On-farm use of recycled liquid ammonium sulphate in Southwest Germany using a participatory approach

For political and environmental reasons, there is an urgent need for alternatives to energy-intensive synthetic fertilizers. One solution is the targeted recycling of nutrients within agriculture. In this study, liquid ammonium sulphate (LAS) as a recycling product derived from digestate treatment was compared to calcium ammonium nitrate, manure and original digestates in an on-farm experiment using a participatory approach. Based on regular meetings with the farmers involved, a flexible experimental design was developed which integrated the fertilization legislation and the farmers’ operational structures already in place, such as crop rotation, available application techniques and manure management demands. The aim was to achieve both implementation practicability and acceptance of the study results by the farmers. Results from the year 2020 showed that LAS applied with three-jet nozzles in barley and wheat had significantly lower yields than the other fertilizers. Applied with a slurry tanker trailing shoe applicator in 2021, LAS had comparable yields to the other fertilizers in maize (51.2 t ha −1 ) and comparable yields to digestate in rapeseed (4.4 t ha −1 ). Application techniques that minimize environmental impacts and lower the LAS pH could potentially increase the effectiveness of the fertilizer. We recommend that farmers use this fertilizer not as a single solution but as a mineral compensatory fertilizer in addition to organic fertilizers following local fertilizer legislation. In this case, LAS could potentially substitute calcium ammonium nitrate (CAN).