Evaluation of realistic yield expectations in the North Carolina piedmont and coastal plain

by Lohman, Mindy M.

LOHMAN, MINDY. Evaluation of Realistic Yield Expectations in the North Carolina Piedmont and Coastal Plain. (Under the direction of Deanna Osmond and Jeffrey G. White.) Realistic Yield Expectations (RYE) have been developed in North Carolina to assist in site-specific farming decisions that will improve N-use efficiency and reduce N contamination of ground- and surface water, especially in the Neuse River Basin. This study was conducted to determine whether correlations exist between soil chemical properties, actual yields, soil map units, transition zones at map unit boundaries, and RYEs. One site-year each of corn (Zea mays L.) and wheat (Triticum aestivum L.) yield data was collected in one Piedmont field; wheat was sampled for one year in a second Piedmont field, and corn (Zea mays L.) sampled for one year in a third Piedmont field. Two years of soybean and one year of wheat yield data were collected in one Coastal Plain field. Soil surveys of the fields were completed in 2002 at an approximate scale of 1:3500 (“remapped” soil map units) and compared to existing county soil surveys (“original” soil map units). Samples from equilateral triangle grid soil sampling were analyzed and used to map the spatial distribution of soil pH, P, K, and lime requirement. Interpolated maps were created to display the spatial distribution of the investigated soil chemical properties. To represent zones (transition zone or map unit interior), 20-m buffers centered on map unit boundaries were created in order to investigate these potentially variable areas. Interpolated nutrient maps showed visual correlations between soil map units and soil K values in the Coastal Plain, but no other relationships between soil chemical properties and soil map units or zones were visually apparent for either location. Yield maps showed visual relationships with soil map units in the Coastal Plain but not in the Piedmont. Remapped and original soil map units and zones were analyzed as fixed effects to determine their effectiveness in capturing the variability of soil chemical properties and crop yield. Analyses of variance with and without spatial covariance models included were utilized to analyze the data. The analyses incorporating spatial covariance models were determined to be more efficient than those presuming independent and identically distributed errors in capturing a significant proportion of the variability for tested soil chemical properties and crop yield in both locations. The remapped soil map units were more effective than the original soil map units in capturing this variability in most cases. Soil K was different among the remapped soil map units in the Field 7 in the Piedmont where the model r2=0.82. In all locations, other investigated parameters also displayed differences, but none as highly significant as soil K in Field 7. Even though differences was discovered in other fields, management decisions would not likely be affected, as most differences were small and the means were usually classified in the same nutrient status category. In the Piedmont, RYEs were found to be less than actual yields, while in the Coastal Plain, RYEs were greater than actual yields, implying that the RYE database needs further study to determine if values are reasonable. Evaluation of Realistic Yield Expectations in the North Carolina Piedmont and Coastal Plain By Mindy M. Lohman A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science