Some folks are a bit taken aback by the use of “parts-per-million nitrogen” (ppm N) as a way to manage their fertilizer application, but I believe it is by far the best way to do so, as it is analogous to counting calories in our own diets.
Let’s start with a general recommendation: we have found that if you feed your plants at every watering, with that being about every three days, a nitrogen concentration of about 35- to 50 ppm appears to be good for a wide array of genera. Therefore, if you feed your plants every other watering, then it seems reasonable to apply the fertilizer at approximately twice that concentration. If you feature plants less frequently than that, it would seem logical to simply use the appropriate multiplier to determine your concentration, but we must keep in mind that too strong of a solution can be damaging, particularly to more sensitive plants. That is one reason we always recommend dilute, frequent feeding as opposed to stronger, more spaced-out feedings. Using our own nutritional analogy, it’s healthier to eat several small snacks then to gorge yourself at one big meal.
However, we don’t need to be all that exact in our measurements, and the following simplified fertilizing dosing guide will work well to help you control your feeding.
For example, if we wish to feed 50 ppm N, and have a 7-9-5 formula fertilizer, then 4 / 7 = 0.57 teaspoons per gallon (it would be safe to go with 1/2), or 5.2 / 7 = 0.74 milliliters per liter (0.7 to 0.8 are fine).