Okay, Adi -- the numbers have been duly crunched, so here you go...
Add the following to 50 liters of water:
- GrowMore 1/2 tsp. x 3
KARATE PLUS 1/2 tsp.
PUPUK Magnit 1/2 tsp.
Potassium sulfate 1/2 tsp. x 3
What you'll get in the watering solution:
- 7 ppm Ammonium N, 16 ppm Urea N, 28 ppm Nitrate N, total N = 51 ppm
The NPK ratio is 1-0.27-1.37, so P and K are well within optimal range, and the NPK dosages per feeding are quite good. Wish I could've increased the amounts of Ca and Mg, but that's the best we can do. Bear in mind that your cacti and succulents can get a good amount of Ca and Mg from the tap water, so every little bit helps -- even the gypsum you add to your pots. Whenever you water with rainwater, the Ca and Mg in the KARATE PLUS and Magnit will be sufficient.
I'm highly confident that you will need to add citric acid when you make 50-liter batches of tap water diluted with the ferts and potassium sulfate. The steps are:
- Test the pH of the watering solution after you add the ferts and potassium sulfate.
- Add citric acid in half-teaspoon increments one increment at a time, and test the pH with each increment.
- Once you get close to the target pH of 6.0, go 1/4 tsp. at a time until you hit the target. If it's a little bit below (say 5.9), no worries.
- "Sneaking up" on the target pH does take some patience, but if you write down the total amount of citric acid you used, you won't have to keep going through the exercise.
A couple of items for your attention. First, we know that pH rebounds up as weak acid in the watering solution neutralizes carbonates -- calcium citrate and magnesium citrate are the reaction products that get Ca and Mg to the roots of your plants. And your acidic mix helps in the process, so watering solution acidified to 6.0 is fine for both of your mix types.
And finally, remember this?

- Mulders_chart.jpg (87.83 KiB) Viewed 137762 times
Mulder's chart was missing something -- Mo increases N availability
from nitrate through the conversion process as I mentioned in a previous post. Relying on just one form of N in a fertilizer will lead to problems, so you have a good balance between Ammonium N-Urea N and Nitrate N. Unfortunately the amount of Mo in GrowMore is so small that it might as well not be there, and I highly recommend that you add a little bit of sodium molybdate to your watering solution. You won't be able to add it directly to your watering solution, so I'll crunch a few more numbers and come up with a sodium molybdate stock solution you can make, then add -- you guessed it -- 1/2 tsp. per 50 liters of watering solution.