The application of straw mulch to organic seed potatoes (Solanum tuberosum L.) has been shown to reduce virus incidence. In order to determine the associated agronomic effects of straw mulch, applied at 2.5–5 t ha−1, on soil nitrate dynamics, weed development, tuber yield and soil erosion, 12 field experiments were evaluated. Experiments were conducted on organic farms over 3 years at two locations in a temperate climate (635–709 mm precipitation/year; 8.1 °C mean air temperature) on loamy silt soils. Tuber yield and tuber size distribution were not influenced significantly by mulching. However, the risk of undesirable post harvest N-leaching was significantly reduced due to the immobilization of nitrate–N after harvest at 6.8–7.0 kg N t−1 straw in two experiments (18–34 kg NO3–N ha−1). There was no consistent effect of straw mulch on number of weeds, weed cover and above ground biomass of weeds. The fact that yield and weed development were not significantly affected by straw mulch is mainly attributed to the relatively low amounts of straw applied. Soil erosion was reduced by >97% in a rain simulation experiment on a potato field of 8% slope with 20% crop cover. Soil loss was greatest (1606 g m−2) in the unmulched treatment, and 31, 42 and 26 g m−2 in treatments with chopped straw at 1.25, 2.5 and 5 t ha−1, respectively.