January 24, 2023

Not All Potting Mixes are Created Equal.

If you’re relatively new to gardening, the world of potting mixes and soils can feel a little overwhelming. If you don’t know what you’re looking for, it can be tempting to assume they’re all pretty similar or to pick one almost at random.

However, there is a lot of variation between potting mixes, and they’re certainly not created equal. Choosing the right potting mix can often be the difference between your plants living and dying.

In this guide, we’ll help you understand the basics of Potting Mix and soil to help you make sure you’re choosing the right blend for your plants.

We’ll talk about what a high-quality potting mix should contain, and the science that goes into understanding how potting mixes work. We’ll also talk about different potting mixes for different types of plants, and a few things you should always avoid in a mix.

What Is Potting Mix?

Potting mix is a combination of different minerals and organic materials that are beneficial to the growth of your plants. Most potting mixes will contain soil as part of this mix, and are sometimes referred to interchangeably as ‘potting soils’. Other mixes are totally soil-free.

The goal of a potting mix is to replicate the ideal soil conditions for particular groups of plants. This is not just relevant for potted plants, but also for outdoor garden beds, where the natural backyard soil conditions may be nutrient-deficient, unsuitable or unpredictable.

Generally, there are three basic types of soil that plants will grow in:

Sandy Soil is loose, gritty and will fall apart in your hand. It’s light and drains very fast, but it doesn’t retain nutrients well.
Clay Soil is tightly packed, heavy and firm. It holds water and nutrients easily but it drains poorly.
Rich Composted Soil is nutrient-rich and contains organic matter (i.e. compost or loam). It’s slightly loose, fertile and drains just right. It should crumble gently in your hand when poked.

While plants from some specific regions will prefer sandy or clay soils, most plants are looking for rich and fertile soil that is relatively loose, drains easily and contain a good amount of decomposing organic matter for extra nutrients. Therefore, most premium general-use potting mixes are formulated to recreate this fertile & loamy soil environment.

What Should a Premium Potting Mix Look Like?

A good-quality potting mix should be formulated to contain the perfect balance of different nutrients in order to encourage healthy plant growth.

The combination of ingredients in a general-purpose potting mix also shouldn’t adversely change the soil acidity, unless that is the desired effect. Finally, a premium mix should also contain a mix of inorganic matter to help loosen up compacted soil and improve drainage. Volcanic rocks like Perlite or Vermiculite are highly popular drainage additives.

When it comes to organic materials, some are more highly prized than others for their various properties. For instance, Peat Moss is considered a premium soil additive as it absorbs a lot of moisture and then decomposes extra slowly, allowing the soil to hold onto water and nutrients for much longer. It’s also slow to grow and farm, making it a rare material.

Other plant-specialised potting mixes – like mixes designed for fruit trees – may decide to use higher amounts of ingredients that are full of useful secondary nutrients, like Calcium and Magnesium. It’s not always as simple as ‘good’ or ‘bad’ potting mix – it’s all about thoughtfully combining ingredients to perfectly meet the needs of the plant at hand.

The Science of Potting Mix and Soil Ingredients

There are several natural elements in soil, many of which are absorbed by plants and converted into available nutrients. Their naturally occurring levels can be supplemented by adding fertiliser or decomposing organic matter like compost or manure to the soil. You’ll want to look for ingredients that contain these important elements when you’re choosing a potting mix for your garden.

NPK

Of the chemical elements naturally present in your soil, the major ones that affect plant health include Nitrogen (N), Phosphorus (P) and Potassium (K). The levels of these three nutrients are called the NPK Ratio, which is outlined on every bag of potting mix. They’re also the key ingredients in most mainstream fertilisers.

All three of these nutrients are naturally produced when bacteria break down the organic matter in potting mix, compost or soil. Together, NPK plays a highly important role in fueling the photosynthesis process and aiding the growth of leaves, root systems, new shoots and flowers.

When choosing a potting mix for its NPK ratio, consider that some plants may have different requirements. For instance, too much nitrogen will cause some flowering plants to divert energy towards growing foliage instead. Meanwhile, some Australian Native Plants (like Banksia) are sensitive to phosphorus. It’s always a good idea to research your specific plant before choosing a potting mix.

Secondary and Trace Elements

Aside from NPK, there are many other elements present in soil and potting mix that can contribute to plant health. The ones that are required in larger quantities – Sulphur (S), Calcium (Ca) and Magnesium (Mg) – can be thought of as Secondary Elements. Other elements – like Iron, Zinc or Calcium – are only needed in trace amounts. Adding more than that trace amount can often cause negative effects.

These other nutrients aren’t as vital as NPK, but they still play important roles. Some are commonly found in premium potting mix, or in specialised potting mixes and fertilisers designed for different types of plants. Important secondary and trace elements include:

Calcium (Ca): Helps to build strength in cell walls, similar to how it functions in our own bones. Most soil will naturally have good calcium levels, but acidic soils may be lacking.
Magnesium (Mg): Magnesium is a vital part of the chlorophyll particle, the key ingredient in the photosynthesis process. Trace quantities of Iron (Fe) and Manganese (Mn) are also required for similar reasons.
Sulphur (S): Helps to form proteins and is also involved in the production of chlorophyll. Adding Gypsum to soil can often improve sulphur levels.
Copper (Cu) and Zinc (Zn): These minerals activate plant enzymes that help to keep leaves green and lush. Deficiencies can be common in highly acidic soils.
Boron (B): Helps to grow tissue in roots, shoots, flowers and fruits. It can be deficient in soils that are too alkaline, which can lead to soft, weak fruit and veggies.

Acidity

Wondering what we mean when we talk about alkaline and acidity? Well, the combination of these elements also contributes to one other important factor – soil acidity.

Soil acidity is measured on a pH scale, which counts backwards. Anything above 7pH is considered Alkaline, soil with a pH of 6 or less is Acidic, and a pH of 7 is Neutral. The lower the number, the more acidic the soil. This applies to potting mixes too.

Different plants will have different acidity preferences, so it’s something you need to keep in mind. You can read more about soil acidity in our Guide to Garden Basics. If you’re interested in a deep dive, this scientific report covers the topic in comprehensive detail.

Diaco’s Premium Potting Mix

As you can probably tell, we have a lot to say about potting mix ingredients and their relative quality! To that end, our team of expert gardeners have used their combined knowledge to create our own Diacos Premium Potting Mix, a highly effective general-use potting soil formula that complies with all Australian standards for a premium-grade potting mix.

It includes premium ingredients like sphagnum peat moss, perlite, vermiculite, and lava rock. These ingredients work together to aerate the soil, retain moisture, and provide essential nutrients for plant growth. It’s also made with slow-release fertiliser pellets that will feed your plants nutrients for up to 3 months – cutting down on the time you need to spend fertilising.

Finally, we’ve added a wetting agent to help the soil absorb water more easily, which is especially important during the hot summer days that we experience here in Melbourne.

A Guide to Buying Different Potting Mixes

Our Diaco’s premium blend is a fantastic choice for general use, but sometimes it’s a better idea to use a mix that’s been specifically designed for the type of plant you are growing. Here are some of the most popular specialised potting mixes that we stock in our online store:

Australian Natives need potting mixes that are low in phosphorus. Australian soils are naturally low in this nutrient, and many popular native plants have adapted to this over time. This makes them extra sensitive to the high amounts found in mainstream potting mix and fertiliser. We stock this fantastic blend from Attunga Native Potting Mix that does a great job nurturing your natives.
Succulents are native to harsh, sandy environments. They tend to prefer a looser potting mix that contains a higher quantity of sandy soil, as well as rocks like Perlite. This provides lots of extra aeration and faster drainage, replicating the perfect semi-arid environment for succulents to thrive. Specialised Cacti and Succulent Mix can also be found in our store.
Fruit and Citrus Trees enjoy nutritious, loamy soil with great drainage. Typically, a fruit and citrus potting mix might include roughly equal parts of organic compost, sand, perlite (or similar), and peat moss to optimise drainage and nutrition. High-performing mixes like Grow Better’s Fruit and Citrus Potting Mix will also contain a slow-release fertiliser full of vital nutrients that help the fruit to grow.
Gardenia, Azalea and Camellia all prefer acidic soil. Therefore, an ideal potting mix for these popular plants will also need to maintain the correct pH levels. We stock a great Gardenia, Azalea and Camellia Potting Mix from Grow Better that contains the perfect blend of nutrients and trace elements to provide nutrition and maintain acidity. The addition of a rapid-soak wetting agent also helps to provide extra hydration.

Potting Mixes to Avoid

Finally, we’ll leave you with a few quick things to avoid. Yes, we did just say that it’s not always as simple as ‘good’ or ‘bad’ potting mixes – but there are definitely some red flags out there! You won’t find any of these issues in our store, but it’s always good to know what to watch out for:

Uncertified Potting Mix.: Potting mix sold in Australia has to pass Minimum National Standards. If a brand of potting mix just looks a bit off, and you can’t find any evidence that it complies with these requirements, it may be worth giving it a miss.
‘Super Cheap’ Potting Mix Healthy, good-quality soil additives cost gardening suppliers money to source or produce. If someone is selling a bag of potting mix at shockingly low prices – think around $5 or less – then it’s likely to be very low quality. It’s not worth the risk of damaging your plants!
Unclear Ingredients. Finally, if you’re not sure what’s in a potting mix, think twice before using it. NPK ratios and even secondary elements should be clearly labelled somewhere on the bag or at least in the store. If they aren’t, it’s a bit fishy. It’s best to avoid it as you don’t know the outcome it could have on your plants.

Understanding the difference between high-quality and low-quality potting mixes can help you to choose the right one for your garden. Ultimately, it will help you to grow stronger and healthier plants. Happy gardening!

Hopefully, this guide has helped to give you a better understanding of what goes into a potting mix, the differing quality between potting mixes, and the way that it all works.

If you’re not sure which Potting Mix to use in your garden, or you’d like to know more about our Diacos Premium Potting Mix, then just ask our friendly and helpful staff. When it comes to finding garden solutions, we’re always happy to help!