An Algorithm for Corn Nitrogen Recommendations Using a Chlorophyll Meter Based Sufficiency Index

Nitrogen fertilizer continues to be the major input influencing corn (Zea mays L.) yield in the Midwest. Improved N recommendations should result in greater N use efficiency and producer profit while reducing surface and groundwater contamination. This study was conducted to develop a plant-based technique to detect and correct N deficiencies during the season. Chlorophyll meter readings and grain yield were collected from corn in irrigated monoculture corn and soybean [Glycine max (L.) Merr.]–corn cropping systems with four hybrids and five N fertilizer application rates in the Platte Valley near Shelton, NE. Normalized chlorophyll meter readings (sufficiency index, SI) were calculated from data collected at three vegetative stages, defined by thermal time accumulation after planting, during each of the 10 yr of study (1995–2004). Highly significant linear correlations between SI and relative yield (normalized yield) indicated both responded similarly to N fertilizer application. Relationships between N rate and SI (at each of the three vegetative stages and combined over stages) were described by quadratic models. The combined model [(SI 5 0.8073 1 0.002(N rate) 2 0.0000056(N rate), R 5 0.70)] can be used to compute N needed to achieve maximum yield. Our procedure gives producers the tools to determine if N is needed, and if so, the amount of N required for maximum yield. In addition if SI is computed for specific areas of the field, N applications can be tailored to those areas, thereby reducing the potential of introducing more N into the system than needed to achieve maximum yield, with spatial and temporal constraints. NITROGEN fertilizer recommendations for corn in the Midwest historically have been centered on yieldbased algorithms. Mulvaney et al. (2006) describe how these yield-based systems were developed and explain many of the assumptions including the widely used mass balance approach. Work by Stanford (1973), Meisinger (1984), and Meisinger et al. (1992) indicates that when yield-based systems are used, a constant N-use efficiency is assumed whether N is taken up from the soil or fertilizer. This assumption becomes somewhat questionable when results like those published by Fox and Piekielek (1995) are examined. Fox and Piekielek (1995) in fact found significant differences in apparent N-use efficiencies on nonfertilized control plots between years with low or high average yields. The differences in N-use efficiency, depending on crop yield identified above, demonstrate not only some of the problems in developingN recommendations for corn but also the complex interactions among the various factors to be considered. Stanford (1982) and Keeney (1982) provide excellent reviews of techniques for creating N recommendations based on laboratory incubations to estimate plant availableNmineralized fromorganicmatter and preplant soil tests to determine inorganic nitrate N. Laboratory incubation methods, inorganic nitrate N soil tests, and other procedures and techniques to determineN availability fromother sources required to understand N management for crops are presented in a book edited by Hauck (1984). Chapters in the book present and discuss factors from N cycling in soil processes to N uptake by the crop affecting N-use efficiency in crop production. This book is just one example of the tremendous past effort by researchers to investigate, describe, and quantify N cycling processes to determine N available for crop production, this effort continues today. In addition to the techniques described above, recent technological developments including global positioning systems, in-season real-time crop sensors, variable rate N applicators, and geographical information systems to analyze large amounts of data have improved our N recommendations. These developments continue to drive the expectation for further improvements in N management resulting in greater N application and use efficiency and producer profit while reducing surface and groundwater contamination. In earlier reports, Varvel et al. (1997a, 1997b) demonstrated that chlorophyll meters provided excellent indications of in-season N status of several corn hybrids in both monoculture corn and soybean–corn cropping systems. In their reports, chlorophyll meter measurements taken throughout the growing season over several years indicated these data might have additional applications; including determination if and how much additional N fertilizer is needed for maximum yield. Therefore, our objective was to develop a plant-based technique to detect and correct N deficiencies during the cropping season with the ultimate purpose of improving N-use efficiency, maintaining or improving yield, reducing N fertilizer costs, and reducing environmental impacts of corn production. MATERIALS AND METHODS A study comparing irrigated monoculture corn and soybean–corn cropping systems was initiated in 1991 on a uniform site in the Platte Valley near Shelton, NE, on a Hord silt loam (fine-silty, mixed, mesic, Pachic Haplustoll). Data for this paper are from this site for the 10-yr period of 1995 to 2004. Before initiation of the study, the site had been in a monoculture corn production system for more than 10 yr. At the beginning of the study, corn stalks from the previous growing season were shredded and the entire area was disked twice USDA–Agricultural Research Service and Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583. Joint contribution of USDA-ARS and the Nebraska Agric. Res. Div. Received 5 July 2006. *Corresponding author ([email protected]). Published in Agron. J. 99:701–706 (2007).