If you want to make your own fertilizer, here is an easy, cheap recipe.
For one cup of fertilizer:
1/3 cup of green sand (potash and minerals)
1/3 cup of rock phosphate or bone meal (phosphorous and minerals)
1/3 cup of alfalfa or soybean meal (nitrogen)
1 Tbl Azomite (70 minerals and trace elements)
This is good for 40 quarts of potting soil. Just mix it in with compost to rejuvenate your old potting soil for this season. You can add up to 50% of compost to the potting soil. Compost brings not only nitrogen, but also microbes that boost the plants ability to take up nutrients from the soil. If you do not want to completely remove the soil to mix in compost, remove at least the top few inches and mix in compost.
Is also great for the garden as well. It is best to either fertilize and then immediately apply mulch at a rate of 5 pounds per 100 square feet. Or you can scratch in 2 tablespoons around the roots of each plant if not applying a mulch to keep the fertilizer in place.
Another great thing about natural fertilizers is that you can't burn up a plant with too much like chemical fertilizers. For fruit bearers, like tomatoes, if you use too much, you will get a jolly green giant plant and no tomatoes because nitrogen stimulates leaf growth. More is not necessarily better.