Agronomic Effects of Underseeding Ribwort Plantain (Plantago lanceolata L.) in Organically Grown Potatoes

In a two year field experiment at Göttingen (Germany) organically grown potatoes were undersown with Plantago lanceolata L. (ribwort plantain) 44 (2002) and 21 (2004) days after potato planting. In the plots which were undersown, U(+), the weeds were suppressed by 59% (2002) and 70% (2004) compared to the plots without underseeding, U(-). Potato yield was only slightly affected by underseeding. However, in the undersown plots the portion of undersized tubers was increased by 15% in 2002 and 11% in 2004. At potato tuber harvest the average mineral soil N quantities (N_min = NO₃-N + NH₄-N) from the ridge and furrow zone (0-120 cm soil depth) were 62.4 kg N ha^-1 in 2002 and 40.1 kg N ha^-1 in 2004 in the U(-) plots vs. 44.6 kg N ha^-1 (2002) and 12.5 kg N ha^-1 (2004) in the U(+) plots. At the last sampling date in late autumn the reduction of the N_min quantities in the U(+) plots was 52% (2002) and 56% (2004). At the end of September 2004 the maximum total root length ranged between 2.21 km m^-2 in U(-) and 6.37 km m^-2 in U(+).