FIGURE 18.6 Relationship between apparent soil electrical conductivity (ECa) productivity zones and plot-scale blocking. (a) An ECa-zone map of a «31 ha field at the Farm-Scale Intensive Cropping Study and (b) a typical plot-scale experiment identified within the field using ECa productivity zones as a basis for blocking.

mean square (MS) error at the FICS was calculated and compared with MS error derived from blocking in the plot-scale experiment. Experimental errors were similar, indicating that field-scale ECa-classified variability effectively estimated soil heterogeneity partitioned by plot-scale blocking. These findings were corroborated at a second and disparate site in central California, an irrigated system with saline soils (Johnson et al., 2005).

Comparisons were also made between ECa-delineated estimates of experimental error and experimental error derived from replication using multiple soil properties measured at the FICS. Again, within-field variance was an effective measure of experimental error for most of the nineteen parameters evaluated. These findings indicate that, for some experimental objectives, within-field variance may serve as a surrogate for replication.

In locations where soil factors contributing to ECa are also yield limiting, ECa productivity zones can be used to design and place plot-scale experiments. They can also be used to design and evaluate field-scale experiments, functioning as an alternative to replication and blocking. This is appropriate because ECa productivity zones are related to outcome (crop yield) differences expected in the absence of treatments, the rationale for blocking.

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