A N Kravchenko

the possibility of using soil electrical conductivity data and topographical information for predicting soil drainage classes, and (3) to compare accuracy and efficiency of discriminant analysis and geostatistical procedures in mapping soil drainage classes on an agricultural field scale.

17.2 MATERIALS AND METHODS 17.2.1 Soil and Topographical Data

The studied area was a square 20 ha central portion of a 259 ha agricultural field located in Central Illinois. The field has been in a corn and soybean rotation for at least 20 years. Soil sampling was conducted on a semiregular grid, and sample locations are shown in Figure 17.1. A total of 107 soil cores, 100 cm deep, were collected from the studied area.

The drainage class for each sample location was determined based on the visual examination of the cores. Drainage classes were assigned according to the depth to the seasonal high water table, as indicated by the presence of low-chroma (<2) mottles (USDA, 1993). The soil was assigned to the well-drained class (WD) if no mottling was observed in the soil core within a 100 cm depth. Soil with mottling occurring at 50 to 100 cm depth was classified as moderately well drained (MWD), and soils with mottling occurring within 25 to 50 cm and within top 25 cm were classified as somewhat poorly drained (SWPD), and poorly drained (PD), respectively. Out of 107 soil cores, 48 were classified as WD soils, 38 as MWD soils, 15 as SWPD soils, and 6 as PD soils (Figure 17.1). Because the number of PD soil cores was insufficient for meaningful quantitative analysis, they were combined in one class with SWPD soils (SWPD/PD).

Soil electrical conductivity data were collected in fall 1999 using a Veris 3100 sensor cart that operates on the principle of electromagnetic induction (Division of Geoprobe® Systems, Salina, Kansas). Geo-referenced electrical conductivity measurements were taken every 3 to 5 m with the

Measured soil drainage classes WD ■ SWPD/PD MWD

Soil series

? Meters

Figure 17.1 Locations of the core samples used to determine soil drainage classes along with the soil survey map and elevation map of the studied area. Each sampling site is labeled according to its drainage class.

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