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FIGURE 19.2 Representative apparent soil conductivity (ECa) map as generated on June 17, 2002.

19.3.1.1 Main plot—Cover Crop

The effect of the main plot treatment, cover crop, is shown in Figure 19.3. Each of the 4 years shows a pattern of -CC ECa values higher at the start of the season, then converging during mid-season, and diverging again after establishment of the cover crop in the fall. Figure 19.3 indicates convergence (data averaged across subtreatments and replicates) (P > 0.05) of -CC, as compared to +CC.

Ferguson et al. (2003) reported that use of a winter cover crop was effective in reducing NO3 levels at depths below 0.5 m since 1998. Nitrate levels from 0.5 m to the surface tend to be higher under the cover crop, indicating that the cover crop is immobilizing NO3-N in the upper portion of the soil profile. Statistical analysis supports an association of ECa and NO3-N (see Section 19.3.2). Sequential ECa graphs bolster the view that the immobilized NO3-N is released when the +CC and -CC curves converge. Each of the 4 years investigated shows a convergence within the crop growing season. The convergence indicates the nitrogen is associated with the organic matter of the cover crop until microbial activity converts the organic N to inorganic NO3-N.

19.3.1.2 Subplot—organic and Commercial Amendments

Treatments MP and CP each had sufficient carryover phosphorus after 1998 so that no manure or compost was applied to these treatment strips in subsequent years, including the 2000 to 2003 study; they became essentially equivalent to the commercial fertilizer check (NCK) treatment. Treatments MN and CN were the only treatments receiving manure and compost application. Each of the 4 years showed the ECa of MN and CN to be significantly greater than the ECa of NCK through major portions of the growing season (Eigenberg et al., 2005).

Day of Year

FIGURE 19.3 Apparent soil conductivity (ECJ for the main treatment of cover and no cover during 2000, 2001, 2002, and 2003.

Day of Year

FIGURE 19.3 Apparent soil conductivity (ECJ for the main treatment of cover and no cover during 2000, 2001, 2002, and 2003.

19.3.2 Statistical Analysis

Three primary dynamic quantities in a cornfield are the soil temperature, soil moisture, and nutrient levels. The ECa data for 2000 to 2002 have been corrected to the equivalent temperature of 25°C (McKenzie et al., 1989). The remaining dynamics of soil moisture and nutrient levels (NO3-N being a primary contributor) are examined in the sections that follow.

19.3.2.1 Main plots—Cover Crop

Cover crop is the main plot of the split-plot design. Survey ECa temperature corrected data were collated with all treatments identified and all survey dates concatenated for each year. Soil data were included in the data file and associated with ECa of replicate 2. A standardized estimate was generated that partitions the percent contribution between the values of soil moisture and NO3-N toward the variability of ECa across survey dates; this test was done for temperature-corrected ECa data for 2000 to 2003, with results shown in Table 19.1. The results indicated that for the -CC treatments,

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