Tuesday, May 28, 2013

What is the best seedling mix?

Here is another post in my ongoing experiment to learn first hand the best way to raise vegetable seedlings. So what is the best seedling mix? But first lets clarify what I mean by ‘seedling mix’, I mean the soil used to sustain an infinite plant, not the material used to germinate seeds. They are very different and I have written previously about the best seed starting mix.

I tested 8 different mixes each on a set of 4 different plants - broccoli, lettuce, beetroot and spinach. Here are the mixes:

  1. 1:1 River Sand, Commercial Compost
  2. 1:1 River Sand, Homemade Compost
  3. 1:1:1 River Sand, Commercial Compost, Peat (fine grained coir fiber)
  4. 1:1:1 River Sand, Commercial Compost, Composted Cow Poo
  5. 1:2 River Sand, Commercial Compost
  6. 1 Commercial Seedling Mix
  7. 1:1 Commercial Compost, Peat (fine grained coir fiber)
  8. 1:1:1 River Sand, Commercial Compost, Dry Horse Poo (uncomposted)

What surprised me the most is that overall they all did ok. I think that goes to show that plants are quite robust and adaptable to their soil conditions. I guess as a plant you would want to be - you never know whose garden you are going to end up in. But when raising seedlings a plant that can be ready for transplant into the garden a few weeks before another is important, so worth while getting the mix right.

Mix #8 on the right
The poorest performer by far was #8 - equal portions of River Sand, Commercial Compost, Dry Horse Poo (uncomposted). No idea why, thoughts?

Mix #5 - 1 part River Sand, 2 parts Commercial Compost - did really well. Comparing to #1 with only 1 part compost, it would appear more compost is better. Sadly I didn’t think to test a pure compost mix, put I would assume there is a limit. Mix #2 with 1 part homemade compost however also did equally well to #5. Which I am pleased to say, indicates my homemade compost is double the strength of commercial compost. This does concur with what my mum always says “nothing beats homemade”.

Left mix #1, Right mix #6
For some time I have been convinced that commercial mixes are not worth the bag they come in. So it brings me great pleasure to announce this next finding. Mix #6 - commercial seedling mix - even though performing ok, did not perform nearly as well to majority of the other mixes, including the simple mix #1 of 1 part sand to 1 part compost. It was very evident in the large pots as seen in the images above. I have of course only tested one brand of mix, but I did select an expensive one. Premade mixes are quick and easy but mixing my own really wasn’t that hard either.

But the winner is...

For overall best performance across the 4 different seedlings the winner is #4 - equal parts of River Sand, Commercial Compost, Composted Cow Poo. Again it is a 2 parts compost to 1 part sand mix, confirming a higher ratio of compost is better. I suspect the commercial compost I bought is made primarily of tree matter thus the inclusion of the manure compost gave it a better balance of nutrients. I now wish I had tested 2 parts homemade compost as I suspect it may have been in the running for first place. My homemade stuff is already balanced with a good selection of organic sources.

Do you have a winning mix?


  1. Hi Jason. Just been catching up with all your experiments having been away from blogging for a few months. What a great idea to test all the different mixtures and even better getting results that you can rely on. I use a homemade mixture and haven't ever bought a commercial compost but I do use both coir and sand as fillers which give great air space needed for root growth. Like you, one of my main problems is forgetting to water sometimes which has been a disaster, especially in summer. And now that the cooler weather has hit, I'm just about to fit out my seedling tray with a plastic dome incubator, similar to yours. I've been using a small plastic lid which fits over a single plastic seedling tray which has really helped to germinate seeds in the cooler weather.

    1. Yes I do think having a filler like sand or coir is a key requirement to allow the roots to breath. From my experiments the sand did slightly better than the coir. Are you willing to share the recipe of your homemade mixture?

  2. Very interesting stuff. I've only ever bought seed raising mix, but you've inspired me to give making it a go. Cheers!

    1. Lovely to have inspired. Why not do a side by side test against the commercial mix. I would love to hear the results.


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