Estimating Agronomic Performance of Recycled Phosphorus Fertilisers – Challenges and Possible Solutions

Rock phosphate is a scarce resource and is seen as a critical raw material by the European Union. In line with the EU Circular Economy Action Plan and related political initiatives, the production of recycled phosphorus (P) fertilisers from secondary phosphate-containing materials is therefore gaining in importance. In order to assess their agronomic performance, reliable test methods are needed. The agronomic performance of recycled P fertilisers is affected not only by the fertiliser itself, but also by soil- and plant-related factors.

Standard chemical extraction methods are insufficiently able to account for the interaction of these factors in the complex processes of nutrient dissolution and transport taking place at the soil / fertiliser / plant (root) / water interface. Therefore, alternative methods based on chemical extractions of incubated soil/fertiliser mixtures have been proposed. Before they can be adopted as reference methods, they need to be calibrated based on vegetation trials. However, the design of a standard vegetation trial for fertiliser testing is not without difficulty, either. This paper presents first results of ongoing research aiming to standardise greenhouse vegetation trials for fertiliser testing. The main goal was to develop a standardised substrate that was (1) poor in the nutrient under study (P), (2) easily reproducible at any time and place (within Central Europe), and (3) able to represent the key factors affecting plant availability of P in an agricultural soil under Central European climate conditions. In addition, a standardised experimental design needs to be developed that provides optimum growing conditions for the main crops cultivated in Central Europe, including a basic fertilisation strategy that enables the observed effects to be reliably traced back to the studied nutrient, with the option to distinguish between short- and long-term effects, as well as easy handling.

From 2018-2020, substrate mixtures consisting of different types of quartz sand, clay and organic components were tested, using ryegrass as a model crop. A mixture comprised of coarse pure quartz sand and bentonite, at a 90/10 ratio, and with the addition of black peat to reach an organic matter content of 2%, was identified as suitable for carrying out fertiliser tests. A pot size of 1,200 cm³ was chosen as a compromise between adequate root space and surface area on the one hand, and tolerable maintenance efforts on the other hand. Future work is needed to optimise the fertilisation strategy and investigate further aspects, including the use of a more sustainable organic component, as well as further characterisation of the test substrate regarding microbial activity and soil chemical and physical parameters.