Dosing of leonardite/struvite compounds as phosphorus fertilizers increased biomass and nutrient uptake in a calcareous soil

Non-renewable sources of phosphorus (P) are running out, so it is necessary to recover P from secondary recyclable materials. This study aims to recover P from domestic sewage sludge as Leo/MAP (Leonardite/magnesium ammonium phosphate) composites and use them as phosphorus fertilizer. The characteristics of Leo/MAP composites, solubility, and kinetic of P-release from them were evaluated. The properties of composites were determined by using XRF (X-ray fluorescence) spectroscopy, as well as structural analysis by using SEM. (Scanning electron microscopy) In addition, the effect of Leo/MAP composites was investigated on the responses of the Moldavian Balm (Dracocephalum Moldavica) under greenhouse conditions compared with traditional P-fertilization (TSP (Triple-superphosphates)) - and unfertilized check in terms of morphological characteristics, and concentration of nutrients (P, Mg, and N) in a calcareous soil. Microscopic images of manufactured composites showed they are free of PO (Parasitic ova) and the presence of FC (Fecal coliforms) in the samples was negative. The results showed that P-Recovery (%) from domestic sewage sludge increased by application of leonardite, and the solubility and the dissolution rate of P, N, and Mg from composites in HCl (0.5 M) and citric acid (20 g/L) are more than that in water. The kinetic of P release from composites was better fitted the Korsmeyer-peppas equation (R-2 = 0.96-0.99) in water, and to the first-order equation (R-2 = 0.87-0.98) in citric acid and HCl solutions. The n value of the Korsmeyer-peppas model showed that the mechanism of P release from the composites is based on the Fickian-diffusion (n < 0.45). The highest positive impact of P-fertilization on morphological characteristics, and concentration of nutrients in Moldavian Balm plants were as follows: L-75/MAP > L-50/MAP > L-25/MAP > L-0/MAP. Therefore Leo/MAP composites, especially with a high amount of leonardite, are applicable to increase the growth of Moldavian Balm in a calcareous soil that is one of the important medical plants with its unique characteristic for human's health.