I only use soluble stuff like "Miracle-Gro" in pots. I save the solid granules for outdoors.
Also, from what I read, and my limited experience with being successful, seedlings can't take much fertilizer at all. For example, add none until they have 1 pair of new leaves, then use 1/4 strength every second or third watering until they have ... 3 pairs of leaves? Even so, you would rather have slow-growing, stocky seedlings than leggy things that shot up due to fertilizer, and then died when they couldn't adapt to going outdoors.
Get them "hardy" and established before you overfeed them. Under-fertilizing young plants will only slow them, and that might be a good thing. Over-fertilizing bur5ns or kills. If they aren't becoming pale yellow, they don't need more food.
(But I hope someone with more knowledge and fewer opinions joins in!)
>> Probably why my veggie yields werent so good I dont know.
Unless you have super-rich, super organic soil, yeah, they will want to be fed after they are big enough to take it. Last year I under fed my Lobelia in containers and they never became thick. But even with adults, the rule is the same: over-fertilizing is much worse for a plant than under-fertilizing.
P.S. I'm n ot the most fervent "organic" advocate i n the world, but they are certainly right about many (or most, or all) things. And that wisdom goes back at least 100 years, and maybe 10,000 years. Compost and humus are NEEDED by soil. Soil needs them to sustain it's structure, water holding capacity and its microorganism (and worms). Plants grow much better in organic soil than in dead dirt.
You "can" grow plants hydroponically with chemicals, but not as well as rich organic soil can grow them. Better to fed the soil with lots of compost and use fewer chemical nutrients. (I fertilize some because I don't buy or make enough compost.)
"Feed the soil, and the soil will feed the plants".
If your outdoor soil isn't alive with a healthy herd of microorganisms, your plants will never thrive. That micro-herd needs organic matter (mostly carbon) and air to live - they eat the OM and combine it with oxygen for energy and to build their bodies, just like we do. Doing so, they release the N, P and K in the compost or humus or manure or leaves. Your plants eat the NPK that was released.
The beneficial soil organisms include root fungi that live symbiotically with plan t roots and increase their ability to draw minerals and water from t6he soil (mycorhyzae, but I can never spell it without looking it up).
