Diagnostic Efficiency of the Blacklayer Stalk Nitrate and Grain Nitrogen Tests for Corn

Protecting water quality and maintaining profitable corn (Zea mays L.) production requires diagnostics that can distinguish between N deficiency, sufficiency, and excess. This study evaluates initial recommendations on blacklayer basal‐stalk NO3–N ranges and critical concentrations for diagnosing N status, and it compares the performance of this test with grain analysis. Observations (428) were collected from 13 N‐response experiments. Linear response and plateau (LRP) and binary logistic regression (BLR) were used to characterize the relationships between yield and tissue‐test values. With the LRP, stalk NO3–N and grain N concentrations separating deficient from sufficient observations were 0.42 and 13.1 g kg−1, respectively, and the success rates of the two tests were comparable (77 and 75%, respectfully). The BLR also identified critical concentrations, but the values increased with decreasing yields, a desirable decision‐rule attribute given that extreme deficiency can result in higher‐than‐expected tissue concentrations. The success 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 superior to grain analysis for distinguishing sufficiency from excess. A constant stalk NO3–N concentration (1.67 g kg−1) separated sufficient from excessive cases, and fertilizer efficiency approached zero at 2.9 g kg−1. Premature sampling resulted in stalk NO3–N levels that were 40 to 600% greater than levels observed after blacklayer formation, with the greatest error occurring when N fertility was low. When not testing for N excess, the advantages of grain analysis are the ease of sampling during harvest and the reduced risk of error associated with premature sampling.