and acknowledge soil health assessment is not an exact science, but there are a few principles that are non‐negotiable. First, to qualify as a meaningful, comprehensive assessment, soil biological, chemical, and physical properties and processes must all be included. Failure to do so, does not invalidate the assessment, but rather limits it to an assessment of “soil biological health”, “soil physical health”, “soil chemical health”, or some combination thereof. Furthermore, although some redundancy may occur, at least two different indicator measurements should be used for each indicator group (i.e., biological, chemical, or physical). To aid indicator selection, many statistical tools are being developed and evaluated to help identify the best combination of potential measurements for assessing each critical soil function associated with the land use for which an evaluation is being made.
There is also no question that any soil health indicator must be fundamentally sound from all biological, chemical, physical and/or biochemical analytical perspectives. Indicators must have the potential to be calibrated and provide meaningful information across many different types of soil. This requires sensitivity to not only dynamic, management‐induced forces, but also inherent soil properties and processes reflecting subtle differences in sand, silt, and clay size particles derived from rocks, sediments, volcanic ash, or any other source of parent material. Soil health assessments must accurately reflect interactions among the solid mineral particles, water, air, and organic matter contained within every soil. This includes detecting subtle changes affecting runoff, infiltration, and the soil’s ability to hold water through capillarity– to act like a sponge; to facilitate gas exchange so that with the help of CO2, soil water can slowly dissolve mineral particles and release essential plant nutrients– through chemical weathering; to provide water and dissolved nutrients through the soil solution to plants, and to support exchange between oxygen from air above the surface and excess CO2 from respiring roots.
Some, perhaps many, will disagree with the choice of indicators that are included in these books. Right or wrong, our collective passion is to start somewhere and strive for improvement, readily accepting and admitting our errors, and always being willing to update and change. We firmly believe that starting with something good is much better than getting bogged down seeking the prefect. This does not mean we are discounting any fundamental chemical, physical, thermodynamic, or biological property or process that may be a critical driver influencing soil health. Rather through iterative and ongoing efforts, our sole desire is to keep learning until soil health and its implications are fully understood and our assessment methods are correct. Meanwhile, never hesitate to hold our feet to the refining fire, as long as collectively we are striving to protect and enhance the unique material we call soil that truly protects humanity from starvation and other, perhaps unknown calamities, sometimes self‐induced through ignorance or failing to listen to what our predecessors have told us.
Douglas L. Karlen (Co‐Editor)
References
1 Alexander, M. (1971). Agriculture’s responsibility in establishing soil quality criteria In: Environmental improvement– Agriculture’s challenge in the Seventies. Washington, DC: National Academy of Sciences. p. 66–71.
2 Bouma, J. (2019). Soil security in sustainable development. Soil Systems. 3:5. doi:10.3390/soilsystems3010005
3 Donahue, R. L., J. C. Shickluna, and L. S. Robertson. 1971). Soils: An introduction to soils and plant growth. Englewood Cliffs, N.J.: Prentice Hall, Inc.
4 Doran, J.W., Coleman, D.C., Bezdicek, D.F., and Stewart, B.A., editors. (1994). Defining soil quality for a sustainable environment. Soil Science Society of America (SSSA) Special Publication No. 35. Madison, WI: SSSA Inc.
5 Doran, J.W., and Parkin, T.B. (1994). Defining and assessing soil quality. In: J.W. Doran, D.C. Coleman, D.F. Bezdicek, and B.A. Stewart, editors, Defining soil quality for a sustainable environment. SSSA Special Publication No. 35. Madison, WI: SSSA. p. 3–21. doi:10.2136/sssaspecpub35
6 Doran, J.W., and Jones, A.J. (eds.). (1996). Methods for assessing soil quality. Soil Science Society of America (SSSA) Special Publication No. 49. Madison, WI: SSSA Inc.
7 Hartemink, A. E. and Anderson, S.H. (2020). 100 years of soil science society in the U.S. CSA News 65(6), 26–27. doi:10.1002/csann.20144
8 Hillel, D. (1991). Out of the earth: Civilization and the life of the soil. Oakland, CA: University of California Press.
1 Laboratory Methods for Soil Health Assessment: An Overview
Steven R. Shafer, Douglas L. Karlen, Paul W. Tracy, Cristine L.S. Morgan, and C. Wayne Honeycutt
The purpose for Volume II is to provide specific methods and guidelines available for individuals and laboratories to evaluate soil health indicators discussed in Volume I. This volume draws on and updates the 1996 Soil Science Society of America Special Publication Number 49 entitled Methods for Assessing Soil Quality that is commonly referred to as the “Green Book” for soil quality and soil health assessment. This volume, however, is not merely a revision of the 1996 book, but rather adds guidelines for several new soil health assessment tests and discusses advances in data interpretation made during the past two decades.
Soil health is defined widely as the continued capacity of a soil to function as a vital living ecosystem that sustains plants, animals, and humans (e.g., NRCS, 2020). In recent years, the concept of soil health has become better understood and more widely accepted in the United States and around the world. An important driver for increased interest and global acceptance of the concept is public recognition that to meet food, feed, fiber, and fuel demands associated with an increasing population, soil degradation through erosion and loss of soil organic carbon (SOC) must be stopped and reversed by enhancing desirable biological, chemical, and physical properties and processes within this living, dynamic resource. Thus, over the past 25 years, soil health has become a focal point for serious attention across a range of public‐ and private‐sector agricultural, environmental, and conservation organizations. Collectively, these groups have identified numerous benefits to farmers; the agricultural industry as a whole; water, air and other natural resources; educators; and the general public. This includes identifying and implementing soil health‐promoting practices (e.g., cover crops, reduced intensity and frequency of tillage, improvements in and expanded use of perennials, site‐specific soil and crop management) that can increase SOC (Ismail et al., 1994; Karlen et al., 1994; Ussiri and Lal, 2009; Varvel and Wilhelm, 2010; Wander et al., 1998), thereby increasing available water holding capacity, enhancing drought resistance and resilience (Emerson, 1995; Hudson, 1994; Olness and Archer, 2005), reducing wind and water erosion, and reducing nutrient loss to surface waters (Langdale et al., 1985; Tonitto et al., 2006; Yoo et al., 1988; Zhu et al., 1989). Additional benefits associated with improvements in soil health include increased suppression of pests and pathogens, increased crop yield and quality, improved return on investment, and many broad, nonpoint environmental benefits. Agricultural productivity, economic return, and environmental goals all benefit from enhancing soil health.
The literature on soil health, including the implementation of practices and technologies that promote it, has exploded in recent decades. In a search of literature covered by Google Scholar, Brevik (2018) identified more than 20,000 references using the term “soil health” from January 2000 through February 2018. This represented more than 93% of the total number of references recovered. However, our understanding of soil health and its benefits did not develop over just the past 20 years; indeed, the idea of promoting good soil health is more than 100 years old and has a surprising history. Brevik writes, “The earliest clear reference to soil health found in this review was made by Wallace (1910), who wrote about the importance of humus, particularly as obtained from manure, in maintaining soil health”. The author of this 1910 reference (a thesis submitted to Iowa State University) was a student who eventually would become President Franklin Roosevelt’s Secretary of Agriculture, then Roosevelt’s vice‐president. The name of this earliest known user of the term “soil health” will be familiar to many,