Dear Ashokkumar

I am not very expert in this subject but I am sure if the soil comes in contact with water having lower pH, the phosphates of the soil are released or desorped because of the production of the formation of phosphoric acid. On the other hand, if some salts of Al3+ or Fe3+ are present in the soil they will adsorb

the phosphates of the soil to give phosphates of these metals.

@lahieb

In my opinion the phosphorus availability (to plants?) in calcacerous soils is affected by precipitation of phosphate with calcium. Precipitated phosphate are not available at all, because it is not water soluble and there is no possibility to be transported to plants. Sorption and desorption plays a minor role.

Regards

Vit