You might want to consider one of the Dynamite controlled release fertilizers (CRF). I've linked to their website below. I think Home Depot stocks some of their products, but I'm not sure which ones. As to your second question, experienced growers here in the Container forum often recommend combining a CRF with a soluble fertilizer like Foliage-Pro. The most often cited application rate is 1 tbsp of CRF per gallon of potting mix (ideally, this is incorporated prior to planting) plus a weak dose of soluble fertilizer at each watering (for Foliage Pro, that would be 1/4 tsp per gallon). Here is a link that might be useful: Dynamite Plant Food

Also can you explain WHY the ideal ratio is 3:1:2 ?? ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I think the reason is the issue of stability of N, P, K. P is the most stable of the three, as it is bonded/attached to soil/medium and sticks around for a good long time and in the garden soil it can accumulate over time. K is semi stable, meaning that it does not leach out readily but it is not as stable as P. N, when available, is water soluble. So every time you water some of it drains out. And also MAYBE certain plants up take more of it too. But generally and specifically in container gardening N depletes the fastest of the three(N P K). Having said all that, I would be careful about using N. It often can fool a gardener by growing lush foliage but can cause late fruiting and LESS fruiting. That is why, as the season progresses I reduce the amount of N( like for matoes, pepps). once the plants have enough foliage for photosynthesis, I don't want to grow more of it. But for leafy plants (cabbage, onions ..) I continue feeding N. Because I want the foliage.

I don't use this product but over the years I've had a nursery, I've had to do this kind of calculation all the time with fertilizers that are labelled for farming. If you do want to be accurate you want to calculate the exact square inches of the top of the pot, but if you think it is a square foot, then what you told us is the same thing I would do. For us, 7 gallon pots are close to 1sf. In reality, this isn't a real hot fertilizer, so if you over do it a bit, it shouldn't hurt anything.

Dyna-Gro has the best all around N-P-K ratio as well as micro nutrients and is the choice of many. tj