Estimating Nitrogen Fertilization Requirement for Grafted Tomato Grown in the Field

Grafting with interspecific rootstocks has been observed to enhance growth and fruit yields of tomato plants. It is likely that fertilizer inputs may be adjusted in grafted tomato production as compared with the current recommendation for non-grafted tomato plants. Because the Crop Nutritional Requirement for N for grafted tomato plants has not been established for drip-irrigated tomato production, the purpose of this study was to determine a mathematical function of applied nitrogen that can best describe the expected marketable fruit yield of grafted tomatoes. Non-grafted and grafted ‘Florida 47’ tomato with interspecific hybrid rootstocks ‘Multifort’ and ‘Beaufort’ were grown in Spring 2010 and Spring 2011 with the combinations of two irrigation regimes (50 and 100%) and six N rates (56, 75, 148, 224, 298, and 372 kg·ha^-1) in sandy soils in Live Oak, FL. The marketable fruit yield data were fitted to quadratic, square root, linear plateau, quadratic plateau, and exponential functions. The goodness of fit of the regression models was compared based on the coefficient of determination (R²), the Akaike information criterion (AIC), and the root mean squared errors (RMSE). The exponential model provided the best fit of the marketable fruit yields of both grafted and non-grafted plants as a function of the N rates. Using the exponential model, estimated minimum N fertilization rates to reach maximum marketable fruit yields ranged from 175 to 269 kg·ha^-1 of  N for the grafted plants in the 2010 study, depending on the irrigation regime. For the non-grafted plants in the 2010 study, the estimated N rates were in the range of 113 to 176 kg·ha^-1 of  N, indicating that higher N fertilization rates were needed to maximize the marketable fruit yields of grafted plants as compared to the non-grafted plants. The corresponding maximum marketable fruit yields were also higher in grafted tomato production ranging from 57 to 67 Mg·ha^-1 in comparison with 42 to 50 Mg·ha^-1 for non-grafted plants. The 95% confidence intervals associated with the estimated N rates were considerably wide, suggesting that care should be taken in formulating N recommendations for growers. The results suggest that higher N rates were required to optimize the marketable fruit yield of grafted ‘Florida 47’ tomato with vigorous interspecific rootstocks than the N requirement for non-grafted plants in field production.