Phosphorus effects of recycled products from municipal wastewater on crops in a field experiment

In this study, the phosphorus (P) fertilizing effects of struvite, one thermochemical-treated sewage sludge ash (SSA) based on Ca-P (Ca-SSA) and one full sulfuric acid-digested SSA based on Al-P (Al-SSA) were analysed in comparison to triple superphosphate (TSP) and a control treatment (CON) without P application in a two-year field experiment. In the field experiment, the effects of the recycling products on crop yield, P uptake and labile soil P fractions were analysed. In addition, the effect of nitrogen and magnesium contained in struvite was investigated in the second year of the experiment compared to TSP and CON. In the first year, spring barley was cultivated in the field experiment; and in the second year, it was forage rye followed by sorghum. In the second year, the relative P effectiveness (forage rye, sorghum) of the recycling products compared to TSP increased in the order: Ca-SSA (81%, 91%) <= Al-SSA (91%, 96%) = struvite (102%, 110%). In addition, an magnesium fertilizing effect of struvite could be demonstrated. The results show that the recycling products from wastewater treatment are appropriate to substitute rock phosphate-based fertilizers.