Phosphorus offtake and optimal phosphorus fertilisation rate of some fodder crops and potatoes in temperate regions

Algal blooming caused by phosphorus (P) losses from agriculture is a major problem in northwest Europe. In order to obtain optimal yields while taking into consideration the environmental impact, legislation and phosphate (P2O5) fertilisa-tion recommendations should consider an equilibrium P fertilisation rate for fields with optimal soil P value, with differentiation for other fields taking into account the soil P value.
In this paper, P offtake by cut grassland (Poaceae), silage maize (Zea mays) and potatoes (Solanum tuberosum) is presented and confronted with the stricter maximum P fertilisation rate in Flanders’ new Manure Action Plan (MAP). Phosphorus offtake by cut grassland and silage maize in field trials was higher than the maximum P fertilisation rate at optimal P soil value. The median P offtake by cut grassland was about 110 kg P2O5·ha-1 (1997–2008) and thus ±15 kg P2O5·ha-1 higher than the maximum P fertilisation rate for Class II soils (target Class). More recent trials show a decrease in P concentration of grass. Although there is certainly no problem at the moment, the P concentration in grass should be monitored, because if P concentrations are too low, this could have a negative effect on the nutritional value. The median P offtake by silage maize has increased significantly from 78 kg P2O5·ha-1 in the last decade of the 20th century to 94 kg P2O5·ha-1 in recent years due to the higher yield. This means that even for Class II soils (maximum 80 kg P2O5·ha-1), there should be a small negative P balance and soil P mining. The maximum P fertilisation rate for potatoes (75 kg P2O5·ha-1 for fields with optimal P soil content) is higher than P offtake (median of 59 kg P2O5·ha-1 in the field trials and 58 kg P2O5·ha-1 used in the legislation). As in other countries, this is linked with their high P sensitivy.
There are large differences in P legislation between European countries, ranging from detailed maximum P fertilisation rates as a function of soil P availablity to no restriction on P inputs. The limited soil P input due to stricter maximum P fertilisation rates will reduce the soil P status and will be critical for reducing P losses in the long term. This is the main reason for the strict P legislation in Flanders. However, soil organic matter content and soil quality can be reduced by this legislation, because inputs of organic material, sometimes containing high amounts of P, are reduced. The stricter maximum P fertilisation rates might stimulate manure separation and mixing. Due to the altered nitrogen to P ratio, the products obtained after separating or mixing manure are potentially more interesting as fertilisers.