Variatie in knolkwaliteit tussen en binnen partijen van consumptieaardappelrassen

For potato tubers destined for French fry production, dry matter and nitrate contents, black spot susceptibility and after cooking blackening as well as fry colour after storage are important quality characteristics. The possibilities to manipulate these characteristics and tuber yield with nitrogen fertiliser level and an additional spring application of potassium (300 kg K2O*ha-1, as sulphate or chloride) were investigated in a number of cultivars, on various locations and in several years. The choice of cultivar proved to have much more relevant impact on characteristics than the ranges of nitrogen and potassium fertilisation. It was analysed to what extent variance of quality characteristics was accounted for by the different experimental factors. Depending on characteristic, about 10 to 30 % was accounted for by cultivar. Nitrogen and potassium fertiliser contributed only 0 to 10 %. The major part of variance was caused by environmental influences related to the factors location, year and their interactions with the other experimental factors. For tuber dry matter and nitrate contents and for black spot susceptibility also variance within lots was investigated. The greater part of the variance was found within rather than between experimental plots. In two experiments for tuber dry matter and nitrate contents variance was analysed down to the level of individual plants, individual stems and individual tubers. Differences between tubers originating from the same stems accounted for 80 to 90 % of total variance found within potato lots. Since nitrogen reduces some quality characteristics its effect on yield was also analysed in order to gain insight into the need for increased financial return by a higher price for increased tuber quality to compensate the grower for lower yield at lower nitrogen levels. The required absolute price increase is rather small, however it is questionable whether buyers are willing to pay the rise since quality benefits of lower nitrogen dressings were also quite small. High potassium levels have positive effects on fry colour, after cooking blackening and black spot susceptibility; the latter was especially reduced when applying potassium in the chloride form. Based on presented yield data and on analysis of historical data it is postulated that in present Dutch practice ware potato growers are too reluctant to use potassium chloride on clay soils in spring in fear of yield depression, caused by high salinity levels. Tuber sucrose, dry matter and nitrate contents as well as ground cover by the crop canopy were determined before and at harvest time and linked to fry colour index after storage, for two cultivars at two locations in three years. None of the parameters at any time of determination correlated well enough with fry colour after storage to allow a prediction of the latter before or at the time of harvest. In many - but not all - cases the fry colour index was increased by increased nitrogen dressing. Effects of nitrogen on fry colour and effects on pre-harvest determined parameters also did not correlate well enough to allow a prediction of the effect of nitrogen on fry colour. Analysis of the manageable factors (cultivar, nitrogen, potassium and chloride level) and the unmanageable factors (location and year) showed that of the manageable factors cultivar accounted for most of the variation in quality characteristics between lots. Nitrogen, potassium and chloride fertilisation accounted for only small portions of the variation between lots. The major portion of the variation between lots was accounted for by location and year and their interactions with the controlable factors. Apart from choice of cultivar growers appear to have little control over the variation in tuber quality characteristics between and within lots.