Tomato Yield, Nitrogen Uptake and Use Efficiency in Relation to Nitrogen Fertilization Levels

The improvement of the fertilizer use efficiency is needed to reduce the environmental impact of vegetable production, especially in the protected cultivation systems. Nitrogen is one of the most important nutrients for its effects on yield and fruit quality, nevertheless it has a great potential for ground water pollution. Therefore a deeper knowledge on tomato yield response to nitrogen levels is important to increase the N use efficiency for agronomic and environmental aspects in mild winter climate. Two cultivars of baby plum tomato were cultivated in a cold greenhouse located in the south of Italy. Five levels of nitrogen were investigated: 0, 75, 150, 300 and 500 kg ha(-1). The raise of nitrogen rate determined a linear and quadratic increase of yield. The production rose from 57 t ha(-1) in the unfertilized control to 108 t ha(-1) in the treatment with the highest N dose (500 kg ha(-1)), which corresponded to the curve peak. The yield variations were more related to the unit fruit weight than to the number of fruits per plant; no difference was showed between the studied cultivars. As expected the nitrogen uptake increased at the rising of nitrogen input (from 118 to 364 kg ha(-1)). Less evident was the N uptake related to fruit production (kg of nitrogen consumed per ton of tomato produced). The nitrogen efficiency ratio was significantly reduced with the increasing of nitrogen supply (-40% between the control and the highest fertilized treatment). Even our studies showed that the N level to have the maximum production is about five-fold higher than N supply allowed by European politics, the efficiency indexes of nitrogen use were dramatically reduced by N fertilization increase. The lowest decline was between 150 and 300 kg ha(-1) N.