Diagnostic efficiency of the blacklayer stalk nitrate and grain nitrogen tests for corn

Protecting water quality and maintaining profitable corn (Zea mays L.) prod uction requires diagnostics that can distinguish between N deficieny, suffi ciency, and excess. This study evaluates initial recommendations on blackla yer basal-stalk NO3-N ranges and critical concentrations for diagnosing N s tatus, and it compares the performance of this test with gain analysis, Obs ervations (428) were collected from 13 N-response experiments. linear respo nse and plateau (LRP) and binary logistic regression (BLR) were used to cha racterize the relationships between yield and tissue-test values. With the LRP, stalk NO3-N and grain N concentrations separating deficient front suff icient observations were 0.42 and 13.1 kg(-1), respectively; and the succes s rates of the two tests were comparable (77 and 75%, respectfully). The BL R also identified critical concentrations, but the values increased with de creasing yields, a desirable decision-rule attribute given that extreme def iciency can result in higher-than-expected tissue concentrations. The succe ss rates of multiple BLR functions using yield and stalk or grain analysis as factors were again comparable (88 and 87%, respectively), but they were significantly greater than with the LRP analysis. Stalk analysis was superi or to grain analysis for distinguishing sufficiency from excess. Ii constan t stalk NO3-N concentration (1.67 g kg(-1)) separated sufficient from exces sive cases, and fertilizer efficiency approached zero at 2.9 g kg(-1). Prem ature sampling resulted in stalk. NO3-N levels that were 40 to 600% greater than levels observed after blacklayer formation, with the greatest error o ccurring when N fertility was low. When not testing for N excess, the advan tages of grain analysis are the ease of sampling during harvest and the red uced risk of error associated with premature sampling.