EVALUATION OF IN-SEASON NITROGEN TISSUE TESTS FOR DRIP IRRIGATED LEAFAND ROMAINE LETTUCE

Chemical analysis of selected plant tissues as a nitrogen (N) fertilit y diagnostic technique has been established for many irrigated horticu ltural crops, but not for recently popular high value specialty vegeta bles such as leaf and romaine lettuce (Lactuca sativa L.). Three field experiments were conducted in southern Arizona during three years to determine the plant part and N form that is most responsive to soil N supply, and to formulate in-season N status interpretations based upon appropriate tissue tests. Fertilizer N treatments were applied throug h subsurface drip irrigation tubing at scheduled intervals to leaf and romaine lettuce to provide N levels ranging from deficient to excessi ve. Plant samples, which included the midrib or leaf blade from the yo ungest full-sized leaf and whole plant tissues were collected througho ut the growing season with midribs tested for NO3-N and the remaining tissues for total nitrogen content. Marketable fresh weight yields wer e recorded at harvest for all N treatments in each experiment. In Expe riment 1, it was determined that of the plant N testing methods evalua ted, midrib NO3-N concentration in the youngest mature leaf was the mo st responsive to differences in soil N supply. Experiments 2 and 3 foc used on the midrib NO3-N method of testing to develop season-long inte rpretations for evaluating the N fertility status of both crops. The s low root and shoot development inherent in each lettuce type and the m inimal uptake of N through mid-season did not contribute to well-defin ed differences between deficient, sufficient, and excessive midrib NO3 -N levels. Resulting interpretations of midrib NO3-N concentrations fo r leaf and romaine lettuce feature a greater sensitivity and practical ity as a N fertility diagnostic tool during the latter one-half of the growing season.