Does legislation mitigate the impact of legacy soil phosphorus on water quality in horticultural fields?

Phosphorus (P) is a major factor in eutrophication of surface waters as P is typically the limiting factor for algal blooming in freshwater systems. In some European countries, agriculture has become the main source of P in water bodies. In 2019, European experts were contacted to obtain national and regional regulations regarding P fertilization. Maximum P fertilization rates or P application standards have gradually been introduced in the legislation of many European regions. In most regions, the maximum P fertilization rates depend on the soil P concentration and the crop. The number of soil P classes varies between 3 and 10. In case the soil P status is above the target zone, the P application standard should be below the P offtake, resulting in P soil mining. Many extraction methods are used to measure the soil P concentration depending on local soil conditions. In most regions, the P capacity (P available at long-term) is measured, which changes slowly. Nevertheless, the validity of the soil samples is limited to 3 to 6 years. Agricultural production is theoretically calculated by the P offtake. However, although the P offtake varies significantly e.g. ±20 kg P2O5 ha‑1 is exported by spinach while up to ±60 kg P2O5 ha‑1 by Brussels sprouts, the P application standards for these vegetables are only different based on the soil P class. The benchmark study shows that in several European regions further fine-tuning of the P legislation is possible. It is recommended that the risk of P losses is measured directly by runoff and leaching experiments and that a distinction in P application standards for vegetables as well as for the cultivation of multiple vegetable crops in one season have to be considered.