Determination of phosphate beans
What is the digestion techniques for the determination of phosphate in beans
Outside United States
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.