Estimating nitrogen nutritional crop requirements of grafted tomatoes under field conditions

• Grafting with vigorous rootstocks affected tomato yield responses to N fertilization rates.
• N-CNR determination differed among the five yield response functions used.
• Grafting led to greater N-CNR estimations and higher predicted marketable yields.

An experiment was conducted to test the hypothesis that the enhanced yield possible with grafted tomato (Solanum lycopersicum L.) under field conditions will also increase the nitrogen (N) crop nutritional requirement (CNR). Determinate ‘Florida 47’ tomatoes were grafted onto interspecific hybrid rootstocks (‘Multifort’ or ‘Beaufort’) and grown in a sandy soil with six N rates (56, 112, 168, 224, 280, and 336 kg ha−1) under plastic mulched bed and drip-irrigation systems during the spring seasons of 2010 (March–June) and 2011 (April–July). The N-CNR for grafted and non-grafted tomatoes was assessed using five yield response functions: exponential, linear-plateau, quadratic-plateau, quadratic, and square root. Over the two seasons, the estimated N-CNR ranged from 165 kg ha−1 with the quadratic-plateau model to 324 kg ha−1 with the square root model. Confidence intervals (CI) around these N-CNR ranged from 125 to 585 kg ha−1 using the bootstrap method and from 98 to 440 kg ha−1 using the delta method. Analysis of these CIs gave N-CNR rates of 239–246 kg N ha−1 for grafted plants, and 196–197 kg N ha−1 for non-grafted plants. Predicted maximum marketable yields were similar between the models, ranging from 56 -71 Mg ha−1 for grafted plants, and 43–53 Mg ha−1 for non-grafted plants, over the two seasons. Overall, while the actual N-CNR is likely to vary with season, soil types, and management practices, the results indicated that grafted tomato plants had a greater N-CNR than non-grafted plants together with an increase in predicted marketable yield. The yield response curves also showed that at a fixed marketable yield goal within the estimated range, the N fertilization rate required was lower for the grafted tomato plants as compared with the non-grafted plants. This study demonstrated that N fertilization program for optimizing tomato production may be modified when grafted plants are used.