Figure There is no increase in the bioavailable calcium when eggshell precursor (CaCO3) is applied in soil, but this concentration increases to 30.97 mg/kg after 30 days for 2 h/600 rpm sample, corresponding to around 8% of calcium recovery from sample (Fig. ). These results corroborate with citric acid solution release essays and attest efficient use of 2 h/600 rpm sample as a source of bioavailable calcium compared to the precursor eggshell -CaCO3. The slower release for calcium and phosphorus for 2 h/600 rpm sample and the P radial diffusion are probably related to hydroxyapatite formation -(Ca10(PO4)6(OH)2) (PDF database n° 9-432 indicated by green columns) -identified from 3 until 30 days in soil conditions (Fig. ). In an additional experiment, samples ground at 600 rpm were suspended in water, dried and then characterized by XRD, in which the phase transition to Hydroxyapatite was observed for all of them (Fig. ).