et al. (2019) in a study of Missouri claypan soils across four watersheds for 3 yr. They were able to detect expected depth differences in soil properties, with surface samples having greater soil biological activity, as indicated by higher enzyme and organic matter values. However, this trend did not occur for glucosaminidase, since quantities of that enzyme actually increased with depth. Overall, the expected soil health indicator improvement expected for no‐till plus cover crops in comparison to tilled, no cover crop treatments did not occur presumably because of unfavorable growing conditions.
Another common short‐term soil health evaluation is to compare two widely varying production systems from opposite ends of a disturbance continuum (e.g., perennial grassland vs. a tilled field). Transitioning from grassland to a tilled field typically causes shifts in soil biological activities, sometimes within the first month after tillage commences. In Texas, Cotton and Acosta‐Martínez (2018) documented a 52% decrease (505 to 241 mg kg−1 soil) in soil microbial biomass in the top 10 cm of the soil profile. After the first growing season, soil organic carbon (SOC) within the top 30 cm of the profile declined by an average of 20%, although the decline in the surface 10 cm (11.60 vs. 7.28 g SOC kg−1 soil) was substantially greater than within the 10‐ to 30‐cm increment (6.76 vs. 6.17 g SOC kg−1 soil). This was not unexpected since in most soil health studies, soil property changes are greater near the soil surface because this portion of the profile is most directly affected by changes in tillage and plant root conditions.
Soil management practices that increase microbial food supplies and reduce disturbance to their habitats will tend to have greater soil microbial activity than soils with limited food sources (i.e., SOC), due to low crop residue or root carbon inputs, excessive crop residue removal, or excessive tillage. Sustaining adequate soil microbial activity is important because biologically mediated processes such as nutrient cycling and SOM dynamics are critical components of several soil ecological functions (Dick, 1992). For example, in comparisons between conservation reserve program (CRP) fields and active croplands, Li et al. (2018) found that fungal abundance increased in proportion to the length of time since CRP practices were implemented. They found increases in fungal abundance up to 15 yr after establishment, followed by decreases relative to bacterial abundance. The shifts in microbial community composition were attributed to historical soil conditions, abiotic factors and climate properties. Soils in CRP had less stressed soil microbial communities as indicated by fatty‐acid methyl ester biomarker (FAME) profiles, in which the ratio of saturated to mono‐unsaturated fatty‐acids decreased (Li et al., 2018). The mono‐unsaturated fatty‐acids indicated active metabolic processes were occurring in the soil compared to higher saturated fatty‐acid profiles that are indicative of slower metabolic processes which can occur when water or nutrients are in short supply for microbes.
Soil Health Limitations
There are at least three related components that can limit the utility of soil health research and implementation efforts. First are the logistical limitations including the cost of a project, access to samples or instrumentation, and time for the study to be conducted. Next, are the philosophical limitations that can occur when an assessment project is designed, especially regarding the scope and questions of interest. These types of limitations generally occur if someone considers the project’s approach to be insufficient to answer what is often potentially a broader or different question. Logistical and philosophical limitations do overlap. For example, consider the two questions: “Can we measure what we want to measure”? and “Why do we want to conduct the measurement”? A third important limitation involves the number of management changes that are needed to fully execute a comprehensive soil health management approach.
Phosphorus loss from agricultural fields provides an excellent example of this third limitation. Growing cover crops and reducing tillage are often promoted as soil health improving practices because of their impact on SOC, but they may not be sufficient to reduce P losses from agricultural fields. Since either high soil‐test P levels, excessive P applications through fertilizer or animal manure, or high soluble P in senescing cover crop vegetation can all contribute to increased soluble P runoff, soil health management practices focused on this problem must be coupled with changes in the way P is applied to the fields. This could easily involve new materials, timing of application, and/or equipment (Duncan et al., 2019). Therefore, an effective soil health research project may require substantial changes to all dynamic soil properties and associated management practices, which from a practical perspective can be a limitation when multiple fields and/or producers are involved.
Other potential limitations to useful soil health research and technology transfer include factors such as producer interest, economic limitations, time requirements, and the magnitude of change needed with regard to soil and crop management practices and/or desired with regard to soil properties. The utility, however, is emphasized by the numerous potential endpoints that exist, especially when balancing productivity with a broader environmental perspective. Is the ultimate endpoint of improved soil health increased yield, long‐term sustainability, water quality, economic viability, community development, or all of these goals? The length of time for which soil health indicators must be tracked and whether or not changes can be documented will be determined by the ultimate goal(s). This also determines the magnitude and type of change that must be measured. Without any doubt, research studies can document findings that are both statistically significant and practically important. However, depending on (1) how changes are measured, such as with an in‐field test, commercial, or research laboratory test, (2) the inherent soil variability and (3) the analytical soil test variability, one or more of those factors can potentially mask any true soil health effects. It is not surprising, therefore, that all of these challenges (i.e., endpoints, time, magnitude of change) reflect various trade‐offs. Research projects tend to be funded for relatively short periods of time, often measured in two to five year increments, research budgets are not unlimited, and every sample that needs to be analyzed requires careful collection, appropriate preparation and adequate processing time. Obviously, these challenges are not unique to soil health research, but recognizing them may help diffuse some of the discussion between those who view the efforts as either useful or futile.
Conclusions
Documenting benefits from soil health approaches first requires defining what is the benefit of interest and then selecting ways to measure and document the response. The principles associated with soil health are not new as evident by centuries of soil management, conservation, condition, tilth, quality, and other terms. Among the well‐known and generally accepted approaches for improving soil health are the goals of keeping the soil covered, reducing disturbance, maintaining plants year‐round, and diversifying the mix of plant species. Implementing these goals can increase the quantity of plant residues and root exudates returned to the soil, boost microbial activity, and ultimately lead to a cascade of soil improvements, including increased SOM, more stable soil aggregation, and efficient nutrient cycling. How well these benefits can be documented depends on the magnitude of change (generally determined by inherent soil properties and/or initial conditions) as well as the type of soil health test selected (i.e., in‐field, commercial or research laboratory, remote sensing), and the scale at which comparisons are to be made and meaningful (i.e., from landscapes down to finely sieved and crushed soil samples). Interactions between inherent and dynamic soil properties can also make documenting soil health benefits difficult, since spatial and temporal variability can mask potential changes associated with new soil and crop management practices, such as annual cover crop establishment, which must be given adequate time for measurable effects to occur. Extreme weather conditions, such as too much or too little rainfall, early or late frost, or above normal temperatures can hinder the effectiveness of new or alternative management systems and prevent them from becoming established and changing soil properties in subsequent years. Without question, researchers have documented numerous benefits from soil health approaches. As the concept evolves, the core questions of defining what constitutes an important benefit and selecting reproducible methods to measure that benefit will remain a constant challenge and an important research goal.
