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 practices that account for not only economic goals such as productivity but also how the entire landscape is maintained (Schnepf and Cox, 2006). Although soil health approaches per se have become recognized research and technology transfer topics during the past two decades, to many readers the concept is not new but rather another term transferable to previous efforts defined as soil conservation, soil quality, soil condition, soil tilth, or simply soil management. For example, Table 3.1 lists four soil and plant management strategies often presented as 21st century soil health approaches that have been considered to be good soil management practices throughout not only the 20th century (King, 1911; Keen, 1931; Lowdermilk, 1953; Keen, 1931) but from the time of Plato and before (Fream, 1890; Hillel, 1991; Diamond, 2011; Montgomery, 2007). Without question, those four management practices are in no way comprehensive, but they do provide a general framework under which many soil health approaches can be listed. A common link between the practices is that they strive to improve soils by keeping them covered and protected by erosive forces of wind or water and they strive to keep soil biological and chemical cycles as active as possible (Table 3.2). Keeping soils covered with living plants minimizes runoff, encourages proliferation of roots, production of root exudates and recycling of senesced vegetation as food for soil microbes, and provides a biotic pump for moving water through the soil–plant–atmosphere continuum.
Table 3.1 Timeless generic strategies for improving soils.
| Management Practice | Year | ||
|---|---|---|---|
| 1937 a | 2017 b | 2097 c | |
| Keep soil covered | ✓ | ✓ | ✓ |
| Reduce soil disturbance | ✓ | ✓ | ✓ |
| Keep plants growing year round | ✓ | ✓ | ✓ |
| Diversify | ✓ | ✓ | ✓ |
a Adapted from Rule, 1937.
b Taken from guidelines presented by the USDA‐Natural Resources Conservation Service (NRCS), 2019b.
c Our projection of soil health approaches that will continue to be emphasized.
With regard to 21st century soil health approaches, two questions emerging from the generic guidelines in Table