The short answer is experience. Buy some good quality soil and use it in your planters. Learn how the plants grow and thrive - if they slow growth or do not bloom as much it may be the weather or it may be because it's lacking nutrients. You can also simply give them some fertilizer. If it doesn't make much of a difference it was probably not needed.
If possible you should renew your soil, not be replacing it, but by mixing it with good manure or even just a bunch of leaves. I have experience with manure from rabbits and horses - both can be directly mixed with the earth.
If possible you should renew your soil, not be replacing it, but by mixing it with good manure or even just a bunch of leaves. I have experience with manure from rabbits and horses - both can be directly mixed with the earth.