I created the below overview and while back and thought I would share here also.
It provides guidelines on how to fertilize annual plants / vegetables grown from seeds indoors in winter.
Sometimes it is beneficial to be able to dose fertilizer accurately. Often we resort to using the same strength for all seedlings, and may not even change the dosing when we switch brands.
When it comes to choice of type of fertilizer seedling, you can use both organic or inorganic (does not matter).
In general, when fertilizing, most seedlings benefit from higher levels of nitrate nitrogen (vs ammoniacal or urea) nitrogen if available. Check for this on the ingredients on the lable. If you can't find it, that is ok, too.
Here are the general rules when it comes to dosing:
As general rule
very light: 50 ppm
light: 100 ppm
regular 200 ppm
heavy: 300 ppm
Very small tender seedlings can benefit from 50-75 ppm fertilizer (young begonias, newly germinated seedlings)
Most young seedlings and light feeders from 100 ppm nitrogen per watering (impatiens / salvia for example)
Larger seedlings 200 ppm, and heavy feeders 300 ppm (This applies to the absolut largest group of summer annuals and vegetables we grow indoors. Wave Petunias can even take more)
The below picture (click for larger version) will show how many tsp you must add to a gallon of water to create the desired strength fertilizer.
Say, you bought 5-6-7 seedling fertilizer, find the "5" under Nitrogen-pct, and then go across in the table until you find the desired PPM (from 50-300 depending on your purpose). The amount of teaspoons (TSP) you should add to create that strength is listed horizontally on top.
e.g. if you use a fertilizer with 10% nitrogen and wish to apply 200 ppm, go accross and find the closest figure (195). Go up and read that you must add 1.5 tsp to the gallon container of water to make this strength.
