If you want something water only, I have done some runs in Sohum Living Soil and it does everything it says on the tin... Match the soil amount to Size of plant and it will go to the end on just rain water..
This only had rainwater and its seriously packed and covered in resin.
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However it is 45 quid a bag

I am also doing a run now with biobizz light mix and this
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Check out my current diary as I have just started top dressing with the Grow amendment. And have used some of the bloom 2 and barley on my purple queen which seems to have had a very positive impact.
Hope this helps a bit mate