Hi there Arsha,

I would try starting here

Boström, B., Persson, G. and Broberg, B., 1988. Bioavailability of different phosphorus forms in freshwater systems. In Phosphorus in Freshwater Ecosystems (pp. 133-155). Springer Netherlands.

Available P in soils are estimated using Bray extractant or Olsen method depending on the pH of the soil. If the soils are acidic you can use bray method (less than 6.5) and for neutral and alkaline soils you can use olsen method.

References 

Bray, RH & Kurtz, LT 1945, Determination of total, organic, and available forms of phosphorus in soils. Soil Science, 59: 39-45

Olsen, S. R., C. V. Cole, F. S. Watanabe and L. A. Dean. 1954. Estimation of

Available Phosphorus in Soils by Extraction with Sodium Bicarbonate. U. S.

Department of Agriculture Circular No. 939.

You can find easily in following links

http://vlab.amrita.edu/?sub=2&brch=294&sim=1550&cnt=1

Available phosphorus content of soil consists mainly of Ca-, Al- and Fe-P. Under neutral and alkaline soil particularly Ca-P is the dominant fraction. A large number of extraction reagents from Dyers 1% citric acid to some of the multinutrient extractents, buffer solutions, acids and chelating agents have been suggested for available P estimation from time to time. However, no single extractant appears to be suitable for all types of soil. Two types of extractent method are more popularly adopted as stated by Dr. Surendran. Under soil acidic conditions, Brays P-1 (or Bray No. 1), which involves soil extraction with a solution 0.03N NH4F and 0.025N HCl is widely followed. The fluride complexes AL and Fe in soil, thus releasing some bound P besides the easily acid soluble P (largely Ca-P). This extractant is suitable for soils containing less than 2% calcite or dolomite because in calcareous soils, carbonates quickly neutralize the acid, resulting a less extraction of P.

The other most widely used extratant is the 0.5M NaHCO3 solution at pH 8.5. The reagent is most suitable for neutral to alkaline soils and is designated to control the ionic activity of calcium through solubility product of CaCO3 thus extracting the most reactive forms of P from AL-, Fe-, and Ca-phosphates. Phosphorus in the extract can be suitably determine after colour development and measuring the color intensity at an appropriate wavelength.

The term available phosphorus refers to the inorganic form occurring in soil solution which is almost exclusively ‘Orthophosphate’. This Orthophosphate occurs in several forms and combinations. The phosphate contributions in soil are governed by hydrogenous equilibria in which it takes part. This can be represented as,

Phosphorus absorbed in soil phase ⇌ P in soil solution ⇌ Precipitated P

you can also refer to the link http://vlab.amrita.edu/?sub=2&brch=294&sim=1550&cnt=1

Extraction with 0.5 M sodium bicarbonate is most suitable for measuring soil P availability in neutral to alkaline soil. Soil : extractant ratio and period for extraction (on shaker) may cause some variation in the amount of extracted P and  can be optimized. The extract can be used for developing color and blue color intensity can be measured at appropriate wavelength.

For your soils of pH 6-7.5 the Olsen test is appropriate as said by others. Make sure the pH of the extractant is very close to 8.50.  Be sure to analyse the extracts on the same day or store them in the freezer.

What you should also know is that there is no 'citical level' that applies to all soils.  This is established by 'calibration trials'. 

 Thank You Dr.Ceth W. Parker, Dr.Surendran U, Dr. Safdar Bashir, Dr. J. C. Tarafdar, Dr. Suja Prabu, Dr.Sunita Gaind and Dr. Paul Milham, for your valuable reply.