For estimation of any nutrient element in plant first it has to be digested using di or tri-acid mixture. After that a definite volume has to be made of that digested plant sample. Now for estimation of phosphorus in degested plant sample, a definite amount of aliquot (5-20 mL) has to be taken in 50 mL volumetric flask. Then 10 mL of vanadate molybdate solution has to added to that and volume has to be made upto 50 mL by distilled water. It has to be mixed well and after 10 mins absorbance reading has to be taken at 420 nm. From standard curve phosphorus concentration has to be calculated using reading (absorbance).

Calcium content in digested plant sample has to be measured either by AAS or by versenate titration.

Yes..I do agree with answer of Mandira Barman. For estimation of calcium and phosphorous, the sample has to be digested using triacid. Diacid digestion is generally done for sulphur.one gram of finely ground tissue is digested with 20 ml of triacid mixture till a clear solution. After cooling add 30-40 ml distilled water, filter and makupto 100 ml. Then store in polypropylene bottles till further analysis using AAS.

For estimation of Ca,  500mg of dry finely ground of the brinjal  part was digested with using triple acid mixture. After digestion, add 30 mi of distilled water and filtered by using Whatman No.1 and finally make up to 100 ml with distilled water. The sample was stored in the polypropylene bottles. Ca  content was estimated by using AAS by selecting suitable wavelength.

If you don't have AAS, there are other methods like using flame photometer for estimation of Ca and Phosphorus estimation using an Autoanalyzer or Spectrophotometer using mixing of reagents.