gerald wrote: ↑Sat Mar 27, 2021 9:52 pm
Both Chempak 4 and 8 have trace elements so no need to add anything else.
Chempak 8 contains no calcium at all. It is probably designed to use with John Innes and other soils that have ground chalk or limestone added. They sell a soluble calcium supplement as well, probably calcium nitrate but they wouldn't tell me.
Have a look on the side of the box for Chempak 4.
The actual elements in your pumice probably vary depending on the volcano that produced it. Here is a typical analysis of a pumice.
https://www.srilava.com/pumice-stone/?lang=en There is plenty of calcium but it is probably as the silicate, which will only leach out into the rhizosphere very slowly compared to the carbonates. The aluminium and titanium are not so good for the plants, hopefully they are also as silicates. Though I think alumina does not get soluble until you get down to about pH 4.
Note the sodium content, which is why you should soak and rinse your pumice very carefully with abundant water before use. Unless you are growing a halophyte.
gerald wrote: ↑Sat Mar 27, 2021 9:52 pm
Don't add Epsom salts or anything else for that matter because you will cause a salt imbalance which could be toxic. In this case too much Epsom salts (magnesium) means the plant will have a hard time taking up calcium.
As Chempak just have "Also contains Magnesium." on the packet it is, unfortunately, impossible to know how to adjust for different plants that may have requirements for extra magnesium.
gerald wrote: ↑Sat Mar 27, 2021 9:52 pm
You're better off giving a low dose of feed (say 50% strength) with every watering.
I definitely agree with that, five meals a day rather than one huge supper.
gerald wrote: ↑Sat Mar 27, 2021 9:52 pm
Use a proper pH meter if you can, the cheap test kits are useless. Vinegar will do it but so will citric acid which is easy to find and very cheap.
Bromothymol blue liquid (usually sold as pH 6.0 to 7.6) is good enough for me and does not need £60, regular cleaning, calibrating nor batteries. Clear results but should be read in natural light. The Chempak should adjust most waters to the correct pH.
What a shame Mount Etna is in Europe. The councils in Sicily have a huge pumice waste problem at the moment.
https://www.bbc.co.uk/news/world-europe-56344311