Improved Phosphorus Recycling in Organic Farming: Navigating Between Constraints

Phosphorus (P) is an essential element for all living organisms. At the current rate of extraction, global reserves of mineable deposits will be exhausted within the next few centuries. This publication aims to summarize the current knowledge on P recycling for organic farming. The evaluation of recycled P fertilizers (RPFs) includes (i) a chemical characterization, (ii) assessment of their plant P availability and added effects in the soil, (iii) life cycle assessments, (iv) a risk assessment of their long-term impacts on soil pollution, and (v) the compilation of other environmental impacts of different treatment approaches to produce RPFs. The highest nutrient recovery rates for P are achieved by rather simple process approaches of P recycling, while more sophisticated approaches often result in lower P recovery rates (e.g., chemical approaches for P precipitation), lower plant P availability in the final product (e.g., most thermal approaches), and losses of organic matter and nutrients like nitrogen and sulfur (e.g., thermal approaches). The plant P availability of many RPFs is higher than that of phosphate rock. Each P recycling approach has strengths and weaknesses. We conclude that any decision not to use a potential recycled P source or to introduce sophisticated treatments may have consequences compromising the ability of future generations to meet their needs. Nevertheless, we need to minimize risks for current and future generations caused by contamination linked to fertilization. Therefore, any management of nutrient recycling requires navigation between constraints. The challenge for the organic agriculture sector is to assess RPFs using a balanced approach that compromises neither the principle of ecology nor the principle of care.