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APFUK Mix and Magnesium levels . . .


13 Nov 2019
I don't have a specific deficiency I'm looking into, this is a simple theoretical question, does the standard APFUK suggested mix provide enough Magnesium for EI dosing targets?

I have always assumed the answer must be 'yes', however I have just had a go at playing with the calculators on this site, and they suggest it is drastically under dosing?

The standard APFUK mix adds 6tsp to MgSO4 to 500ml of water, with a resulting dose of 10ml per 50 litres of tank water, three times per week - standard EI schedule, and of course the 50% WC once a week. According to the calculator, this results in the addition of just 0.32mg of Mg per dose:


Plugging this into the Accumulation Calculator, and removing all other variables (plant uptake etc) - suggests a long term average level of around 1.5ppm.


This is way below the generally suggested EI target of 5-10ppm, and if you take account of plant uptake, potentially bottoms out completely during the week (obviously I appreciate this will depend on Mg content in tap water).

Further, if I use the calculator to attempt to hit a target ppm of, say, 7.5ppm (the middle of the EI range), you'd need to be adding around 1.5ppm per dose (which also aligns to the weekly dose of 4-5ppm suggested by Tom Barr here):


If I plug that back into the Dosing Calculator:


That results in approx 140 grams of MgSO4 to be added to the 500ml of water to reach the required levels. That's over 3.5 times what is included in the APFUK mix. (I appreciate we should account for Mg in the tap water, but since many water companies don't disclose the separate Mg content we have no way of knowing.)

I can only assume I have made an error in the use of the calculators, or have misunderstood the Mg requirements otherwise more people would be questioning this I guess. So, is the APFUK mix adding sufficient Mg?

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