Influence of Nmin target values on fertiliser need, yield and Nmin residues in asparagus (Asparagus officinalis L.)

Asparagus with more than 14,000 ha cultivated in 1998, covers the largest growing area of all vegetables in Germany. For this reason, solutions to reduce environmental pollution of the groundwater by nutrients have become more and more urgent. Asparagus takes up nitrogen from the end of June until the end of September under German conditions. In the other months, nitrogen is at risk of leaching. To reduce this risk, the fertiliser applied to asparagus should depend on the N-MIN content in the soil. A field trial was carried out to test four different N-MIN target values: 30, 60, 90 and 120 N kg ha(-1), without irrigation. The yield and quality of spears was determined daily over eight harvest years. On average 6 kg/ha N fertiliser had to be applied in order to reach the N-MIN target value of 30 kg, 33 kg for the target value of 60 kg, 62 kg for the target value of 90 kg and 85 kg for the target value of 120 kg. Total yield and number of spears showed no significant differences between all target values. However, the cumulative yield of grade I spears (diameter >16 mm) increased by more than 10 % with the 90 kg target value, compared to the 30 kg value. When annual N-MIN residues were analysed over the years, the influence of weather was higher than the influence of different fertiliser amounts. In all years, the tendency for high N-MIN at harvest was observed only at the highest N-target value.