Problems with home made potting soil


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I make my own potting soil because it's a way to use my compost, it's cheaper, and I like the idea of it being more sustainable, etc. But I'm finding that at least some plants don't do as well in my potting mix compared to a commercial one. I decided to test the idea with two pots of parsley seedlings from the same punnet. The pots were same, size, placed next to each other, etc. The one with the commercial mix has plants that are now twice the size as the one with my own mix. So that's a pretty clear indicator that something is off with what I'm doing.

My first thought was that pH problems might cause nutrients to be less available in my home made mix. So I bought a less-cheap-but-still-cheap pH meter that measured both mixes at 7.0. I doubted that, so got out some vinegar and bicarb soda to do a basic test and neither visibly reacted with either mixture, so maybe the pH of both is actually neutral. There goes that idea.

I'm not sure what else I can try to measure to work it out. I've considered adding fertilizer to my mix in varying amounts and seeing if I can see a difference in plant growth: maybe my mix doesn't have enough nutrients in it, but if it has too much I guess I won't see that.

My mixture has, in roughly equal proportions,
- compost (mix of worm castings and compost from my tumbler)
- coconut coir (much cheaper than peat moss, where I am, also more sustainable, I gather)
- vermiculite.

Plus it always ends up with a bunch of worms in it from the worm farms.

Does anyone have any ideas? This is bugging me!
 
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For sowing seeds or taking cuttings no fertiliser is needed. My mix consists of fine grade peat and vermiculite in equal parts with fine horticultural grit for extra draining.
For potting up small plants - rooted cuttings etc or larger seedlings, I add some Osmocote slow release fertiliser (the best!!!) to the same recipe as above.
Home made compost from any other source is used on the garden plots to enliven and improve the soil.
I know what all the TV gardeners tell us to do is use non peat based compost, but I`m afraid that`s what my plants have to have...good old peat :joyful:

Sometimes trying to do it cheaper turns out to be a false economy.
 
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Thanks Tetters,

Unfortunately, in Australia, peat moss is prohibitively expensive. It costs $6 (that's £3.38) for 5l, and I have large pots to fill (my apartment terrace garden is all pots). I can get 30l of coir for that.

Does anyone else have a suggestion?
 
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I didn`t realise that you are unable to buy peat in Australia - this is a bit different to the peat moss you are referring to. The other way I have made potting compost is by using bulk bags of loam which has been treated to eliminate unwanted weed seeds etc.
@DirtMechanic - I use some fine grit as well as vermiculite lately as it helps the initial watering of very dry peat..... don`t tell Zigs though, it`s supposed to be for his precious cacti :giggle:
 

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