needed to accurately represent a site (Fig. 2.1). Sources associated with this type of error (also referred to as sampling error) are well known (Das, 1950). This error can be minimized by using a sampling plan that accurately captures patterns of interest. Selection error can also be reduced by increasing the number of samples collected within or among subareas if using a stratified sampling approach (see below).
Figure 2.1 Error components associated with soil property assessment.
Processing error are errors made while collecting, handling, and preparing samples for evaluation (Fig. 2.1). Reducing this error requires consistent application of approved protocols tailored for the specific type of analyses, inclusive of storage conditions. Closely aligned with processing error is measurement error, which arises from an improper application of analytical methods or evaluation techniques. Consistent use of consensus protocols will ensure accuracy and precision of each measurement. For laboratory analyses, use of blanks, internal standards, and reference samples is necessary to detect potential contamination and bias.
Once data have been collected, interpretation error can further confound errors from site selection, sample processing, and measurement (Fig. 2.1). Interpretation error results from accidental or systematic misinterpretation or improper application of data. Reduction of interpretation error relies on the evaluator’s knowledge to accurately decipher data in context to the sampled site, while concurrently ensuring data outcomes are not extrapolated beyond inherent spatiotemporal constraints or methodological limitations.
Site Characterization
Preliminary site characterization is important and encouraged, especially when spatial variation of inherent soil properties and/or previous land use is unknown. If the site is intended for long‐term monitoring, preliminary site characterization is essential. Referencing maps and/or imagery of the site prior to in‐field assessments may elicit attributes not visible from the ground. Preliminary field assessments can be made rapidly using hand probes, augers, or a shovel to identify variation in morphological attributes. More intensive profile assessments will provide additional information but will increase the investment of time and labor. Profile assessments are especially useful if framed by knowledge of pedogenic processes (Boone et al., 1999), recognizing such assessments are generally beyond the scope of most soil health assessments. Characterizing site geomorphology, however, may be helpful for interpreting landscape attributes and their relationship to ecosystem processes (Gringal et al., 1999).
Collection of metadata (Vol. 1, Chapter 4) is an important part of site characterization and will facilitate interpretation of measurements. At a minimum, geographical, landscape, soil, vegetation, climatic, and management attributes should be recorded and maintained for future reference (Table 2.2). When sampling as part of a long‐term study, more metadata is required to accurately characterize each sampling site (see Table 1.2 in Boone et al., 1999).
Table 2.2 Suggested minimum metadata for site characterization.
| Category | Metadata |
|---|---|
| Geography | Site identifier/nameState/Province, county, city, postal codeLatitude, longitude, and elevationRegional characterization (e.g., Farm Resource Region, Hydrologic Unit Code, Major Land Resource Area) |
| Landscape, Soil, and Vegetation | Landscape position, slope, and aspectSoil series and taxonomic descriptionNative vegetation |
| Climate and Weather | Mean annual temperatureMean annual precipitationLength of frost‐free period |
| Management | Current managementManagement history |
Sampling Designs
Following preliminary site characterization, the evaluator must decide how to sample the site. The design used for sampling should reflect the goals of the evaluator, recognizing and reconciling tradeoffs associated with different designs (Table 2.3). Typically, available time and resources dictate the selection of a sampling design.
Judgement Sampling
Judgement sampling selects sites presumed to represent larger areas (Table 2.3). Since site selection is based on the evaluator’s knowledge and skill, it is subject to considerable bias. As outcomes are inherent to the evaluator, application of this design for long‐term use is limited as changes in evaluator judgement are possible over time. Data collected using judgement sampling has limited statistical validity especially when isolated from other sites sampled by the same evaluator.
Despite its drawbacks, judgement sampling can be useful when resources are scarce and demands for accuracy and statistical rigor are relaxed. This design may also be applicable when samples are difficult to collect, or an evaluation involves attributes with high analytical cost.
Table 2.3 Definitions and attributes of different soil sampling designs.
| Description | Advantages | Disadvantages | Potential application |
|---|---|---|---|
| Judgement sampling | |||
| An approach to sampling that bases site selection on the knowledge and skill of the evaluator. Effective use of approach requires prior knowledge of site and attributes being assessed. Sites are generally selected to avoid nonrepresentative locations with the larger area. | Uses expert knowledge of site.Sampling decisions informed by previous experience.Limited sampling can save time and resources. | High degree of evaluator bias. Outcomes strongly associated with the individual doing site selection.Samplings over time are compromised by changes in evaluator judgement.Limited application to statistical analyses.Less legally defensible. | Initial site screening when resources for sampling and analyses are limited and demands for accuracy and statistical rigor are relaxed.In some instances, this approach may be useful for the evaluation of very expensive attributes when only a restricted number of samples can be collected. |
| Simple random sampling | |||
| A sampling approach whereby each sample collected has an equal chance of being selected. If used, prior knowledge of site is not required. | Approach minimizes bias associated with site selection.Relatively easy to apply.Approach is statistically valid. |
Approach treats all points as equally important regardless of inherent
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