Assessment of Alternative Phosphorus Fertilizers for Organic Farming: Chars, Ashes and Slags

The application of sewage sludge to agricultural land, a common practice to recycle nutrients in the past, is currently prohibited in organic farming due to the risk of soil contamination from pollutants (e.g. toxic and potentially toxic elements, persistent organic pollutants, etc.). Incineration or disposal to landfill of dewatered sludge is costly, leads to greenhouse gas emissions, and eliminates the possibility of nutrient recovery, especially phosphorus. Options to treat dry organic matter are combustion and gasification, among other technologies to produce ashes or chars and to recover the residues for nutrient recycling. Dependent on the technology applied as well as the input material, nutrient concentrations and concentration of contaminants vary widely. Due to organic matter degradation, non-volatile elements including mineral nutrients like phosphorus, potassium, magnesium and calcium are enriched in ashes and chars. Thermal processes have high P recovery rates of 80 to 90 % depending on the treatment procedure. For organic farming, ashes from sewage may allow the recycling of P from the food chain by rather simple technologies. The risks of reducing the soil quality by xenobiotic compounds may be minor with incinerated products. Permission to use such fertilizers in organic farming in the EU would contribute to better nutrient cycling between rural and urban areas, reduce problems of declining phosphorus concentrations in some organic farming systems and could also contribute to the development of practical methods for processing sewage. This fact sheet describes different types of chars and ashes with their characteristics and possible applications. Beyond that it indicates ways to enhance the use of ashes in organic agriculture