Development of a Physicochemical Test Kit for On-Farm Measurement of Nutrients in Liquid Organic Manures

Optimised use of liquid organic manures (LOM) can reduce the consumption of mineral fertilisers and help reduce the emission of nutrients into nonagricultural ecosystems. To achieve this, farmers need to be able to measure the greatly variable nutrient composition of LOMs as accurately as possible on-farm. Since existing on-farm test methods either need to be precisely adapted to each LOM type or take a long time to perform, a test kit was developed to measure the nutrients of different LOM types within a short time. For the study, 619 LOMs (391 pig slurries, 139 cattle slurries, and 89 digestates) were collected from farms in northwest Germany and analysed in the laboratory for total N, ammonium, phosphorus, and potassium. The samples were analysed in parallel using the on-farm test kit consisting of ion-selective ammonium and potassium electrodes and an automatic moisture analyser to evaluate the comparability of the data. Each measurement could be performed in less than 15 min. Regardless of LOM type, regressions with an R² > 0.9 could be generated for total nitrogen, ammonium, and potassium, while the models for phosphorus were not as reliable