One of the top objectives of the table and food processing potato production is good tuber quality. The fact that potatoes are grown for a number of different uses makes the quality requirements high, yet also varied, both in terms of morphological traits and the chemical composition. Throughout the production cycle, potato storage is the most difficult stage of maintaining good quality of tubers. During long storage, tubers are affected by processes leading to quantitative and qualitative changes. The present three-year field experiment (2003-2005) investigated the effect of varied mineral fertilisation (N, K and Mg against a fixed P dose) applied over the plant vegetation period and the influence of storage (3 and 6 months) on the content of dry matter and starch in tubers of mid-early potato cultivars: Bila and Triada. The samples were stored in a storage chamber for 3 and 6 months (temperature +4oC, relative humidity 95%). The content of dry matter and starch was significantly affected by both the fertilisation and the storage time. With increasing N, K, Mg doses, a significant increase in the content of dry matter appeared, as compared  with the control, and its highest content was reported for the fertilisation with 100 kg of nitrogen (3.5% increase), 80 kg of potassium (7.7% increase) and 100 kg of magnesium per hectare (6.2% increase). However, as for the content of starch, the doses of 100 kg of nitrogen, 160 kg of potassium and 100 kg of magnesium per hectare turned out to be the most favourable. The losses of dry matter and starch in potato tubers calculated from the balance were the highest after 6 months of storage and accounted for 8.1 and 15.4% for the magnesium experiment, 6.6 and 10.1% for the nitrogen experiment, 6.7 and 7.9% for the potassium experiment, respectively.