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Criteria for managing fertilizer nitrogen (N) applications for lettuce ( Lactuca sativa  L.) based on midrib nitrate-N analysis were developed 25 years ago. However, this test had not been recently evaluated for the newer cultivars of lettuce currently grown or the higher yield potential now obtained. More recently, quick sap nitrate-N tests have been correlated to the traditional dry midrib test and preliminary criteria for making diagnosis based on these sap tests have been proposed. Field experiments were conducted at 20 locations across the low desert region of the southwestern United States from 1996-1999 to evaluate the traditional dry midrib and sap nitrate-N tests. Tissue samples were collected before each sidedress N application and diagnostic accuracy was evaluated by determining lettuce growth and yield on both N-treated and untreated plots and comparing predicted to actual response. Overall, the variability associated with the quick sap test seemed to limit its application as a predictive N management tool in the low desert. Although less variable than the quick sap test, the high frequency with which the dry midrib test resulted in incorrect diagnosis suggests that either this test needs revision or that it is an unreliable N management tool.

452 Evaluation of Midrib Nitrate-N Tests for Desert Lettuce