The crux of the matter is how you sample plant tissue from the species/ecosystem. See attached for herbaceous plants and my mangrove articles for evergreens.

Article Nitrogen and phosphorus limited production of cereals and se...

Total P can be estimated by the complete digestion of samples with the acid/ acid mixture and then total P  can be measured with the help of spectrophotometer after development of colour. Lots of protocol are available, please refer based on your sample properties.

With regard to total phosphorus in plant samples , the procedural steps are well laid out  , which can be seen through any standard Plant analysis  Manual . Usually , the acid digests of plant samples are analysed through vanadomolybdophosphoric acid ( Yellow color  ) method  using spectrophotometer at 420 nm transmittance .   

Thanks a lot , I found a complete protocol for Wet digestion  using  A mixture of HNO3, H2SO4 and HClO4 in the ratio of 9:4:1 and the Method of  spectrophotometric vanadium phosphomolybdate. in Guide to laboratory establishment for plant

nutrient analysis FAO FERTILIZER AND PLANT NUTRITION BULLETIN 19