This paper focuses on the economic and qualitative effects of site-specific application of potassium to potato fields as seen from the farmer's perspective.In 2002, a pilot study was carried out to investigate what effect site-specific potassium application has on potato quality. In 2003, three ordinary plot trials with different potassium applications were performed on one potato field and, in 2004, the trial was performed once again, this time in a different field. The trials showed that site-specific potassium application led to improved potato quality with regards to after-cooking darkening, strong sogginess and weak sogginess. On the other hand, site-specific potassium application had no influence on yield levels. In the trials, a benefit up to 600 Euro/ha was achieved in the field sections with the largest divergence from the mean amount of fertiliser potassium.