Justin, plants only use PO4, P is just the elemental form and is always
oxidized.

You should try and find some KH2PO4(monobasic potassium phosphate) from
a hydroponic vendor down there.

1 kg will last....oh ....about 50 years.
You can use the SeaChem product(just KH2PO4 in water) and add about
1ppm of PO4 3x a week or so.

Regards,
Tom Barr

www.BarrReport.com