Phosphate leaching

Yes, it is Organic soil, since in Organic soil there is a very limited (or no) amount of either Fe, Al, and Ca ions that are all responsible for phosphate fixation. Organic soil also has a very limited (or no) amount of silicate clay minerals. In addition, Organic soil tend to have a large number of negative charges (organic matter has pH-dependent charge, and Organic soils tend to have very acidic pH due to various organic acids being released in their limited decomposition), which in turn leads to electrostatic repulsion of phosphate anions, keeping them in soil solution and making them susceptible to leaching.

Thanks maja!