Phosphorus in soil is vitually non-mobile.It is to difficult to measure its mobility. If you have necessary facilities to use isotope in your experiment it would be best way to to do it. If you don't have those facilities then it is better to use distructive column. Make a colum from 0.2-0.5 cm rings then fill it with soil at specific bulk density. Ad your P solution or leaching solution for a while depending on the Project purpose. then take samples by slicing in the rings and analyse either geochemical fractions or available fractions.
Thank you dear doctors for your contribution. I would like to test an organic amendment with 8% phosphorus on a sandy soil. I would like to have an idea about the effect of irrigation on the mobility of phosphorus.
you may look at the following list of oral communication that I attended today at EGU2019 in Vienna. Some of them are considering your question. The difference is the research investment and the analytical facilities which are huge compared to what we have as available techniques in Morocco. You can download the abstract and select the pertinent ones then I will send you the authors contact since I have the EGU2019 App (Android).
you must do soil column experiment. do in situ ( with out disturbance of natural soil) soil sampling by using PVC in the field, and design the application of P fertiliser and irrigation according to the local soil and rainfall as simulation. make saturated each soil column before application of P fertiliser solution. i think is is the best method to look the mobility/ mechanism of P in column method. have a look below the attachment related your question please