Determination of phosphate beans
The paper by Morris and Hill (1996) in the Journal of Food Composition and Analysis (Volume 9, Issue 1, pages 2-12) has a description of the technique they used to estimate phosphate/phosphorus. They were interested in the form of the P - particularly the phytate content because phytates are associated with binding of some other nutrients such as zinc - so a high phytate content can reduce zinc bioavailability.
If you are only interested in the absolute amount of phosphorus, I suggest you look to have the grain analysed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) - see http://en.wikipedia.org/wiki/Inductively_coupled_plasma_atomic_emission_spectroscopy. This method should be available at most modern laboratories at reasonable cost.
If you do not have access to this type of equipment, there are some "wet chemistry" techniques - see the methods described in the article by Bilyeu et al. (2008) at http://www.plantphysiol.org/content/146/2/468.full
I hope this helps.