A Survey of Soil Attributes in North Dakota by Landscape Position

Land surveys of soil attributes can provide valuable information on the geographic distribution of important soil attributes and summarize the levels found. Landscape position can have a major influence on soil attributes. Consideration of field landscape variables in selecting sampling locations and interpreting results, even in large-scale surveys, should help to reduce unexplained variability in many soil attributes. We report the results of a survey of selected soil attributes in agricultural fields across North Dakota at sites selected within each field by landscape position. Our objective was to determine if this sampling design might contribute to a better understanding of the distribution of soil attributes. Soils from two or three fields within each of the 53 counties of North Dakota were sampled in 1996. Within each field, three samples of surface soil (0–15 cm) were obtained. One sample was collected from an upland position, one from a slope, and one from a depression. Each field was georeferenced using a differentially corrected GPS receiver. The samples were analyzed for DTPA (diethylenetriaminepentaacetic acid)-extractable Cu, Zn, and Cd; water soluble B and Se; and soil pH. Mapping and analysis showed distinct regional patterns of all soil factors. Some soil attributes, including pH and extractable Zn, Cu, and Cd, exhibited a strong relationship to field landscape position, while soluble B and Se were less related. The results suggest that separating field sampling locations into upland, sloping, and depressional areas will reduce the confounding effects of field landscape position on larger-scale spatial trends in soil attributes. MAPPING IS A POWERFUL TOOL for understanding the geographic distribution of soil attributes at any scale. National maps help delineate regions that tend to have higher or lower levels of certain properties (Holmgren et al., 1993; White et al., 1997; Gustavsson et al., 2001). Within regions, states, or counties, smaller areas with specific characteristics can be more accurately delineated at an appropriate scale (Holmgren et al., 1993; Wu et al., 2000).Within agricultural fields, relatively small areas can be identified as having differing soil attributes or differingmanagement needs (Franzen et al., 1998a;Wu et al., 2000). Whatever the geographic scale, there is always a smaller scale at which variability exists (Franzen and Berglund, 1998; Pierce and Nowak, 1999; Webster, 2000). Failure to understand sources of small-scale geographic variability simply increases the unexplained variability in data collected over larger geographic areas. Within-field variability in soil characteristics is common. Soil P and K levels (Peck and Melsted, 1973; Wollenhaupt et al., 1994; Franzen and Peck, 1995a), soil pH (Franzen and Peck, 1995b), soil organic C (Bhatti et al., 1991), chloride and Zn (Franzen et al., 1998b), and soil nitrate (Franzen et al., 1998a) have all been found to vary spatially within farm fields. Knowledge of withinfield variability in attributes contributes to more effective agricultural soil management and assists also with the design of efficient strategies for studying the distribution of soil properties over larger areas. There is substantial evidence that within agricultural fields, the plant nutrients and soil factors of interest in a general soil survey of North Dakota might be related to landscape position. Copper, Zn, B, and Cd accumulate in organic matter (Stevenson, 1991), while soil pH of the region (Knuteson et al., 1989) and Se and Cd availability are related to some degree by hydrology, which in turn is related often to landscape position (Ruhe, 1960; Mikkelsen et al., 1989; Seiler, 1998; Wu et al., 2002). Previous site-specific soil sampling for plant nutrients revealed a tendency for several nutrients, including NO3, Zn (Fig. 1), and chloride to be related to landscape position (Franzen et al., 1998a, 1998b). The first objective of our survey was to measure and map the distribution of DTPA-extractable Zn, Cu, and Cd; water soluble B and Se; and pH in soils at three different landscape positions in fields across the state of NorthDakota. Extractable forms of these trace elements measured by soil-testing methods were chosen because these measures are much better related to plant responses than are total concentrations of elements. Second, we wished to determine if the differences in these measured soil characteristics in samples from upland, sloping, and depressional locations within the field landscape were significant and sufficiently large to justify including landscape-based sampling in statewide surveys of soil characteristics. MATERIALS AND METHODS Soil samples were collected from each of the 53 counties in North Dakota during the summer of 1996. Samples were obtained from two to three cropped fields within each county (Fig. 2) Pasture and range fields were not included in the survey. Each field was georeferenced using a differentially corrected GPS receiver (Omnistar 7000 with satellite differential, Omnistar, Houston, TX), which recorded in latitude and longitude using the coordinate system WGS-84. Within each field, separate samples were collected from a typical upland, sloping, and depressional location. Each sample consisted of at least eight soil cores taken from a 0to 15-cm depth with a 2.5 cm-diam. sample tube and was composited together in the field. No lubricant was used to obtain the samples. The subsample cores for each sample were obtained from an area of about 100m within each landscape position. The samples were then air-dried and ground to pass a 2-mm sieve. The samples were analyzed for soil pH (1:1 soil:water paste); DTPAextractable Zn, Cu, and Cd (Lindsay and Norvell, 1978) with D.W. Franzen and T. Nanna, Dep. of Soil Sci., North Dakota State Univ., Fargo, ND 58105-5758; and W.A. Norvell, USDA-ARS, U.S. Plant, Soil, and Nutrition Lab., Ithaca, NY 14853. Received 10 Oct. 2005. *Corresponding author ([email protected]). Published in Agron. J. 98:1015–1022 (2006).