Sources of variability in nitrogen availability in Iowa cornfields
Studies were conducted to learn more about the spatial structure of soil NO[subscript]3[superscript]- concentrations in cornfields in late spring and to identify some major causes of variability in these concentrations. This information is specifically needed to aid in design of efficient sampling strategies for the late-spring test for nitrogen (N) availability, which is a new management tool that enables site-specific evaluation and adjustment of fertilization practices. One study assessed the importance of windrows of plant residue left by combines as a source of variability in soil nitrate (NO[subscript]3[superscript]-) concentrations for the next crop. Observations made in several fields having distinct windrows showed that yields of corn were significantly affected, but the effects on soil NO[subscript]3[superscript]- concentrations were minor compared to other unidentified sources of variability. A second study evaluated rows of young corn plants as a potential source ofvariability. Results from several fields revealed cyclic patterns in soil NO[subscript]3[superscript]- concentrations along transects perpendicular to rows. A third study evaluated nonuniform distributions of fertilizers across the width of large applicators as a possible source of variability. Observations made in several fields revealed cyclic patterns in NO[subscript]3[superscript]- concentration that seemed to be caused by fertilizer applicators and other farming operations. Models were developed that enabled integration of the observed cyclic patterns into geostatistical models commonly used to describe spatial structure. Procedures were developed for partitioning variability caused by two or more superimposed cycles;Overall, the results show that cyclic patterns are an important component of the spatial structure of soil NO[subscript]3[superscript]- concentrations in cornfields. Consideration of these cyclic patterns should enable more complete characterization of the spatial structure of soil NO[subscript]3[superscript]- concentrations in fields and, therefore, enable design of more efficient sampling strategies to assess NO[subscript]3[superscript]- concentrations within these fields. In addition, the ability to detect previously unrecognized cyclic patterns in soil NO[subscript]3[superscript]- concentrations gives the ability to demonstrate the benefits of using practices that minimize formation of troublesome cyclic patterns in fields.