Effect of intra-row spacing, gibberellic acid concentration and N:P<sub>2</sub>O<sub>5</sub>:K<sub>2</sub>O ratio on growth and yield of potato variety Royal

The potato variety Royal is suitable to make French fries due to its frying qualities. However, the tubers lack the long-oval shape which is preferred in processing of French fries. Despite the lower nitrogen (N) requirement of variety Royal, farmers apply same recommendation ignoring the varietyspecific needs. Thus, two experiments were conducted to (a) evaluate the possibility of changing tuber shape by manipulating intra-row spacing and gibberellic acid (GA3) application and to (b) identify the best N:P2O5:K2O ratio to optimize growth and yield. The first experiment was conducted with 45 cm inter-row spacing, as a two-factor factorial RCBD. Factors one and two were intra-row spacing (15, 20, 25, 30, and 35 cm) and foliar-applied GA3 concentration (0, 150, 200, and 250 ppm), respectively. In the second experiment, recommended N:P2O5:K2O ratio, 8.0:3.5:8.5, (Department of Agriculture, Sri Lanka) with organic manure (control) and the same without organic manure were compared with three other N:P2O5:K2O ratios. Each treatment had 4 replicates, with 12 plants per replicate in both experiments. Data were analysed with Two-Way ANOVA in the first experiment and One-Way ANOVA in the second, using Minitab 17. Tukey’s mean comparison test was used to compare the mean values of both experiments. Intra-row spacing and GA3 interaction did not significantly (P>0.05) affect the tuber yield and shape. By increasing IRS, yield per plant increased, but yield per unit land area (YPL) decreased. The effect of GA3 on tuber yield was not significant (P>0.05). GA3 at 200 ppm recorded the highest number of long-oval tubers (62% increment). N:P2O5:K2O ratio had no significant (P>0.05) effect on growth, yield, and quality. Therefore, a fertilizer ratio with low nutrient input will be more sustainable and cost-effective for potato variety Royal with lower nutrient needs. Closer intrarow spacing (15 cm) with 200 ppm GA3 can be recommended for higher YPL with higher number of long-oval tubers.