The availability of P is often insufficient and limited by accumulation in soils. This led tothe necessity of solutions for the recovery as well as recycling of secondary P resources. Batchexperiments were conducted with CaCl2 and citric acid to characterize P release kinetics fromvivianite, hydroxyapatite, and bone char. While the P release during the CaCl2 treatment was solow that only vivianite and hydroxyapatite showed a slightly higher release with increasing CaCl2concentration, the

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... e of dissolved P was more pronounced for citric acid. The application ofcitric acid resulted in a 32,190‐fold higher P release for bone char. Fourier‐transform infraredspectroscopic data suggested higher instability of hydroxyapatite than for bone char. The kineticdata showed that bone char, especially at a lower particle size, had a higher long‐term P releasethan hydroxyapatite or vivianite. The suitability of hydroxyapatite and bone char as a poorlysoluble, but sustainable P source is better than that of vivianite. However, the efficiency as a Pfertilizer is also dependent on present soil P mobilization processes. The results underline theimportance of the accessibility of fertilized or naturally bound P for plant roots to benefit from theexcretion of organic acids.