A very complicated question as different nutrients in different strengths are needed at different stages of growth, different frequencies, and with different growing conditions. Lots of variables - size of container, mix in the container, soil test results for the in-ground garden soil, climate, age of the plants, watering frequency.

You'll find many discussions here about "favorite" that the search will pull up but the things most have in common are liquid and/or water soluble, 1/4 to 1/2 strength for seedlings, well-balanced including micro-nutrients, and much more frequent for container plants than in-ground plants (frequency determined by size of container and frequency of watering).

Dave

I plant peas or beans or green manure cover crops in between rounds of warm weather crops. In the spring, I put down a layer of compost topped by bark mulch for looks, and then after planting give the baby plants a couple of half strength shots of Miracle-Gro to get started. It works for me.

Right now I'm trying to select a liquid fert for seedlings. I'll be potting up shortly as they're getting their first true leaves. Also some are leggy since my bulbs were delayed. I'm mapping out my needs for the season and looking into the different needs at different times.

i only use organic fertilizers as i'm a believer that not only do they contribute to flavor of whatever is being grow, in the long run you'll spend less money and effort in maintaining a garden. how is that possible? well, synthetic fertilizers might grow you plants, but they do absolutely nothing and add no value to your soil.. that bag of organic fertilizer not only feed the plants, but also feeds the soil and conditions it for the future.
the price of the two are not as much as many people think these days.. and again, in the long they will improve your soil.

another great thing about organic fertilizers is they can be made very easy.. i live less than a mile from the ocean and i pay it a visit.. i gather up seaweed and kelp that washes up on the shore.. rinse off the salt, throw it in a bucket with some fish, molasses and end up with the best fertilizer money can buy ;-) tomatoes go nuts over it. i find pleasure and it's rewarding putting things back into the earth with things that came out of it.

no ocean? no problem.. check your area for local farms. i bet many will either give you animal waste (chicken, horse, cow manure,etc) for free or for a small fee.

seedlings don't need much fertilizers, so i don't bother buying anything specific for them. diluted fish/kelp emulsion works. more than half of the available nitrogen in fish is available to plants almost immediately.. the rest of it will get to work feeding the plant once it's planted out in the garden with other microbial life.

"Right now I'm trying to select a liquid fert for seedlings. I'll be potting up shortly as they're getting their first true leaves. Also some are leggy since my bulbs were delayed. I'm mapping out my needs for the season and looking into the different needs at different times."

For young seedlings the two most frequently mention brands are Miracle Grow for synthetics and Fish Emulsion for organics - both diluted to no more than 1/2 strength or less. How much and how often depends on the mix used in the pots and the size of them plus how often you have to water.

MG has the advantage of also containing all the micronutrients that fish emulsion doesn't but if you want to use only organics then you can accomplish the same thing by mixing fish emulsion with kelp products and/or any other of several blends on the market.

Once the plants go into the ground the program changes greatly. In ground plants are hopefully planted into well amended soil that is already rich in composted nutrients - something one needs to know upfront - so minimal supplements may be needed. And if they are one can continue to use MG at full strength or mix up all sorts of organic mixes or teas to do the job as Stevie already mentioned. If using synthetics feed only as the plant indicates a need with symptoms. Some will feed after first bloom and fruit set and then again mid-season.

Container growing is a whole different ballgame when it comes to feeding with its own set of guidelines.

For season long planning the key word is "flexibility" as there are too many variables involved for any one set plan to work.

Dave

I choose MG over organic fertilizer for my starter fertilizer because the soil bacteria that is needed to break down organics into their usable components is less active at cooler temperatures. Organic material from compost and cover crops provides nutrients for the rest of the season.

"synthetic fertilizers might grow you plants, but they do absolutely nothing and add no value to your soil"

That would be news to your soil, which would find itself seriously enhanced in nutrients that are beneficial to both plants and soil bacteria. There might be reasons to stick to organic fertilizers, but the idea that inorganic ones don't contribute to soil isn't one of them. When confronted with a nitrate or phosphate, the soil biome isn't asking where it came from.

Maybe you could add "long term" before value.

I enhance my soil with Espoma products like Plant Tone. A little Miracle Grow is added when the green tomatoes arrive.

Seedlings, a very weak solution of MG liquid. Emphasis on WEAK, because I've killed small seedlings with too much. Containers, MG liquid also, as strong and as much as the plants can stand. A little Calcium Nitrate and Epsom salts added. For in ground, 10-10-10, and some liquid MG.

As Dave pointed out for seedlings an container plants in soil less mix you have to provided balanced nutrients (NPK , Ca , S, Mg) and trace elements. Also the frequency in feeding container plants is more often due to watering more often. So I will do 1/2 strength.

In ground is different story. Eg, my soil test came in. There is more than enough P and K. All needed is N.