scale in addition to date and hour. Data describing where may include slope position, GPS coordinates, a description of position relative to the crop (e.g., mid‐ or within‐row); soil samples, including for root biomass, should have depth and depth interval recorded. Information related to how the sample was collected includes sampling method, e.g., implement used or if hand collected, whether the sample represents a composite, and frequency of sampling for repeated‐measure parameters. Sample handling and preparation documents details such as drying temperature, sieve size, grinding, and storage conditions (e.g., duration, temperature). These details may not be included in a meta‐analysis but are important for assuring that like data are being compared and that the data meet quality control standards. For example, oven drying instead of air drying will compromise many biological properties. Another aspect of how refers to reporting the methods of analysis. For standard methods, a very brief description and citation may suffice, whereas novel methods should be described with substantially more detail.
Discussion
The categorial data used in several meta‐analyses correspond to the metadata recommended in Table 4.1. For example, a meta‐analysis of soil microbial biomass response to C amendments included the following categorical or metadata: microbial biomass method, crop type, cropping system, years of application, type of amendment, soil pH, soil organic content, clay content by depth interval, rate of C input, and rate of N inputs via organic amendments (Kallenbach & Grandy, 2011). Another meta‐analysis used soil pH, texture, SOC, and biochar production conditions (feedstock, pyrolysis temperature) and application rate, plus experiment type (field or pot) and study duration to understand soil microbial biomass responses (Zhou et al., 2017). Laganière, Angers, and Paré (2010) included previous land use, climatic zone, clay content, soil pH, pre‐planting disturbance, plantation density, and tree species planted in a meta‐analysis assessing C accumulation due to afforestation of former agricultural soils. Mahal, Castellan, and Miguez (2018) conducted a meta‐analysis using potential mineralizable N as a soil health indicator and an extensive cadre of metadata including crop type, fertilizer type and rate, cover crop, tillage system, and duration of the experiment in years plus documentation of the incubation method, soil sampling time, soil sampling depth, soil type (texture), pH, bulk density, soil organic matter, SOC, soil total N, NO3 and NH4 concentrations, mean annual temperature and precipitation, crop yield, and state and country where the study was conducted.
Summary
This chapter emphasizes the importance of collecting metadata that provide adequate information to support the wide range of soil biological, chemical, and physical measurements being quantified for various soil health assessments. Examples of metadata or categorical data that may be useful are suggested but should not be viewed as either extensive or exhaustive. Furthermore, it should not be considered necessary to record every item in the list but rather those data that may contribute to understanding the range of soil health indices being measured for a specific assessment.
Acknowledgments
The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the USDA or the ARS of any product or service to the exclusion of others that may be suitable. The USDA is an equal opportunity provider and employer.
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