Evaluation of in‐season nitrogen tissue tests for drip irrigated leaf and romaine lettuce

Abstract

Chemical analysis of selected plant tissues as a nitrogen (N) fertility diagnostic technique has been established for many irrigated horticultural crops, but not for recently popular high value specialty vegetables 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 through subsurface drip irrigation tubing at scheduled intervals to leaf and romaine lettuce to provide N levels ranging from deficient to excessive. Plant samples, which included the midrib or leaf blade from the youngest full‐sized leaf and whole plant tissues were collected throughout the growing season with midribs tested for NO₃‐N and the remaining tissues for total nitrogen content Marketable fresh weight yields were recorded at harvest for all N treatments in each experiment. In Experiment 1, it was determined that of the plant N testing methods evaluated, midrib NO₃‐N concentration in the youngest mature leaf was the most responsive to differences in soil N supply. Experiments 2 and 3 focused on the midrib NO₃‐N method of testing to develop season‐long interpretations for evaluating the N fertility status of both crops. The slow root and shoot development inherent in each lettuce type and the minimal uptake of N through mid‐season did not contribute to well‐defined differences between deficient, sufficient, and excessive midrib NO₃‐N levels. Resulting interpretations of midrib NO₃‐N concentrations for leaf and romaine lettuce feature a greater sensitivity and practicality as a N fertility diagnostic tool during the latter one‐half of the growing season.

Notes

Contribution of the University of Arizona Agricultural Experiment Station, Tucson. This work was supported in part by the Tennessee Valley Authority and the Arizona Department of Environmental Quality.