BEST SUITED STATISTICAL MODEL FOR DESCRIBING YIELD RESPONSE OF POTATO (SOLANUM TUBEROSUM) TO APPLIED NITROGEN FERTILIZER

Under the backdrop of growing environmental concerns estimation of optimum fertilizer rates as accurate as possible is of great interest to the researchers. Trials would be conducted by spending all sorts of inputs including time, capital, labour etc., but if the generated data could not be interpreted properly, all these efforts go waste. Response functions or statistical models are used to estimate the optimal fertilizer recommendations. In the present study, three models were evaluated viz., Quadratic, Square root and Mitscherlich’s models describing the yield response of three potato (Solanum tuberosum L.) varieties (Kufri Jyoti, Kufri Swarna and Kufri Giriraj) to four different rates (0, 50, 100 and 150 kg ha-1) of nitrogen application in each of three years (2004 to 2006) in Nilgiris, a hilly tract in Tamil Nadu, India. Economic optimum N rates (Nop) varied among varieties and models. The proportion of variability (R2) explained by the three models was almost similar. The quadratic model, calculated a minimal Nop value (131 to 140 kg N ha-1) for different varieties than those calculated by the Mitscherlich’s (164 to 174 kg N ha-1) and square root (166 to 192 kg N ha-1) models. Regression residues of the quadratic model were closer to a normal distribution than those of the other two models, indicating a less systematic bias. Economic losses were greatest when the quadratic model was the most appropriate model, but the data were fitted to the square root (loss of Rs.83.4–274.2 ha-1) or Mitscherlich’s model (loss of Rs.1501–2354 ha-1). We conclude that the quadratic model is the most appropriate for describing the potato yield response to N fertilizer and predicting Nop for areas with a ratio of the cost of N fertilizer to the price of potatoes similar to that in Nilgiris.