Nitrogen Rates Effects on the Yield, Nutritional Status, Fruit Quality, and Profitability of Tomato Grown in the Spring with Subsurface Irrigation

Abstract

With increasing environmental concerns, the sharp cost increase of fertilizer and the absence of a soil test to predict nitrogen (N) needs of tomato (Solanum lycopersicon L.) grown on Florida’s sandy soils, a partnership was created with growers, state agencies, and the University of Florida, Institution of Food and Agricultural Sciences (UF/IFAS). The objectives of this study were to identify a range of N rates that would result in highest yields and postharvest quality, and maximum economical return for tomato, grown with subsurface irrigation (management of a perched water table above an impermeable soil layer or hard pan) during the spring season (low probability of leaching rain events). The study was conducted in Spring 2007 and 2008 in Palmetto, FL, with N rates ranging from 22 to 470 kg·ha−1 at pre-plant as ammonium nitrate (NH4NO3). Weather conditions were typical of a dry spring season in central Florida with no leaching rain events recorded in either year; however, rain patterns were different between the 2 years. In the absence of leaching rain and frost protection (either may raise the water table), petiole sap NO3-N decreased over time and the rate of decline depended on the N fertilizer rate. Extra-large and total marketable fruits yields showed a quadratic plateau response to N rates with maximum yields at two harvests (97% of the yields) grown with 172 and 298 kg N/ha in 2007 and 2008, respectively. During subsequent ripening, N rate did not correlate consistently to fruit ripening rate, fruit firmness, nor compositional quality at table-ripe stage. The high value of tomatoes relative to the cost of N fertilizer created a situation in which the profit-maximizing rate of N was not significantly different from the production-maximizing N rate. Whether the profit-maximizing level of N was higher or lower than the UF/IFAS-recommended rate depended on the growing season. With favorable growing conditions (i.e., conditions in 2008), a grower’s net return would have decreased between $1000 and $2000 per hectare by using UF/IFAS-recommended rates depending on market conditions. However, if the UF/IFAS-recommended rate of 224 kg·ha−1 resulted in the highest yield, applying upwards of 300 kg·ha−1 would have increased grower production costs by at least $67/ha. Although fertilizer costs are known before the crop is grown, tomato prices are realized only at the end of the growing season and profit margins can only be calculated after the fact.