Redistribution of soil phosphorus from grassland to cropland in an organic dairy farm.

Limited knowledge is available on inner farm nutrient transfer from organic grassland to arable land in organic farms. This study quantifies the phosphorus (P) mobilization of permanent grassland and different arable crops for inner farm P transfer and discusses in how far P reserves in grassland soils can be a component of sufficiency P management in organic farming. A North German organic dairy farm with sufficient soil P supply is analyzed. Over three years its P balance showed an average deficit of 7.9 kg ha-1 yr-1 in permanent grassland and 10.9 kg ha-1 yr-1 in arable land. Maize (30.5 kg P ha-1 yr-1), grass-clover (23.9 kg P ha-1 yr-1) and mixed faba bean and oats (19.8 kg P ha-1 yr-1) had the highest P uptake in cropland. At grassland, grazing intake of P by livestock was 15.9 kg P ha-1 yr-1 and via storage feed and manure it directly fed arable land with 64 kg P yr-1 (average 1 kg P ha-1 yr-1). Especially on grassland, soil P mining does not endanger soil fertility yet, according to sufficient available P-contents in the soils (CAL-extract averages [mg 100 g-1 P]: grassland 14.7, arable land 6.7). Generally, the inclusion of unexploited grassland sites with high soil P contents in farm nutrient flows (via feed conserves for livestock or biogas) would address unused soil P reserves for redistribution.