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Description of data sets† used for validation of the Maize-N model to determine the economically optimum N rate (EONR).

 
Data set Location, year Location Tillage‡, soil series, previous crop Soil organic C§ Hybrid¶ Plant population Measured yield# Observed EONR
lat., long. % plants m−2 Mg ha−1 kg ha−1
Central Nebraska (Roberts, 2009) Merrick County, 2007 41.277, –97.992 NT, Ipage loamy fine sand, soybean 0.68, 0.86 P33N08 6.6 11.3–12.1 172–202
Merrick County, 2008 41.257, –98.015 NT, Thurman loamy fine sand, soybean 0.91, 1.33 P34R67 6.7 10.7–15.1 127–235
Hamilton County, 2007 40.775, –98.123 RT, Crete silt loam, maize 1.75, 2.03 P34R67 6.7 13.8–14.5 107–148
Hamilton County, 2008 40.803, –98.219 RT, Hasting silty clay loam, popcorn 0.87, 0.87 HH NG6783 6.7 11.9–14.3 219–245
Eastern South Dakota (Kim et al., 2008) Aurora, 2002, 2003, 2004 (each with irrigation and rainfed treatments) 44.300, –96.667 NT, Brandt silty clay loam, soybean–wheat 2.51 DK 44–46RR 6.8 8.7–10.8 116–129
Western Nebraska (Blumenthal et al., 2003) Banner County, 1999 41.583, –103.452 NT, Tripp very fine sandy loam, wheat 0.70 P3893 5.7 6.3 89
Box Butte County, 1999 42.157, –103.208 NT, Creighton very fine sandy loam, wheat 0.64 P3893 5.7 4.0 107
Cheyenne County, 1999 41.231, –103.020 NT, Duroc loam, wheat 1.83 P3893 4.7 6.5 67
Box Butte County, 2000 42.147, –103.184 NT, Alliance loam, wheat 0.68 P3893 3.7 3.8 79
Data were not used in model development and calibration of Maize-N; number of observations (n) = 18.
NT, no-till; RT, ridge tillage.
§Two values for soil organic C (SOC) at the central Nebraska sites represent SOC levels in two distinct soil zones within the experimental sites.
P, Pioneer; HH, Heartland Hybrid; DK, DeKalb.
#Measured yield from the treatment with the highest yield in well-managed field experiments.



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Maize N fertilizer recommendation methods used in different states and regions of the Corn Belt.

 
Approach N rate determination Reference
University of Nebraska-Lincoln (UNL) Empirically based on expected yield, inorganic soil N (residual soil NO3 based on soil test analysis), soil N supply from organic matter, and N credits from previous crops and manure applications. Slope of N rate vs. expected yield is 67.2 kg grain kg−1 N (1.2 bu grain lb−1 N) Shapiro et al. (2008)
Kansas State University Similar to UNL approach but with greater sensitivity of N rate to changes in expected yield. Also takes into account the profile depth of residual soil NO3 measurement. Slope of N rate vs. expected yield is 89.6 kg grain kg−1 N (1.6 bu grain lb−1 N) Leikam et al. (2003)
South Dakota State University Empirically based on expected yield, inorganic soil N (residual soil NO3), and N credits from previous crops and manure applications. Does not take into account soil N supply from organic matter. Slope of N rate vs. expected yield is 50.4 kg grain kg−1 N (0.9 bu lb−1 N) Gerwing and Gelderman (2005)
University of Missouri Empirically based on expected yield, plant population density, soil N supply from organic matter, and N credits from previous crops and manure applications. Does not require inorganic soil N test. Slope of N rate vs. expected yield is 50.4 kg grain kg−1 N (0.9 bu grain lb−1 N) Brown et al. (2004)



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Soil N credits based on soil texture and organic matter content.

 
Soil texture Organic matter Soil N credit†
% lb N acre−1
Sand–sandy loam ≤0.5 20
0.6–1.4 40
≥1.5 60
Silt loam–loam ≤2.0 40
2.1–3.9 20
≥4.0 80
Clay loam–clay ≤2.0 20
2.1–4.9 10
≥5.0 50
Multiply values by 1.12 to convert data to kg ha−1.