How to Make the Best Potting Mix for Starting Seeds

, written by Benedict Vanheems gb flag

Making a potting mix

It’s that time of year again: seed sowing time! Sowing the first seeds of the growing season is incredibly exciting – just think of all that fresh, tasty homegrown produce to come!

To sow into containers you’ll of course need some seed starting mix, and a good mix can prove costly. Unless, that is, you make your own, which is what we’re going to do in this video and article.

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Seed Starting Potting Mix Recipe

The perfect seed starting mix mustn’t be too high in nutrients, which could harm delicate seedlings. The mix should also hold onto moisture without becoming soggy. Overly wet conditions can rot seeds and encourage fungal diseases such as damping off.

Our seed starting mix uses a soil-less recipe, so it’s beautifully light and fluffy and will promote good, strong growth and happy seedlings.

Begin with two parts compost as your base. All parts are measured by volume, so it doesn’t matter what you use to measure your ingredients, so long as you’re consistent. The compost slowly releases nutrients into the mix, which will help to feed seedlings as they grow. You can use your own garden compost, or buy some in. Break up clumps with your hands or, better still, screen or sieve the compost to get a fine, even texture.

“Mixing
A simple potting mix of compost, coir and perlite gives seeds an excellent start in life

Now add two parts coir (coconut fibre). Coir is extracted from coconut husks, making it a sustainable, plentiful alternative to peat or peat moss. Extracting peat damages fragile ecosystems and contributes to climate change, so we like to avoid using it. If your coir has come in a block, rehydrate it first by soaking it in a bucket with water until you can easily break it apart. If you prefer, you could substitute well-rotted leaf mould in place of the coir. Both coir and leaf mould contribute bulk to the seed mix, and are great for moisture retention.

Finally, add one part perlite, which will both lighten the mix and improve its air content. If you prefer not to use perlite then you could substitute sand, though it will give a heavier mixture.

“Perlite”
Adding perlite to a potting mix helps to improve its structure

Use a spade or your hands to mix all of the ingredients together. Take your time and be thorough – you want a consistent mix, with all of the ingredients evenly distributed. Once you’re done, store the starter mix in either a lidded container or in old compost sacks (or any other strong plastic sack) with the top rolled down tightly and secured. Store your mix in a dry, cool place.

Using your Seed Starting Mix

Moisten the seed starting mix before you use it, so it’s damp but not sodden. The mix can be used in module trays, plastic pots, seed flats, or any recycled containers suitable for seed sowing.

Gently press down your seed starting mix as you fill your container, and take particular care to properly fill at the corners. Top up with more mix if required. Don’t worry, this mix isn’t easily compacted so don’t be shy about firming it down so there’s enough mix for roots to explore.

“Sowing
Sowing into module trays of potting mix is an easy way to start seeds

Sow your seeds according to the packet instructions, then water. Watering requires some care – you don’t want to blast the mix out of the container, so use a mister or a watering can fitted with a very fine rose. Alternatively, make a watering bottle by piercing holes into the cap of a plastic bottle using a pin. Fill with water, screw the cap back on and you’re good to go.

Once the seedlings have germinated it’s best to water from below. Sit your containers in shallow trays of water for a few minutes until you can see the surface of the mix is moist. Remove containers from the water once you’re done so excess water can drain away.

Many seedlings need potting on into larger containers at least once before it’s time to plant them out. Most will be happy potted on into the same seed starting mix but hungrier seedlings like cauliflowers or tomatoes will appreciate something a little richer. Adding some worm compost to the mix like this gives it just the nutritional boost you’re after.

Potting Mix for Containers

Try this simple potting mix for plants to be grown on in larger containers. Thoroughly combine two parts garden compost with one part coir or, better still, leaf mould. Now add some perlite for drainage – about two to three generous handfuls to every 10 gallons (45 litres) of the coir-compost mix. A similar amount of worm compost can also be added for hungry plants, or incorporate a slow-release organic fertiliser according to the manufacturer’s instructions.

“Filling
Plants grown on in containers need a richer potting mix

Plants grown in the same container for a long time need a potting mix that holds its structure and is buffered against nutrient imbalances. Loam or good quality garden soil offers this. Simply combine one part loam – or screened or sieved garden soil – with one part garden compost then add some slow-release organic fertiliser. And that’s it – a versatile potting soil suitable for many containerised fruit trees, bushes and perennial vegetables.

Making your own seed and potting mixes like this can save you a lot of money. But perhaps most appealing of all is that you can tweak these recipes for what you’re growing. Of course, there are lots of other seed sowing and potting mixes out there, so if you’ve got one that works for you, why not share it in the comments section below?

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Comments

 
"Enjoy all of the info and ideas!"
Kathy Kutscherousky on Friday 9 March 2018
"Thanks Kathy, glad you've found it useful."
Ben Vanheems on Friday 9 March 2018
"DIY to me does not mean going to the shop, buying bags of stuff, bringing it home and mixing it. DIY means 'using whatever you have'! I don't have ANY of the ingredients you recommend, so I might as well buy it ready-mixed!"
My Name on Monday 29 October 2018
"That's a fair point. If you have your own leaf mould, however, you can really save a lot of money, adding only a few choice ingredients to make your own potting mix. Leaf mould is very much the 'DIY' element to this. In all cases, though, making your own potting mix, even when using bought-in ingredients, should save you money."
Ben Vanheems on Monday 29 October 2018
"In the east of Scotland, leaf-mould from deciduous trees needs two years to rot properly by itself; don't use evergreen tree leaves, e.g. Holly, as they rot much more slowly. Sycamore is good. Rake up in Oct., and Nov., make heaps, the bigger the better, near where you want to use it; aerate now and then by stirring and digging from the bottom; this definitely speeds things up. It's ready when you can put it through a quarter- inch sieve with ease. Lovely stuff, lovely smell! "
Megaloikos on Friday 4 January 2019
"Sounds like you've got leaf mould making down to a fine art. It is wonderful smelling stuff isn't it!"
Ben Vanheems on Friday 4 January 2019
"We started a worm bin a couple of months ago - can the worm castings be used instead of the compost?"
Kat on Friday 25 January 2019
"Worm castings (also known as vermicasts) are much higher in nutrients that compost, so aren't a perfect substitute for general compost. Worm castings won't 'burn' or damage plants though, so they are safe to use in quantity and are a great way to give a boost of nutrients to plants. However, it's quite a precious material - you don't get much of it generally - so use it wisely as a top-dressing around plants or lightly forked into the soil, in much the same way you would with any fertiliser. General compost is best used in bulk - to also improve soil structure, as well as nutrient content. "
Ben Vanheems on Monday 28 January 2019
"MAKE THE BEST SEED COMPOST. Thank you for the info on seed compost, I will be using that ingredients this year [2019] what I would like to know is what size sieve do you recommend, I have just bought a 3in1 sieve 3mm 6mm and 9mm"
les on Friday 1 February 2019
"A 3mm sieve is probably too small for sieving compost, so go for the 9mm or 6mm gauge sieve. The latter would be best for very find seed compost."
Ben Vanheems on Monday 4 February 2019
"If making your own compost from loam / garden soil, what advice would you give to get rid of the "resident" weed seeds that are already in the soil before planting your seedlings? Is it worth to allow the soil to cultivate the weed seeds for a few weeks prior?"
John Clarke on Saturday 16 February 2019
"If making your own compost from loam / garden soil, what advice would you give to get rid of the "resident" weed seeds that are already in the soil before planting your seedlings? Is it worth to allow the soil to cultivate the weed seeds for a few weeks prior?"
John Clarke on Saturday 16 February 2019
"You have a few options. You can spread it out and allow any weed seeds to germinate before removing and using. Or you can sterilise the soil in an oven or microwave, by raising the temperature of it to kill of any lurking weed seeds. Of course, sterilising in this way is only really practical for smaller batches of soil."
Ben Vanheems on Thursday 21 February 2019
"Thank you for your advice. Could you use vermiculite instead of perlite. Also, in regards to the measurements. If I add a 9L coir-peat. Then what would 2 part compost be - 18L? Thank you for your help "
Helen Nomlatyu on Saturday 9 March 2019
"Hi Helen. You could use vermiculite in place of perlite. And yes, two parts would be twice the amount - so if it's one part coir, then two parts compost would be twice the amount of coir. "
Ben Vanheems on Monday 11 March 2019
"Great article! Would this suggested seed starting mix be considered organic for veggie seedlings?"
Stephanie on Monday 26 August 2019
"Yes, if the ingredients used to make your mix all come from herbicide and pesticide-free sources and contain no artificial fertilisers, then you can consider the mixes to be organic. Certainly the seed starting mix would be organic, as you are only using compost and coir or leaf mould, plus the perlite."
Ben Vanheems on Friday 30 August 2019
"Very helpful, thank you"
Anthony Worthington on Saturday 30 November 2019
"What is the best soil to buy that is already prepared? Can you use glass containers to sow the seeds in?"
rt on Sunday 1 March 2020
"It's hard to recommend specific brands, as these will vary across regions. But if you are looking to sow seeds then any seed starting mix/propagation mix would be perfect. Or for more general use an all-purpose/multi-purpose potting mix, which you can sieve for seed sowing. You can use any container for sowing into, so long as it has drainage holes at the bottom."
Ben Vanheems on Monday 2 March 2020
"Thanks for the information and clear pictures! Have not started from seed for many years, so wanted a brush-up and you inspired me to switch from peat to coconut coir. Thank you for helping this gardener run a cleaner garden with fewer environmental impacts. What kind of containers are you using and what do you recommend?"
Christina on Friday 27 March 2020
"For sowing I recommend a few types of container. Simple 3-4in (7-10cm) wide containers are great for sowing seed like tomatoes, which are then pricked out/transplanted into their own pots once they are big enough to handle. For vegetables or flowers you will be growing a lot of, a seed tray works well, giving more surface to sow into. But I wouldn't be without plug trays, also known as module trays. You can sow one seed or a tiny pinch of seeds per plug/module, then grow the seedlings on until they are ready to plant out. This avoids having to prick out seedlings and is great for many crops, from salads to cabbage."
Ben Vanheems on Monday 30 March 2020

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