Challenging Approaches to Nitrogen Fertilizer Recommendations in Continuous Cropping Systems in the Great Plains
2005
Schlegel, Alan J. | Grant, Cynthia A. | Havlin, John L.
Cropping systems in the Great Plains have evolved over the past two decades from reliance on summer fallowing to continuous cropping under reduced or no-tillage. Most N recommendation models were developed in fallow systems under conventional tillage and were based on average yield goal, with adjustments for soil profile N content. The objective of this review is to examine the impact of continuous cropping on N requirements. With high-residue continuous cropping systems, N requirements may increase because of increased annualized production, reduced contribution of N mineralization, and increased immobilization and volatilization potential of surface-applied fertilizer N. Mitigating these effects on N availability and supplemental N requirements are the reduction in yield per crop, reduced nitrate (NO₃) leaching potential, increased N use efficiency (NUE), and increased rates of N mineralization due to higher soil organic matter (OM) content. Unfortunately, increased year-to-year yield variability with continuous cropping increases the difficulty in accurately estimating yield goals. Also, reducing the frequency and duration of fallow may reduce the usefulness of the preplant soil N tests in estimating N availability. Recent research has evaluated the use of optical sensors during the growing season to assess N stress and to estimate crop N requirements. If proved feasible for many crops, this would provide a drastic change for determining N recommendations. In the absence of a reasonable yield goal and known residual soil N content, a fertilizer N rate near 70 kg N ha⁻¹ or less was generally sufficient to optimize small-grain or oilseed yields in several continuous cropping studies.
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