Using soil attributes and GIS for interpretation of spatial variability inyield

Precision farming application requires better understanding of variability in yield patterns in order to determine the cause-effect relationships. Thi s field study was conducted to investigate the relationship between soil at tributes and corn (Zea mays L)-soybean (Glycine max L.) yield variability u sing four years (1995-98) yield data from a 22-ha field located in central Iowa. Corn was grown in this field during 1995, 1996, and 1998, and soybean was grown in 1997. Yield data were collected on nine east-west transects, consisting of 25-yield blocks per transect. To compare yield variability am ong crops and years, yield data were normalized based on N-fertilizer treat ments. The soil attributes of bulk density, cone index, organic matter aggr egate uniformity coefficient, and plasticity index were determined from dat a collected at 42 soil sampling sites in the field. Correlation and stepwis e regression analyses over all soil types in the field revealed that Tilth Index, based upon soil attributes, did not show a significant relationship with the yield data for any year and may need modifications. The regression analysis showed a significant relationship of soil attributes to yield dat a for areas of the field with Harps and Ottosen soils. From a geographic in formation system (GIS) analysis performed with ARC/INFO, it was concluded t hat yield may be influenced partly by management practices and partly by to pography for Okoboji and Ottosen soils. Map overlay analysis showed that ar eas of lower yield for corn, at higher elevation, in the vicinity of Ottose n and Okoboji soils were consistent from year to year; whereas, areas of hi gher yield were variable. From GIS and statistical analyses, it was conclud ed that interaction of soil type and topography influenced yield variabilit y of this field. These results suggest that map overlay analysis of yield d ata and soil attributes over longer duration can be a useful approach to de lineate subareas within a field for site specific agricultural inputs by de fining the appropriate yield classes.