Question: What fertilizer do you use? I have only been able to locate Miracle Gro for African Violets and it is 7-7-7. I have found no 15-15-15 or 20-20-20. Do you have any suggestions? At this time I am doubling-up onthe Miracle Gro.
Answer: The numbers you are referring to represent the percentage of the three major nutrients; nitrogen, phosphorus, and potassium, respectively, contained in the fertilizer. In this case, the 15-15-15 fertilizer contains a bit more than twice the relative amounts of these nutrients than does the 7-7-7. The 20-20-20 fertilizer contains almost three times the relative amounts. Since all three fertilizers contain the same amounts of nutrients relative to each other (i.e. all are “balanced” formulas), you could double (or triple) the usage of the 7-7-7 fertilizer to achieve the same diet as the others.
That said, you need to ask “double” what? What unit of measure is being used? One may call for a quarter teaspoon of dry fertilizer while the other calls for 7-10 “drops”, or a quarter “capful”. Rather than get a headache trying to figure all of this out, its best just to follow the instructions on the label for whatever fertilizer you choose to use–the manufacturer has already done all of this math for you. Most fertilizers will give directions for every-watering use (what we’d suggest), or for occasional (2-4 weeks) use. Use whatever dilutions apply for your watering and feeding schedule. If you use constant-watering systems, such as “wicking” or “self-watering” pots, use half, or less, of the recommended amount, since your violet will be processing much greater volumes of water than one that is only periodically watered.