PMDD doesn't contain any phosphate at all. It was devised back when folks all believed that phosphate causes algae, which it doesn't. And, you shouldn't add any to that mix because it tends to precipitate the iron out. If you haven't done so already, look up EI fertilizing here and see if you want to try that. Back to your original question, plants do use up phosphate, if they have enough nitrates, traces, CO2, and micros to grow well. Once the plants are short of one of the required fertilizers the growth doesn't keep going, so the other ferts don't get used up fast. That's why the EI system makes sure you always have more of each fertilizer element than you need - then only the light limits the growth.