much of North America. Methods addressed in this volume are applicable to both efforts and reinforce the concept that data on physical, chemical, and biological properties and processes all must be obtained for a full understanding of a soil’s health.
Both the SHI and NRCS‐SHD efforts obtained input from researchers, farmers, soil‐testing laboratories, non‐governmental organizations (NGOs), and representatives of state and federal agencies starting in 2013, when the two longest‐serving agricultural foundations in the United States (Farm Foundation, established 1933; and The Samuel Roberts Noble Foundation, established 1945) partnered to design and initiate the Soil Renaissance effort. Several workshops were organized and facilitated to identify and strive for consensus regarding appropriate indicators of soil health. Each workshop was attended by a different mixture of university, government, and private industry scientists, field conservationists, and farmers. Technical discussion papers were written by teams of scientists from the U.S. Department of Agriculture and land‐grant universities. Between 2014 and 2016, many measurement‐related issues and challenges were assessed, including the status of existing soil health measurement frameworks; the benefits of a “tiered” approach for measurements at different stages of development and reliability; service lab adoption issues; data needs; communications plans; data interpretation, including issues related to different regions; sampling protocols; sample archives; quality assurance/quality control (QA/QC) protocols; and sampling frequency. To further the vision of the Soil Renaissance and implement its findings, the Soil Health Institute was created in 2015. In June 2017, the SHI used input from the Soil Renaissance effort to conduct a survey of 179 individuals who were active in the measurement‐related workshops organized by the SHI and/or the Soil Renaissance over the three years. A consensus emerged among the 48 respondents that many of the measurements used to characterize soil conditions for many years are also valuable for assessing soil health. These measurements– physical and chemical, supplemented with a few key biological– are well‐accepted in the scientific community and thus were designated by the Soil Renaissance participants as “Tier 1” indicators. They can be used directly or as ancillary factors needed to improve the interpretation of yet other measurements. They include:
Physical:Soil textureWater‐stable aggregationBulk densityWater penetration resistanceVisual rating of erosionInfiltrationAvailable water holding capacity
Chemical:Routine inorganic chemical analysis (N, P, K, micronutrients, pH, cation exchange capacity, base saturation, electrical conductivity)Soil organic carbon
Biological:Short‐term carbon mineralization (respiration)Nitrogen mineralizationCrop yield
The Soil Renaissance, SHI, and NRCS‐SHD communities also identified a group of measurements that have been designated “Tier 2”, mostly biological properties or processes in soil, for which there is scientific consensus that they are related to soil health but are less standardized with regard to measurement methods, interpretation, and known thresholds for management action. These indicators are identified in the SHI Action Plan (www.soilhealthinstitute.org, accessed February 20, 2020) as targets for research to develop sufficient response data to complete their development as reliable measurements. To achieve those goals, the Tier 2 indicators listed below need further development, testing, and evaluation on working farms so they can eventually be transferred and communicated to landowners, operators, and retailers as tools for improving soil and crop management practices. They include:
Beta‐glucosidase activity (organic matter decomposition)
Macro‐aggregate stability (water partitioning)
Permanganate oxidizable carbon (carbon food source for microbes)
Soil protein (bioavailable nitrogen)
Ester‐linked fatty acid methyl ester; phospholipid fatty acid (microbial community structure, diversity)
Nematode population densities (trophic levels)
Pathogenic fungi populations or bioassays (pathogen activities and host ranges)
The SHI, SHD and Soil Renaissance communities also identified a category of measurements designated “Tier 3”, which are primarily measurements of soil biological properties or processes for which, again, there is scientific consensus that they are quite likely related to soil health, but they still require major research and development investments to determine whether they reveal information that can be used to improve soil and crop management decisions. Fundamental biological and agricultural principles suggest Tier 3 indicators may be very useful eventually for assessing soil health and making management decisions, provided significant research investments in their development are aggressively pursued. Therefore, Tier 3 measurements are worthy subjects of further research on long‐term research sites and on‐farm evaluations where there are detailed records of environmental conditions and management practices over enough years that Tier 3 measurements can be interpreted reliably. Prominent among such measurements are metagenomic analyses to reveal information about soil microbial populations, community structure, and diversity, as influenced by the status and trends of soil health and in relation to the history of environmental conditions and management practices on exceptionally well‐characterized sites.
Consensus on what to measure is just part of the research associated with soil health measurements. There is also a need to reach consensus on how to measure each indicator, which can be very challenging and even contentious within the soil science and agronomic research communities. In the case of Tier 1 indicators, many analytical methods for measurements are widely accepted, for example, Soil Science Society of America Book Series 5, Methods of Soil Analysis, second edition– Part 1, Physical and Mineralogical Methods (1986); Part 2, Chemical and Microbiological Properties (1982); Part 3, Chemical Methods (1996). Variations in specific methods have been adapted in response to recommendations from research conducted in university, government, and private laboratories to obtain optimal, meaningful results for different soils collected from widely different locations and environments. These methods are in use for several different frameworks for soil health assessment (e.g., Karlen et al., 2014; Moebius‐Clune et al., 2016).
Methods for Tier 2 indicators are under active development and evaluation, and the research community is not in full agreement on methods and interpretation. Tier 3 indicators, however, as might be expected, are still very much in development, and their interpretation and value as soil health indicators that can be used to guide soil and crop management practice decisions remain uncertain.
To develop consensus that would support research on Tier 1 and Tier 2 indicator evaluation, in early 2018, the SHI assembled a panel of experts in soil health measurement from USDA agencies (Agricultural Research Service, Natural Resources Conservation Service), universities, and a private laboratory to meet and recommend a specific protocol for each indicator listed below. The goal for this gathering was to assemble a definitive list of widely‐applicable, effective indicators for evaluating soil health and the specific methods to use for each indicator in many production environments across a wide geographical scale. To accomplish this, SHI is partnering with numerous investigators at long‐term agricultural research sites (with appropriate experimental designs, controls, documented management histories, production records, etc.) that are being sampled and analyzed for over 30 soil health indicators (www.soilhealthinstitute.org/north‐american‐project‐to‐evaluate‐soil‐health‐measurements/) (accessed February 20, 2020). Together, the indicator methods described in USDA‐SHD Technical Note 450–03 (Stott, 2019), information provided in this volume, and methods under evaluation in the wide‐scale SHI project (Tables 1.1 and
