Garden

Make your own potting mix

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Over the last week, I have been asked at least once a day for my recipe for potting soil. So I thought this was a good opportunity to share it with you guys, but also to talk about it and help us all get to grips with all the terms we see out there. Firstly, let’s tackle the elephant in the room.

Although you might hear the term potting soil… actually it very rarely contains soil, generally we are referring to a mix of ingredients perfect for your plants growth, a potting mix. Let me explain.

What is soil?

When we say soil, usually we are talking about the stuff that our lawns and garden plants grow in. But you might also hear folk mention potting soil. Unfortunately, this is a bit of a misunderstanding, so let’s get comfortable with the basics so we can work out what we are talking about. Soil is a mix of organic matter and inorganic matter that forms over time. It’s mostly inorganic like clay, silt, sand and rock particles with a small percentage of organic matter like rotten leaves and other plant material. Because it’s mostly inorganic particles it’s quite low in nutrition and can be quiet heavy. Potting mix, however, is the opposite. It’s mostly organic materials like composted garden waste with a smaller amount of inorganic material added so it’s much higher in nutritional content and can be created to meet the needs of specific plants. However, it usually contains absolutely no soil ( not always, but usually). The reason it can sometimes be referred to as potting soil is because soil is used as a generic term for ‘dirt to grow stuff in”.

So what is potting mix or potting soil?

Now, when we grow plants in containers and pots, we don’t use soil from the garden as obviously this is a whole different environment and might not suit our needs. Generally what we do is we use a potting mix, which is a mix of specific organic and inorganic materials to suit our plants’ needs. After all, we wouldn’t grow a cactus and a fern in the same pot, would we?

Now some of you have also been talking about how confusing all these different terms are, especially when you are in the garden centre looking at the bags of “soil” they sell. Why the different names and why the different prices?

So lets put it this way. Soil – usually called topsoil is for the garden. It’s mostly inorganic materials and of low nutritional content and can be quite heavy. Compost is 100% organic – it’s basically decomposed vegetation like garden waste or kitchen waste or could be well rotten manure. Usually, you will see either peat or coir mentioned on the packaging. Now you will also hear potting mix, seed sowing mix, grow bag, or even soil improver – I’ve had a look at my local garden centres and found that most things are actually 100% compost regardless of the name. Except when it says mix or even soil, then it tends to be compost with additives. Basically, they are all a mix of organic and inorganic materials to suit a specific need. But I would 100% recommend checking the ingredient before you buy, as doing a bit of research for this video, I found at least 8 differently named products that were all 100% compost. But a fancy name meant they were way more expensive than the same brand item labelled compost. Just be aware before you part with your cash.

So mix… compost with additives. Let’s talk specifically about that so that you can decide how to make the perfect potting mixes for your growing.

So as I said – potting soil/mix – is actually not soil but a mix of materials which may include soil or may not, hence why you may also see it labelled potting mix or container mix. So in our last video, I mentioned how seed sowing doesn’t require a mix of any particular ingredients, just something very fine, very light and free draining. However, once you start talking about actual plants, they do need a little bit more from their “dirt”. Now, you can just pot up plants in compost with absolutely no additives, and that is exactly what I do for everything I grow in my greenhouse. Because I grow in a system where the plants are watered from below using wicking matting, I don’t need to consider additives to improve the environment for the plants. But for the pots out in the garden, I do. So what I’m going to do, is take you through the various additives you can use to make potting mix and their use and that way you can decide on your own recipe, perfect for your plants. Because there is no benefit in learning to copy someone else’s recipe by rote, it’s much better to understand why they have that recipe so you can adapt it or not to suit your needs.

What’s in a mix?

Well, basically the majority as I’ve already said is organic in the form of compost or coir. Peat used to be the standard in all compost because it is great for water retention as well as providing nutritional elements but these days folk are tending to look for alternatives for environmental reasons, so you are more likely to see coir which is made from the husks of coconuts. That is your organic base.

If you’ve never used coir before, you can buy it as these compressed bricks. Then you add water to let it rehydrate. You’ve be surprised just how much hydrated coir one little brick gives you.

Then in smaller amounts, you could see perlite added, which helps to keep the mix very free-draining – you could replace this with fine-grit if you want but that would be a heavier mix as perlite is very, very light.

You may also see vermiculite which also helps to keep things free draining, but is more water retentive than perlite so a bit of an in-between and lastly, you might see some sand added. Sand helps to keep the mixture loose but it’s more compacted than grit or perlite and can make your mix tight and heavy, so don’t go mad with the sand. These are your inorganic materials, they are usually there to help add drainage, airflow and bulk but don’t offer any nutritional value. So you have your organic base and your inorganic amendments to change the characteristics to suit your needs. On that note – with peat-based products, you may also see lime added, this is because peat is slightly acidic and the lime is needed to bring the balance back.

My potting mix recipe

So we’ve already said anything you use for seeds/seedlings needs to be super fine. But also, you may adjust things to suit, for examples, cactus or succulents… you wouldn’t want to add any moisture retention, in fact, you may actually go completely in the other direction. Whereas for a fern, you may want rich organic mixes so that they hold the moisture. For me, I am mostly growing small shrubs, fruit bushes, and the occasional flowers so I have a pretty standard mix. I like a bit of drainage because we get a lot of rain in Scotland, but I also want something rich in nutrients. So my basic mix I use for most of my plants is

  • 2 parts coir for bulk and air
  • 2 part compost for water retention and nutritional value
  • 1 part perlite or horticultural grit for drainage

How do you decide what should be in your mix?

Well start with a basic recipe (feel free to use mine), and then you can decide if you want it to behave a little differently. For example – you notice it’s a bit too wet for your liking, so you want it to dry out quicker. In that case, you might want to add more perlite or grit. What if your mix dries out too quickly? Well, that’s ok, just increase the amount of organic materials – compost or even add vermiculite. It’s all about getting your perfect recipe 😀 And the benefit of having your own recipe? It’s often a lot cheaper to make up your own than buy it ready-made, especially if you have a lot of pots to fill.

One thing to remember though, if you are using this for indoor pots and containers, remember to sterilise your organic materials like compost to kill any beasties you don’t want to bring inside.

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2 Comments

  1. Great article Eli. Just what I was looking for. I cannot find Coco coir in my area ( I have to buy it online) which makes me not buy it because I do not like to pay for shipping (or pay for Prime). So I tend to go with the Peat because I can find it. I do have several compost bins (which is not my strong suit but I will do better) which I add in addition to my potting soil mixes. My big question is — do you have to dump your soil out of your pots? I would think so if there was a problem and what you were trying to grow had bug issues or something bad —then yes but what if there was not a problem? Could you just sterilize the soil (and when I say sterilize I mean pouring hot water in the soil)? Thanks

  2. Hey Renee,
    You don’t have to replace your soil unless there is an issue like disease. What I do when required is to take the top layer of soil and replace it with compost to add organic matter back. Once your pots have been planted up, the plants pull the nutrients from the soil and organic matter breaks down and reduces, so you are simply replenishing it.

    If you have disease, I wouldn’t risk reusing the soil. But if you are really wanting to reuse soil and wanting to make sure you are getting rid of fungus or other nasties, I would go with heat to sterilise instead of boiling water.
    To give you an idea…

    Preheat an oven to 180c

    Put your soil in an oven proof dish in a layer about 3 inches thick, loosely cover with foil and bake.
    Use a thermometer to check the tperature in the centre… 30ins when the centre is
    50c this will kill most moulds
    At 65c it will kill most pathogens, fungus, bacteria, worms, slugs, centipede etc
    75c plant pathogens and soil insects
    85c most weed seeds
    100c heat resistant viruses and weed seeds

    O used to be one of those people that always replaced soil but the first lockdown taught me to waste less and reuse when appropriate.

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