Apparent soil electrical conductivity (EC a ) has shown promise as a soil survey tool in the Midwestern United States, with a share of this interest coming from the precision agriculture community. To fully utilize the potential of EC a to map soils, a better understanding of temporal changes in EC a is needed. Therefore, this study was undertaken to compare temporal changes in soil EC a between different soils, to investigate the influence of changes in soil water content on soil EC a , and to explore the impacts these EC a changes might have on soil mapping applications. To this end, a 90 m long transect was established. Soil EC a readings were taken in the vertical and horizontal dipoles at five points once every one to two weeks from June until October in 1999 and 2000. At the same time, soil samples were collected to a depth of 0.9 m for volumetric soil water content analysis. Soil EC a readings were compared to soil water content. At four of the five sites linear regression analysis yielded r 2 values of 0.70 or higher. Regression line slopes tended to be greater in lower landscape positions indicating greater EC a changes with a given change in soil water content. Two of the soils had an EC a relationship that changed as the soils became dry. This is an item of concern if EC a is to be used in soil mapping. Results indicated that soil water content has a strong influence on the EC a of these soils, and that EC a has its greatest potential to differentiate between soils when the soils are moist. Soil water content is an important variable to know when

yield data collected over many years in the same field and a larger set of measured soil and terrain variables Farmers will be better able to implement site-specific management would have a better chance of accomplishing this goal. practices when they understand the causes of spatial and temporal variability of corn (Zea mays L.) and soybean [Glycine max (L.) Data collection and analysis, however, are costly and Merr.] yield in their fields. Our objectives were to determine if a data labor-intensive, and questions remain about the kind set containing 20 soil and terrain variables could explain spatial yield and amount of data that are needed to adequately unvariability better than a subset of seven more easily measured variables derstand spatial yield patterns. Additionally, it is unclear and to determine whether the relative importance of factors in exwhether different variables should be measured for soyplaining yield variability differed between corn and soybean or bebean and corn or for wet and dry years. tween wet and dry years. Yield data were collected for 11 yr in a 16-ha Growing season precipitation often interacts with terfield in central Iowa. Soil and terrain variables measured included: A rain attributes and soil properties to influence crop yield horizon depth, carbonate depth, pH, coarse sand, sand, silt, clay, (Timlin et al., 1998; Jaynes et al., 2002; Kaspar et al., organic C, N, Fe, K, P, and Zn; and seven easily measured variables: 2003). In years with below average rainfall, field areas electrical conductivity, soil color, elevation, slope, profile curvature,higher on a hillslope with greater slopes and convex plan curvature, and depression depth. Factor analysis of the variables followed by regression of yield on the resulting factors showed that curvatures usually have less available water and lower the 20-variable set explained more of the spatial variation in yield than yield than areas lower on the hillslope, with lesser slopes the subset of seven variables. Further, the analysis of the 20-variable and concave curvatures (Ciha, 1984; Halvorson and data set showed that soybean yield was affected more by pH, more Doll, 1991; Afyuni et al., 1993;Timlin et al., 1998; Jaynes by closed depressions in wet years, and less by curvature in dry years Kaspar et al., 2003). Similarly, eroded soils, than corn yield. Similarly, yield was negatively affected by closed which commonly have substantial slopes, convex curvadepressions and lower landscape positions in wet years, whereas these tures, and shallow topsoils (Pennock and de Jong, 1987; factors had either no effect or a positive effect in dry years. Alternately, Lindstrom et al., 1992), show a greater yield decline curvature had a negative effect in dry years and no effect in wet years.

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The zone of saturation plays significant roles in decisions regarding soil classification systems and land use both present and potential. This study was conducted to determine the status of water tables, morphological features, and groundwater hydrology and to evaluate the zone of saturation on selected soils (Aquolls and Hapludolls) of a Mollisol catena. Water table depths and the amount of precipitation were measured for a 10‐year period on five soils formed in glacial till or till‐derived sediments in central Iowa. Results show that the depth of the water table, duration of saturation, morphological features, and recharge and discharge to groundwater varied with geomorphic position in the Mollisol catena. In general, soils on summit and shoulder positions (Hapludolls) are not saturated and have high chromas without redoximorphic features in the B horizon, have deeper water tables, and have maximum fluctuation of water table levels. The soils on toeslopes and depressions (Aquolls) have the shallowest water tables, have the longest time of saturation, and have B horizons with gray matrices, bright mottles, and Fe‐Mn concretions. Nicollet soils (fine‐loamy, mixed, mesic Aquic Hapludoll) on backslope positions have intermediate characteristics. Redoximorphic features present in these soils are correlated with the fluctuating depth of the present‐day water table. The extent of recharge and discharge and artificial the drainage are contributing factors to the water table level. Data for water table level and calcite/dolomite ratio indicate that ephemeral recharge and discharge of groundwater occur in the soils on lower landscape positions. Recharge is the dominant process in the soils of higher landscape. This study was designed to contribute to the understanding of drainage class‐water table‐morphological features of soils in this region. The apparent drainage conditions and the redoximorphic features contribute to the testing and clarification of soil classification and soil taxonomy.