Mr.Long,the phosphate potential(PP) the equilibrium phosphate potential(EPP) and buffering capacity can be obtained based on the method of White and Beckett(1964).The brief procedure is as follows:Five gram soil samples were equilibrated for 72 hours with 50 mi of 0.002M CaCl2 solutions containing P ranging from 0 to 10 ug P/ml.Phosphorus,Ca and pH analyses were made in equilibrium extract and phosphate potential(1/2pCa+pH2PO4)were then computed.The difference between the phosphate concentration of each solution before and after equilibration gave delta P expressed as ug P /g soil.Equilibrium phosphate potential(EPP) values were obtained by plotting phosphate potential values against +_delta P(ug/g)values and by interpolating at delta P=0.The buffer capacity was computed from linear portion of the curve i.e.,Delta P/delta PP. Studies on phosphate potentials of soils.Part 1.The measurement of phosphate potential by R.E. White and P.H.T.Beckett (1964)Plant and Soil 20(1)pp.1-16.

The relationship between resin-extractable phosphorus and phosphate potentials for sandy soils of Hutton and Clovell forms Agrochemophysics 4,77-82 (1972).Available on line.This reference gives more details of calculations and graphical presentations.

Check this one: Ehlert et al (2003) Potential role of phosphate buffering capacity of soils in fertilizer management strategies fitted to environmental goals. J. Plant Nutr. Soil Sci. 166, 409-415

Please study the following papers those will help solve your problem.

I think references attached by Dr.Ghafoor will definitely help you.One more important reference is as follows:Interlaboratory comparison of a standardized phosphorus adsorption procedure.By Nair,P.S and others.J. Environ. Qual.,vol.13,no.4,591-595(1984)

You should be aware that a physical interpretation of parameters in  the Langmuir  models as applied to P sorption data for soils is discredited, e.g. "binding energy" constants.  That is, the model is purely descriptive, so any model that fits the data well and simplifies slope estimation is, if anything, more useful than the Langmuir model, which is intrinsically curved.  The log transformation of the Freundlich model satisfies these requirements over a large range of sorbed and solution P concentrations.  So I suggest that you might investigate its use.

Whatever comments the above draws, for fertility rather than pollution risk purposes, whatever the modelling approach you take, the critical region for slope estimation appears to be about 1-10 micromolar P in solution.

Two comments:

• Be sure to use a potent antimicrobial during the the time the suspended soil is reacting with P;
• You need to measure the P sorption behaviour under conditions relevant to the cropping situation you wish to study.  For example, I would expect very large changes during the transition from wet soils in the Vietnamese summer to aerobic conditions northern winter. 

I have a personal interest in learning about the seasonal changes since I am involved in vegetable production in Lao Cai Province as part of an international project.  So if it is your intention to investigate the transition phase please stay in touch: [email protected]

I think you have references the model will be definitely help you search w/c was published be me