/17: We are currently in discussions with Scotts Australia regarding the performance of the Scotts and Debco products in our experiment. Stay tuned for an update on this in due course.

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.

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