the reason it says that is because the soil people normally put it in already has calcium and magnesium...promix does not have nutrients, so i'd go with what Hobie says with an ounce(1/8th cup) per pot...if you start getting deficiencies at all you can always sprinkle some on the top of the soil and water it down in later.on my bag of dolamite it says to use like a tsp. for a single plant.