Measuring On-Farm Phosphorus Fertiliser Use—Lessons Learned from Surveying Data of Five Regions in Northern Germany

Phosphorus (P) is essential for agriculture; however, local P surpluses can have adverse environmental effects, such as eutrophication. Optimal P fertiliser use, therefore, means balancing these effects. Although P fluxes from soil to plants are key research areas, little is known about on-farm use of P fertilisers. As, typically, not all fields or crops are treated with P annually, the transferability of measurements for other nutrients, such as nitrogen, appears limited. This study aims to close this knowledge gap. On-farm P use was described using the farm records of 50 farms in five regions of northern Germany for the period of 2010 to 2018. All sources for P fertilisation were taken into account as either P mineral, P organic or P total. Two indicators for on-farm P use were suggested: frequency, which gives the percentage of the treated area, and amount, which gives the quantity used per ha of the treated area. The frequency for P total ranged from 55.9% to 93.1% of the total farm area being fertilised. Amounts between 24.8 and 41.6 kg ha−1 P total were applied on the treated area of the farm. The results supported the separation of the quantity and frequency in on-farm P use. No decrease in P use was found during the period investigated. Using mixed models, the results further show that explanatory variables, including the farm characteristics and crop choice, explain the substantial variations in P use. It is recommended for the example of Germany to establish an official digital database for P fertilisers that can be updated professionally and is mandatory for all documentation on P use.