Nitrogen Requirements of Drip-irrigated Processing Tomatoes

As growers of processing tomato (Lycopersicon esculentum Mill.) adopt drip irrigation, plant vigor and fruit yield typically increase, suggesting a need for re-evaluation of established nitrogen (N) fertilization practices. Trials were conducted in California in 2007-2008 to evaluate growth and N uptake dynamics of drip-irrigated processing tomatoes across N fertigation regimes ranging from deficient to excessive. Whole plants were collected at 2-week intervals for determination of biomass and N content, recently matured whole leaves for total N and petioles for NO3-N. Additionally, six commercial fields were sampled at 3- to 4-week intervals to document N uptake and crop N status under conditions representative of the industry. A seasonal N rate of approximate to 200 kg.ha(-1) appeared adequate to maximize fruit yield across the range of field conditions encountered. The four highest-yielding fields (143 Mg.ha(-1) mean fresh fruit mass) averaged 14 Mg.ha(-1) of above-ground biomass with fruit representing 62%; these fields averaged 296 kg.ha(-1) biomass N, of which 71% was in fruit. The rate of biomass development and N uptake peaked during the period between early fruit setting and early red fruit development (a period of approximate to 6 weeks) during which N uptake averaged 4 to 5 kg.ha(-1).d(-1). Leaf N concentration was highly correlated with whole plant N (r(2)=0.83) and provided a reliable indicator of plant N sufficiency throughout the season. Petiole NO3-N did not reliably discriminate between crops with adequate or deficient N availability; current petiole NO3-N sufficiency guidelines are unrealistically high.