I have a bunch of old-school mineral fertilizers that I would like to use for container plants. The problem is that the dosage on them is grams/m2, lets say for simplicity it is 100 grams/m2. How do i convert that dosage for container plants ? Taking 0.2 meters as depth (and this is pretty low i think) 1 m2 with 0.2 meters depth becomes 200 liters of volume. My biggest container is 20 liters, so i only need to put 10 grams of fertilizer per container ? This seems pretty low, especially since this is supposed to feed the plant for 6-8 weeks.
The one fertilizer I use inside and out is MiracleGrow. There are several different mixtures of this stuff, but many say:
For Outdoor Plants: Mix 1 tablespoon of Miracle-Gro for every gallon of water. For Indoor Plants: Mix 1/2 teaspoon (not tablespoon) per gallon of water.
I use a teaspoon per gallon indoors. Of course, your mileage may vary, but half strength is a good place to start.
Late to this, but your math checks out.
For container gardening, the problem isn't that the amount is low, it's that the amount you put in will leach out over time as you water. Depending on the fertilizer, I tend to refill a little more often than the recommended rate to account for leakage. You can't just add more to account for leakage because too high an amount at one time will burn the roots.
For water soluble fertilizers that don't have low measurement amounts listed, I cut dosage substantially and apply more often.
If in doubt, give less fertilizer and replace with better potting soil in the pot.