Nutrient analysis of potato leaves in early growth is not sufficient for a reliable prediction of tuber yield. This hypothesis was verified based on a field experiment conducted during 2006–2008. The experimental factors were: nitrogen (N) rates (60, 120 kg ha−1), fertilizers (Urea, Urea + inhibitor—NBPT ([N-(n-butyl) thiophosphoric triamide]), and sulfur rates (0, 50 kg ha−1). Plant material for nutrient determination (N, P, K, Mg, Ca, Fe, Mn, Zn, Cu), which included leaves, stems, and stolons + roots (R+S), was sampled at BBCH 39/40. The marketable tuber yield (MTY) was in the ranges of 43–75, 44–70, and 24–38 t ha−1, in 2006, 2007, and 2008, respectively. The MTY and contents of N, Zn, and Cu, irrespective of the potato tissue, showed the same seasonal pattern, reaching the lowest values in the dry 2008. The N content in stems was the best tuber yield predictor. A shortage of K in stems and Mg and Cu in R+S, due to the opposite effect of Ca, reduced the N content. An N:Ca ratio in stems greater than 10:1 resulted in yield decrease. A reliable indication of nutrients limiting the tuber yield at the onset of potato tuberization requires data on the nutrient status in both leaves and stems.

A reliable tuber yield prognosis requires a complex statistical analysis of potato nutritional status in the fully developed 4^th leaf at the onset of tuberisation. This hypothesis was validated in the series of field experiments conducted in 2006–2008 in Poland. The experimental design was composed of two nitrogen (N) rates (60, 120 kg/ha), two N fertilisers (Urea and Agrotain), two rates of sulfur (0, 50 kg/ha). The marketable tuber yield of cv. Zeus ranged from 31.3 to 59.3 t/ha in 2008 and 2006, respectively. Despite annual variability, the potato presented a good nutritional status. In 2008, the contents of N, Mg, Cu and Zn were about 33% lower as compared to 2006. The stepwise and path analyses indicated N, Mg and Cu as the key yield-limiting nutrients. The diagnosis and recommendation integrated system (DRIS) showed that a slight imbalance of N and Mg did not disturb tuber yield, provided a positive balance of K was maintained. The Mg index, as a result of the DRIS procedure, emerged as the best single predictor of potato yield.

The in-season dynamics of potato tuber biomass (TTB) growth requires effective nitrogen (N) control. This hypothesis was tested in 2006 and 2007. The two-factorial experiment with two rates of N (60, 120 kg ha−1) and sulfur (S; 0, 50 kg ha−1) was carried out in the split-plot design. The third factor was the sampling of plants at 10-day intervals. The collected plant material was divided into leaves, stems, stolons + roots, and tubers. The seasonal trend of TTB was linear, while the biomass of leaves, stems, and stolons + roots was consistent with polynomial regression models. TTB was controlled by (i) the date of potato growth after emergence, when the TTB exceeded the leaf biomass (DAEcrit); (ii) the stem growth rate; and iii) the share of stems in the total potato biomass. TTB growth was reduced when DAEcrit preceded the DAEop for leaf biomass, determining its maximum. This phenomenon appeared in 2007 on plots fertilized only with N. The absolute growth rate of the stem biomass, exceeding ¼ of that of the tuber biomass in the descending phase, resulted in an increased and prolonged share of stems in the total potato biomass, which ultimately led to a decrease in tuber yield. The use of sulfur to balance the N, applied effectively, controlled the growth rate of potato organs competing with tubers.