Phosphor-Bodenbilanzen von Grünland- und Ackerflächen auf einem ökologischen Milchviehbetrieb

Summary

Mineral phosphorus (P) fertilizers from fossil reserves are finite which puts agriculture at risk of losing soil fertility. This study calculates P soil balances of an organic dairy farm in Northern Germany to focus on potentials and variations in the farm P cycle. P flows were different between the years because yield levels and amount of manure application varied. Different plant species showed different P export which depended mainly on biomass export. This can also be seen as their site specific mobilization potential. The highest mean P export between 2010 and 2012 was measured with maize (mean 30.4, range 17.7-38.2 kg P ha-1 a-1) and grass-clover (mean 23.7, range 17.3-29.5 kg P ha-1 a-1). By grazing, permanent grassland showed a mean P mobilization from soil P of 15.9 kg ha-1 a-1. The annual soil P balance was -7.96 kg P ha-1 (range -6.16 to -9.3 kg P ha-1) at permanent grassland and -11.1 kg P ha-1 at arable land (range -1.5 to -17.7 kg P ha-1). Because of the high ranges it is reasonable to analyze a few years to get a better view on P flows at farms. High PCAL values from historical fertilizer inputs can be seen as a P source in short term and can especially be mobilized by permanent grassland. It is necessary to change the P flows into a more sustainable way before the P gap cannot be closed by soil reserves anymore and P deficit will limit plant growth.