Methods

19.2.1 Site Description

A center-pivot irrigated field (244 m x 244 m) of silage corn (Zea mays L.), located at the U.S. Meat Animal Research Center (USMARC), served as a comparison site for various manure and compost application rates for replacement of commercial fertilizer, with the same treatment assigned to field plots for 10 consecutive years. The soil series at this site is a Crete silt loam (fine, montmorillonitic, mesic Pachic Argiustolls), 0 to 1 percent slope.

19.2.2 Field Operations on the Research Cornfield

The study site was laid out as a split-plot design (Figure 19.1), with four replications of the main plot of cover crop (+CC) versus no cover (-CC) (the cover crop was a winter wheat [Secale cereale L.] no-till drilled following silage harvest). Subplot application to treatment strips (6.1 m, eight corn rows wide) was made with two manure sources: beef feedlot manure and composted beef feed-lot manure. Applications were made each spring according to two strategies: (1) to approximately supply the total crop demand for N (Ferguson et al., 2003), denoted MN and CN for manure and compost, respectively, and (2) to supply the approximate crop demand of P (Ferguson et al., 2003), denoted MP and CP for manure and compost, respectively. Treatments MP and CP each had sufficient carryover phosphorus after 1998 so that no manure or compost was applied to these treatment strips for the 2000 to 2003 study.

19.2.3 Equipment

Two different magnetic dipole soil conductivity meters were used in this study: (1) an EM-38, manufactured by Geonics Ltd., Mississauga, Ontario, Canada (2000 and 2001 seasons) and (2) a Dualem-2 manufactured by Dualem Inc., Milton, Ontario, Canada (2002 and 2003 seasons). The EM-38 was operated horizontally and had a response that varied with depth in the soil, yielding ECa that was centered at a depth of about 0.75 m. The Dualem-2 operates in the horizontal and vertical dipole modes simultaneously, with ECa centered at 1.5 and 3.0 m, respectively. Only the horizontal response of each instrument is reported in this study and is designated ECa. The EMI instruments were mounted on a plastic sled and transported through the field, pulled by an all-terrain vehicle (ATV), or by hand when the corn became too tall for the ATV. Surveys were planned on weekly intervals throughout the corn growing season for all 4 years. A Trimble PRO-XR GPS (global positioning satellite) unit was used to obtain positional data.

Soil conductivity responds to soil temperature (McKenzie et al., 1989). An array of thermocouple temperature probes was buried at the cornfield site prior to the 2002 growing season. Probes were installed at incremental depths to 315 cm. The thermal profile was used with known Dualem-2 response (Dualem, www.dualem.com) curves to provide an effective soil temperature correction for each survey date (McKenzie et al., 1989). The soil temperatures demonstrated predictable temperature patterns that allowed temperature corrections to be estimated for the Geonics instrument for the 2000 and 2001 season. Precipitation events were recorded at the cornfield site using a tipping bucket rain gauge and a portable event recorder.

Soil cores were taken on survey dates throughout the growing season with a hand probe to a depth of 0 to 30 cm. The cores were analyzed by a local commercial soil testing laboratory to

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