The use of tissue testing of crop nitrogen status is widespread in vegetable production; given the high value of vegetable crops the interest in nutrient testing has historically been to ensure maximum production and cosmetic quality rather than to achieve high fertilizer use efficiency. In drip-irrigated culture, growers have the ability to inject N throughout the season, and the use of tissue testing to ‘fine tune’ the N fertility program is common. However, increased awareness of the environmental impact of nutrient loss from vegetable fields has introduced an environmental perspective on nutrient ‘best management practices’ (BMP), and caused a reevaluation of tissue N testing. Recent California research with lettuce and tomato has confirmed the validity of leaf total N concentration as a measure of crop N status. In lettuce and tomato, leaf N is strongly correlated with whole plant N across fields, growth stages and cultivars; current leaf N sufficiency guidelines are generally consistent with those predicted from the theoretical whole plant critical N concentration. Leaf N changes slowly over time, allowing the grower to project crop N status into the future. Leaf reflectance has been explored as a more easily measured surrogate for leaf N concentration, but has proven problematic. Cultivar differences in leaf reflectance characteristics, and the confounding effects of factors unrelated to N status (soil moisture status, foliar disease, etc.) confound the relationship between leaf reflectance and leaf N concentration, and limit the practical value of reflectance measurements.
Petiole NO3-N analysis is popular in the California vegetable industry because it is more easily measured than leaf N (even measured on-farm by sap analysis), and is widely believed to represent recent soil N uptake. Unfortunately, petiole NO3-N testing has proven to be an unreliable measure of plant N status. Petiole NO3-N is poorly correlated to current soil N availability or crop N uptake rate. The effects of environmental factors on NO3-N assimilation rate can be profound; coupled with the relatively small amount of NO3-N usually contained in whole plant biomass (typically the equivalent of less than 3 days of crop N uptake), petiole NO3-N concentration can be highly volatile, and does not dependably predict future fertilization requirement. Current petiole NO3-N guidelines are unrealistically high, and fertilizing to maintain those levels will often result in unnecessary fertilization.