Honestly, it can be rather difficult to increase phosphorus and potassium through the use of natural products while you are growing your plants. It may be best, if you are not doing to already, to try a synthetic fertilizer that is made either for vegetables or even for flowers. The important think to look at is the three number strand that it will have on it. The order is Nitrogen-Phosphorus-Potassium (N-P-K for short). You can often find one that is something like 4-12-4 or the like. The most important one is the middle number for flower and fruit developement. Potassium is far less important, but will greatly help reduce the risk of disease, and if you live in an exceptionally hot area, can help the plant tollerate the heat better (it also helps with cold weather tollerance, but thats rarely an issue for the average veggie garden).
Then, this winter, use your ashes to raise the P and K naturally so that you may not need (or atleast will need fewer) soil amendments for next summer's garden.