I haven't tried doing without phosphates, but several years ago I spent a lot of time reading old posts in The Krib, where people would report all kinds of plant grow difficulties, then try phosphates only to find it caused the nitrates in the water to disappear! This was because the plants began to grow as they should, now that they had phosphates, causing them to use up the available nitrates. The clear conclusion was that not dosing phosphates greatly reduced the plant growth rates.
I spent some time on Google looking for other phosphate compounds that you might be able to find, but had no success. Farmers use "super phosphate" among other things, but I don't know if that is usable in an aquarium or not, nor how to dose it if it is. Perhaps someone else here does know.