Hydrolysable phosphorus is the reactive ( available)  phosphorus that can undergo hydrolysis (e.g by the addition of hydrochloric acid) to produce  a combiination of orthophosphate and acid condensed phosphorus. Hope this is useful. Let me know if you need further help.

 Dear Grace Solomon-Wisdom,

Thanks for your response. I need to write this definition in my MSc. thesis so I would be glad if I can find a reference. 

Thank you

Dear Hani Antoun,

Thank you for your answer. I got a better insight