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Following the 1970s oil crisis, new questions regarding the potential use of crop residues for bioenergy began to emerge as a conservation issue directly linking urban and rural communities. Field research designed to quantify the impact of crop residue harvest on SOM and subsequent productivity resulted in the evolution of a soil health assessment framework. An experiment in southwestern Wisconsin that quantified soil and corn yield response to removing, doubling, or retaining crop residue for 10 years (Karlen et al., 1994a) with moldboard‐, chisel‐, or no‐tillage practices (Karlen et al., 1994b). To more effectively interpret the combined biological, chemical, and physical responses to those treatments, a soil quality/health assessment framework that later becomes known as the Soil Management Assessment Framework (SMAF) (Andrews et al., 2004) was developed. Simultaneously, other assessment tools including an expanded Soil Conditioning Index (SCI), AgroEcosystem Performance Assessment Tool (AEPAT), and Cornell Soil Health Test (now known as the Comprehensive Assessment of Soil Health or CASH) also began to evolve.

The soil tilth review (Karlen et al., 1990) prompted advancement of soil quality/soil health concepts through a Rodale Foundation workshop (Rodale Institute, 1991) that was described by Haberern (1992) as coming “full circle” in reference to J. I. Rodale’s 1942 vision of a “soil‐care revolution.” Rodale had stated that greater awareness to soil health was needed to create “a healthy society, a country of prosperous farms, and healthy, vigorous people.” An important outcome of the Rodale workshop was consensus regarding the need for soil assessments that went beyond productivity and included environmental quality, human and animal health. There was also a realization that assessing and monitoring soil health was complicated by the need to consider multiple soil functions and integrate physical, chemical and biological attributes (Papendick and Parr, 1992; Parr et al., 1992; Warkentin, 1995). Discussions regarding subtle differences between inherent and dynamic soil quality indicators were another important outcome of the Rodale workshop.

Soil quality activities around the world expanded rapidly during the early 1990s, driven in part by increasing recognition of the role soils had in buffering and mitigating factors affecting environmental quality (Warkentin, 1992). However, the true global driver and inspiration for the advancement of soil health or quality was Dr. John W. Doran, to whom this book series is dedicated. His perspective stating that “soil health, or quality, can be broadly defined as the capacity of a living soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and promote plant and animal health” (Doran, 2002) is to us, the ultimate goal for soil health. He also emphasized that soil health will change over time due to natural events or human impacts.

In Australia, Powell and Pratley (1991) developed a “Sustainability Kit” that provided guidelines for measuring soil structure, acidity (pH), salinity, and soil/water temperature. This kit also served as a precursor to John Doran’s field‐based soil health kit developed in collaboration with scientists at the Rodale Research Center and tested throughout the country (Liebig et al., 1996). Marketed with a USDA Soil Management Manual, the “Soil Health Kit” described simplified tests for soil respiration (Liebig 1996; Doran 1997), infiltration (Ogden et al., 1997), bulk density (Doran 1984), electrical conductivity (EC), pH, and nitrate‐nitrogen (NO₃‐N) concentrations (Smith and Doran, 1996); a soil structure index and penetration test (Bradford and Grossman 1982); soil slaking and aggregate stability tests (Herrick et al. 2001); and an earthworm assessment protocol (Linden 1994). Since that time, the power and potential for on‐farm testing of soil health indicators has been greatly magnified by development of the internet, smartphones, and applications such as the Land‐Potential Knowledge System (LandPKS; Herrick et al., 2013).

Other developments included a symposium sponsored by the North Central Region Committee No. 59 that focused on SOM at the Soil Science Society of America (SSSA) meetings. That event led to the SSSA publishing two books that became known as the “blue” (Doran et al., 1994) and “green” (Doran and Jones, 1996) soil quality guides that are the precursors for this two‐volume series. Doran (2002) also provided important insight stating that although soils have an inherent quality associated with their physical, chemical, and biological properties, their sensitivity to climate and management practices means that land manager decisions ultimately determine soil health. He continued stating a pivotal role for scientists is translating scientific information regarding soil functions into practical tools that land managers can use to evaluate the sustainability of their management practices. Soil health indicators thus became the tools for making the assessments, but there is no single indicator or technology that will always be appropriate.

Soil health assessment has also been advanced by the NRCS, through databases at the Kellogg Soil Survey Laboratory (KSSL), National Soil Survey Center in Lincoln, NE that currently have analytical data for more than 20,000 U.S. pedons and at least 1,100 more from other countries (Brevik et al., 2017). Collectively, morphological descriptions are available for about 15,000 pedons (https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/research/?cid=nrcs142p2_053543#:~:text=Summary,1%2C100%20pedons%20from%20other%20countries) (verified 6‐12‐2020).

The NRCS also has extensive data on basic soil properties, landscape characteristics, and interpretations for use and management. Those databases, describing inherent soil properties, provide a resource to match various land uses with inherent ability of individual soils to perform critical functions (Karlen et al., 2003a). Building upon those resources, the NRCS created several cross‐cutting teams during the 1990s, including the Soil Quality Institute (SQI) whose scientists developed many of the first‐generation soil quality/soil health scorecards, assessment tools, and information packets. The SQI compiled soil quality information for NRCS staff to help them integrate soil quality concepts into conservation planning and resource inventory activities for their stakeholders (SQI, 1996). The SQI also provided leadership and collaboration for research studies designed to evaluate soil quality indicators at several scales, including Natural Resources Inventory (NRI) sites within four Major Land Resource Areas (Brejda et al., 2000a, 2000b, 2000c).

Linkages between soil and environmental quality were significantly strengthened by the National Academy of Sciences Soil and Water Quality: An Agenda for Agriculture publication (NRC, 1993). It prompted Dr. L.P. Wilding, 1994 president of the SSSA, to appoint a 14‐person committee (S‐581) with representatives from all SSSA Divisions. Appointees were asked to define the concept of soil quality, examine its rationale, determine if pursuing its development should be a core SSSA activity, and identify soil and plant attributes that would be useful for describing and evaluating soil quality (Karlen et al., 1997).

One of the first comprehensive soil quality/health assessments was used to quantify benefits of the CRP. Following passage of the 1985 Food Security Act, that program took 14.7 million hectares (36.4 million acres) in 36 states out of production (Skold, 1989). Recognized as environmentally sensitive or highly erodible land (HEL), the primary goal was to reduce soil erosion. Secondary CRP goals included: protecting the nation's ability to produce food and fiber, improving air and water quality, carbon sequestration, reducing sedimentation, fostering wildlife habitat, curbing production of surplus commodities, and providing income support for farmers (Allen and Vandever, 2012; FAPRI, 2007; Follett et al., 2001; Li et al., 2017). In exchange for retiring HEL for 10 years, USDA paid CRP participants (farm owners or operators) an annual per‐acre rent and half of the cost of establishing a permanent land cover (Young and Osborn, 1990).

Soil quality assessment thus emerged as an evaluation tool as many first‐round contracts began to expire (Karlen et al., 1998). Since then, a plethora of studies have evaluated the CRP effect on soil health (Baer et al., 2002; Knops and Tillman, 2000; Matamala et al., 2008; Mensah et al., 2003; Reeder et al., 1998; Rosenzweig et al., 2016; De et al., 2020). Most have focused on insensitive or slower changing soil health indicators (e.g., soil organic carbon [SOC] and N pools). Only a few have evaluated more management‐sensitive active carbon pools, such as potentially mineralizable carbon (PMC), potentially mineralizable nitrogen (PMN) microbial biomass carbon (MBC) and soil microbial communities (Baer et al., 2002; Matamala et al., 2008; Haney et al., 2015; Rosenzweig et al., 2016; Li et al., 2018; De et al., 2020).

In Canada, the concept of soil quality/soil health was revived nearly a decade after its introduction by Warkentin and Fletcher (1977) when the Canadian Senate Standing Committee on Agriculture prepared a report on soil degradation (Gregorich, 1996). The research branch of Agriculture and Agri‐Food Canada began to work with federal and provincial governments, universities, and the private sector to develop a Soil Quality Evaluation Program (Acton and Gregorich, 1995). Collectiveluy they sought to identify sensitive and reliable indicators (Larson and Pierce, 1991; Haberern 1992; Doran et al., 1994; Doran and Parkin, 1994; Karlen et al., 1994a, 1994b; Karlen et al., 1996). Soil quality assessment began to be interpreted as a sensitive and dynamic way to document soil conditions, response to management, or resistance to the stress imposed by natural forces or human uses (Arshad and Coen, 1992; Haberern, 1992). Their efforts focused on indicator development since soil quality/soil health per se cannot be measured directly. This resulted in efforts to identify a minimum set of biological, chemical, and physical measurements (Doran and Parkin, 1996) that provided useful and meaningful information regarding how a specific soil was functioning in response to a specific use and/or management practice (e.g., tillage system, crop rotation, stover harvest, irrigation management, or land‐use changes).

In New Zealand, soil quality/soil health assessment was also a rigorously studied topic with one example being “Visual Soil Assessment” (VSA) guidelines for areas characterized as “flat to rolling” or “hill country (Shepherd, 2000; Shepherd et al., 2000a; Shepherd and Janssen, 2000; Shepherd et al., 2000b). Those publications provided instructions, photographs, and scorecards for using VSA to assign scores of 0, 1, or 2 for poor, moderate, or good, respectively, for a variety of soil and plant indicators. Guidelines to help land managers respond if soil quality/soil health was deemed to be either moderate or poor were provided. Inherent site characteristics including land use, soil type, texture, moisture condition, and seasonal weather conditions are also recorded.

Soil quality quickly became closely aligned with good soil management in New Zealand. Furthermore, because land‐based industries were the main generator of export income, the use of soil quality assessment as a sustainability indicator generated a substantial amount of research and technology transfer activities throughout that country (Beare et al., 1999). Increasing economic pressure to intensify land use, possibly beyond the margins of sustainability further increased farmer demand for more information, better monitoring tools, and improved soil management practices. Ultimately this led to development of a soil quality monitoring system (SQMS) by Crop & Food Research Ltd. and the Centre for Soil and Environmental Quality (Beare et al., 1999). Several soil quality web sites and other technology transfer activities also evolved during that era.

Many other New Zealand‐based soil quality studies that contributed to the foundation upon which current soil health activities have evolved were conducted, but even listing them is beyond the scope of this chapter. One study (Reganold et al., 1993) does warrant discussion because it helps illustrate that soil quality evolution during the 1990s was not without conflict and strong differences in scientific opinion. Reganold et al. (1993) evaluated soil quality and financial performance of biodynamic and conventional farms in New Zealand. They concluded that per unit area, biodynamic farms had better soil quality and were as financially viable as neighboring conventional farms. Within a year a critique questioned the statistical analyses that had been used (Wardle, 1994), but a reanalysis of the data (Reganold, 1994) confirmed the original conclusions and added information indicating that measurements collected from two of the farm pairs had twelve times more earthworms (by number) with biodynamic management than their conventionally managed counterparts.

Bouma (2000) broadened the definition further to include land quality (Bouma et al., 2002) and also explored soil quality effects on the global food supply (Bouma et al., 1998). Meanwhile, Schipper and Sparling (2000) favored the term soil condition, while many German‐language publications indicated scientists in that country struggled with differentiating between soil quality and attributes associated with soil fertility (Patzel et al., 2000). Ultimately the German Federal Soil Protection Act [https://germanlawarchive.iuscomp.org/?p=322 (verified 30 June 2020)] was passed to protect or restore soil functions on a permanent sustainable basis. That Act and others in Europe reflected viewpoints that contaminant levels should be the focus for soil quality debates (Singer and Ewing, 2000) and that the concept should be used to improve land use decisions associated with industrial and urban waste and by‐product disposal. The goal was to prevent soil property changes that would disrupt natural functions of the soil. Meanwhile, Bujnovskỳ (2000) stressed that for the Slovak Republic, soil quality assessment should focus on deriving a fair soil price and monitoring for degradation. With regard to biomass production, he concluded that was only one of many critical soil functions.

In Australia, soil quality/health research during the 1990s was focused on issues similar to those in New Zealand or the northern hemisphere. For example, Aslam et al. (1999) used MBC, MBN, MBP, and earthworm (Apporrectodea caligninosa) populations as biological soil quality indicators to quantify effects of converting pastureland to cropland through either plowing or no‐tillage practices. They concluded that conversion using no‐till practices could protect soils from biological degradation and maintain better soil quality than with moldboard plowing.

Europe and New Zealand were not the only locations where soil quality was intensively debated. In the United States, Sojka and Upchurch (1999) were fearful SQI and similar efforts could lead to premature conclusions advocating a value system as an end unto itself. They argued there was very little if any parallel between soil, air, and water quality, and that there were regional or taxonomic biases. Karlen et al. (2001) rebutted, emphasizing that advocates and early adopters of soil quality were in total agreement that “our children and grandchildren of 2030 will not care whether we crafted our definitions or diagnostics well. They will care if they are well fed, whether there are still woods to walk in and streams to splash in — in short, whether or not we helped solve their problems, especially given a 30‐yr warning.” That philosophical debate is mentioned simply to alert soil health advocates the road ahead may not always be smooth.