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3 The Utility and Futility of Soil Health Assessment
John F. Obrycki and Lumarie Pérez‐Guzmán
Chapter Overview
Documenting benefits from soil health management practices and assessments has been described as both useful and futile because it requires continual observation, some form of data collection, and an assessment protocol. This chapter focuses on the benefits of soil health being evaluated through soil physical, chemical, and biological property measurements. A producer, landowner, or researcher interested in soil health usually wants to know if soil properties are changing from an identifiable condition or point of interest, such as an inherent baseline or an equilibrium condition established by business‐as‐usual soil and crop management practices. When soils are considered within social, political, economic, and environmental contexts, the type of benefits that can be documented expands (Heller and Keoleian, 2003; McBratney et al., 2014; Mena Mesa et al., 2014; Rasul and Thapa, 2004; Steffan et al., 2017; Wolde et al., 2016), but although those assessment scales are important to consider, they are outside the scope of this chapter because such changes, whether positive or negative, generally take several years (perhaps even decades) to be noticeable and/or measurable. This chapter focuses on agricultural research and discusses the general opportunities and limitations associated with soil health management approaches and strategies used to document potential soil physical, chemical, and biological property changes.
Introduction
There are several important questions associated with soil health research (Fig. 3.1). These include issues associated with more clearly defining the soil health concept, determining how to measure and quantify soil health at multiple scales, and using these principles to guide current and future soil and crop management decisions. As discussed in Chapter 2, questions regarding how to achieve effective soil management are not new (e.g., Bennett and Chapline, 1928; Hobbs, 2007; Janvier et al., 2007; Janzen, 2001; Karlen et al., 1997; Karlen et al., 2019; Magdoff and van Es, 2009; Stoll, 2003). Furthermore, several visual, in‐field, and laboratory methods for evaluating soil health have been developed over several decades. Answers to those questions are not simple because the living and dynamic nature of soils results in fiscal, human resource, intellectual, and other research constraints associated with sampling, analyzing, and interpreting how soil biological, chemical, and physical properties and processes affect soil health. Our objectives for this chapter are to provide practical definitions and examples of various approaches for addressing soil health, along with our assessment of current analytical methods, their limitations, and potential research topics that may clarify and help advance the concept.
Definitions
Within the context of soil health, we suggest the term “benefit” refers to a human defined and desired change in soil physical, chemical, and/or biological properties and processes because of their effect on critical functions (i.e., productivity, filtering and buffering, water entry, retention and release) that soils provide for humankind. For example, a decrease in soil compaction, and an associated increase in soil porosity, would be considered a physical soil health benefit because less compacted soils allow more water to infiltrate and be stored in the soil. Those water and retention benefits subsequently improve productivity as plants are able to extract and use the soil water, and provide environmental benefits because the rate and amount of water running off the site (carrying soil particles, nutrients, pesticides, etc.) is decreased. Soil pH can be used to illustrate the role of chemical properties and processes in a soil health assessment because it affects critical soil functions such as nutrient availability and fitness for plant root growth and development. With regard to soil biological properties and processes, soil organic matter (SOM) or a closely associated measurement (i.e., active carbon, β‐glucosidase, particulate organic matter) can be monitored over time to determine how various soil and crop management practices are affecting the soil (Gebhart et al., 1994; Li et al., 2017). In general, it would be considered a benefit if SOM increases because it can then increase soil water holding capacity, nutrient retention and cycling, and soil aggregation.
Figure 3.1 Key soil health research questions and selected responses (orange boxes).
A second important point when defining soil health benefits is to realize that because of the living and dynamic nature of soils, changes are site‐ and landscape‐specific and therefore when interpreting the relative importance of a change, the phrase “it depends” must be kept in mind. For each soil biological, chemical, and physical soil health indicator, there are ranges over which changes are of most interest and highly influential as well as other ranges where they have minimal to no agronomic, environmental, or other economically important effect.
Previously, soil health indicator benefits have generally been conceptualized as following one of three curve types: “less is better” (e.g., soil compaction), “more is better” (e.g., SOM content), and “mid‐point optimum” (e.g., soil pH) (Andrews et al., 2004; Moebius‐Clune et al., 2016). Therefore, a soil with 500 g kg−1 (50%) organic matter may be a suitable peat or wetland soil with environmental buffering, wildlife, or other positive attributes, but without major investment in drainage water management, it would not be a suitable soil for production of corn (Zea mays L.), soybean (Glycine max [L.] Merr.), wheat (Triticum aestivum L.) or cotton (Gossypium hirsutum L.). Similarly, an acidic soil is desired for high‐bush blueberries (Vaccinium corymbosum L.) or some forest species, but would be toxic for plants that cannot tolerate the high concentrations of soluble aluminum (Al) or manganese (Mn) that can occur under those conditions. Soil health, therefore, does not mean that all soils will have the same properties, but all soils will exhibit health benefits when physical, chemical, and biological properties are evaluated in the context of one or more specific soil functions.
The third and final focus that needs to be defined for this chapter is the phrase “soil health approaches”. We use this term to refer to management systems that consider soil physical, chemical, and biological properties collectively, rather than focusing on only one aspect (Andrews et al., 2004; Moebius‐Clune et al., 2016). For example, adequate nutrient availability for plants provided by routine soil fertility testing and good fertilizer management is not sufficient for a soil to be considered “healthy” if that resource is highly compacted due to excessive or inappropriate wheel traffic, eroded by wind or water, or depleted in SOM compared to its inherent conditions. Soil health approaches must focus on comprehensive management that views soil resources as physical, chemical, and biological systems and uses practices that address all three components. Implementation of such approaches is not difficult and may be accomplished by combining routine soil test recommendations with reduced tillage intensities, controlled traffic planting, and harvest patterns. Collectively, such a soil health approach could improve soil nutrient availability and reduce soil compaction.
Opportunities for Implementing Soil Health Approaches
The primary purpose for developing and implementing soil health approaches is to encourage the use of scientifically‐based, comprehensive soil management
