I've seen a couple people post about this recently. What I've noticed is it seems (the other poster hasn't returned to answer questions about what they're using, so I'm guessing somewhat) that people are conditioned to think of fertilizer as something extra that you "feed" to the plants, usually in a liquid medium like you are now. Combined with potting soil, which is not very nutritious for plants of any size at all, typically, and the tomatoes end up terribly malnourished.
I would encourage you to think, instead, of fertilizer as something
you add to soil to improve a plant's ability to grow in that soil.
When growing any plants, but especially a plant indoors and organically, your job is to think like the plant. What does a plant need to thrive (not just survive)? You have to provide it. Light and water in appropriate quantities, temp, and soil that is adequately nutritious are the main ones, obviously. Miss on any of these and your plants will not be healthy.
If your soil/potting mix/whatever you want to call it is so sterile and nutrient-poor that you have to supplement any liquid fertilizer (whether it is organic or MiracleGro), you haven't created a good soil for your plant to grow in.
So... here's the fertilizer and potting soil I use when growing plants indoors. This is also the fertilizer I add to my vegetable beds. No reason for them to be different - again, I want my indoor-grown tomato seedling to grow in conditions that let it thrive. Those conditions are the same indoors and outdoors - the plant doesn't suddenly need different things to thrive once you put it outside.
First let's mix our fertilizer... the ingredients (fertilizer mix, agricultural (not dolomite!) lime, and gypsum
Note what this fertilizer is made of. This is what's called seedmeal-based fertilizer. If you've never grown veggies with seedmeal, you will be pleasantly surprised. It is a powerful, natural, slow-release organic nitrogen source. This means you can't really over-fertilize, and your plants are fed naturally over a long period of time, instead of given a blast of nitrogen (liquid fertilizer) and then left to starve until the next scheduled "feeding."
Add approximately a gallon of the fertilizer to a bucket, and add one pint lime, one pint gypsum, and mix thoroughly. Leaving you with awesome organic fertilizer for any soil (indoors or out) you want:
Organic potting mix:
The angle on the camera is weird, but this is about 1/2 of a 5 gallon bucket of potting mix, and a slightly heaping spade-ful of fertilizer.
Add the fertilizer to the potting mix, mix thoroughly, and you have powerfully nutritious potting soil that will feed your tomatoes for as long as they are not root-bound in your pots (always transplant to larger pots before they get too root-bound, and use the same fertilized mix when you do - thus adding more fertilizer to your growing plants!).
Here are this year's tomato seedlings 6 days ago when I transplanted to larger pots:
Here's what they look like today:
Nutritious soil = happy tomatoes!