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Three field experiments were conducted during 2002 and 2004 on two sites in Germany in order to examine the impact of preceding crop, pre-sprouting, N- and K-fertilization and cultivar on total tuber fresh yields, tuber DM, glucose and fructose concentration, as well as the colour of crisps and the quality score of French fries at harvest and after storage. Generally, total tuber yields depended very much on the growing season. However, highest yields were obtained when horn grits were applied along with potassium sulphate. Increasing yields after cattle manure fertilization could be attributed to K rather than N. Combined N and K fertilization may cause DM concentration to fall short of the required minimum for crisps. Pre-sprouting and storage increased tuber DM concentration considerably. Cultivars belonging to the very early and early maturity type showed the largest relative increase of reducing sugars due to storage. On the whole, results suggest that the effect of agronomic measures such as fertilization, preceding crop and seed-tuber preparation may be rather small and the response of internal tuber quality and quality of fried products difficult to predict. The quality standards for tuber raw stock can be accomplished best when adequate cultivars suitable for storage are chosen.

The effect of different sources of K and N on graded yields, dry matter, reducing sugars and quality of french fries and crisps, respectively were examined in two maincrop varieties (AGRIA and MARLEN). Fertilizers applied were cattle manure, potassium sulphate (PS), PS + grinded horn, grinded horn (GH) and a control. Results of two-factorial field experiments (2002-2004) on a loamy sand are presented. Cultivar and fertilizer had a significant effect on final yield, combined mineral K and organic N (PSGH) being the most efficient fertilizer with regard to graded yields for crisp (>40 mm) and french fries production (>50 mm). K content of tubers could be increased, but dry matter was lowered by most fertilizers, so that only the unfertilized plots yielded tubers with a dry matter content favourable for crisp production. As a result of the warm and dry weather in 2003, however, dry matter was high enough, but not too high for french fries (AGRIA), no matter if fertilizer was applied, while in 2004 PSGH lead to a decline in dry matter of around 2.5 %. Further quality parameters important for processing (reducing sugars, crude mash discoloration, after-cooking darkening) were not impaired by fertilization, but variety and storage (4 months at 8°C) had a significant effect.