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How much aluminum sulphate for a small potted houseplant?

After testing my Gerbera daisies soil pH, I noticed it was too alkaline. Mixing in some aluminum sulphate to potting soil and repotting them in that seems to be the answer, but I have no idea how much to use. The pH is currently 7 and I want to get it down to about 6.5 for now. I'll only need a handful of soil for repotting,. Does anyone know how much aluminum sulphate would be needed for that? Thank you!


  • DovefromaboveDovefromabove Central Norfolk UKPosts: 82,138
    edited April 2021
    I would simply repot it in some bought balanced multipurpose compost. Trying to adjust the balance of such a tiny amount is nigh on impossible for the amateur gardener. Youre dealing with the sort of tiny measures used in a lab. 
    “I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh

  • NewGardener25NewGardener25 Posts: 47
    edited April 2021
    Ah okay haha 😅 Thank you! 

    Can you suggest some potting soil and how I can tell it would be a suitable pH? After Googling, it doesnt seem that store-bought soil tells you what its pH is.
  • DovefromaboveDovefromabove Central Norfolk UKPosts: 82,138
    A general multi-purpose garden compost from a garden centre will be fine for a gerbera. 

    “I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh

  • JennyJJennyJ DoncasterPosts: 7,666
    pH 7 is neutral. Are the gerberas suffering (eg yellowing leaves)? If they look OK I wouldn't worry.
    One thing my Nan used to do is put the used tea leaves on her potted azaleas - she said it kept the soil acid. I've no idea whether it's true but the azaleas always looked healthy so it might be worth a try.
  • Yeah, the leaves are going yellow on the outsides with slight brown spots. I posted a discussion here asking if someone could tell what nutrient deficiency was causing it but I think it may be that the potting soil is too alkaline for Gerberas. I know the problem isnt overwatering as I let them dry out between watering.
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