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, as it remains before us today on the largest research station operated by the U.S. Department of Agriculture: the Henry A. Wallace Agricultural Research Center in Beltsville, MD. Thus, the concept of soil health has a particularly illustrious pedigree in the history of agricultural science.
Research on soil properties– physical, chemical, and biological– has led to major advances in managing agricultural soils, contributing to significant crop yield increases throughout the 20th and 21st centuries. However, consensus on a holistic approach to understand, implement, and measure outcomes of soil management, with goals that include sustaining production and enhancing soil health, has escaped the scientific community. Reasons for this include: (i) ever‐changing methods of measurement and how to interpret the data, especially for biological properties and processes; (ii) how to adapt analytical methods for soils having different properties, which may alter results and make data comparisons difficult; (iii) the meaning of analytical results for different agricultural production systems and environments; (iv) unclear links among measurements, soil processes, and desired outcomes (ecosystem services such as agricultural yield, nutrient cycling, improved water quality, etc.); (v) complexity and costs for advanced measurement techniques; (vi) differences in sample handling and measurement protocols among analytical laboratories; (vii) producers’ uncertainties about what the data mean and how to adjust management practices in response to the information, including potential risks and benefits; and (viii) inconsistent messaging about soil health and how to manage it to agricultural producers, natural resources managers, educators, policymakers, and other stakeholders.
Stakeholder diversity alone presents significant challenges to the community of scientists, practitioners, producers, and others who advocate making soil health the cornerstone of agricultural and environmental decision making. The needs of different segments of the community demand different kinds of data, information (interpretation of the data), and communication techniques. For example, the interests of a typical agricultural producer are unlikely to be met with a report on 20 to 30 laboratory measurements that quantify a range of physical, chemical, and biological properties of a soil. Such a report may be more than most producers would want to interpret. On the other hand, a small group of indicators, easily obtained and explained, might be helpful to a producer but insufficiently accurate, precise, and process‐oriented for scientific research. The distinctly different needs of various stakeholders provide a critical starting point for any conversation about soil health.
How Can a Farmer Assess Soil Health in the Field?
Many producers are keen to learn about soil health on their farms and how they can alter their current soil and crop management practices to sustain or improve it. This interest has greatly increased opportunities for agricultural experts who can successfully bridge researcher and producer communities and is a key factor driving development of public and private programs that strive to strive for clear communication about soil health. For example, pasture and range scientists affiliated with the Noble Research Institute in Ardmore, OK, often advise farmers to consider five indicators (Jeff Goodwin, personal communication, 2018), which we summarize here as “the Five C’s of Soil Health”. They are:
Color– A healthy soil’s dark brown color indicates the presence of a lot of carbon in the form of decomposed organic matter. In contrast, gray, yellow, or mottled colors indicate soil that has a low carbon content, is poorly drained and poorly aerated, and likely low in nutrients available to plants.
Crumbs– A soil that is crumbly, like coffee grounds or cake crumbs, and holds that aggregate structure is likely in good physical condition supporting soil health. This is structure that allows water movement yet aeration, as well as root penetration. It holds up even when the soil is wet. If the dry soil can easily be ground to dust between the fingers, or it turns into a slick film when wet and rubbed between the forefinger and thumb, the aggregates are not stable and will not support a good crop.
Critters– A healthy soil shows lots of evidence of life. Pulling the crop debris back from the surface should reveal earthworms, or their holes and castings. Turning over the soil with a shovel should uncover insects, pillbugs, and other arthropods essential in carbon and nutrient cycling. A low‐power hand lens might allow observation of smaller arthropods such as mites that feed on debris and microbes, and perhaps even the filamentous hyphae of fungi or the near‐microscopic worms that feed on them. A soil that lacks evidence of diverse life is not healthy.
Cooperation, with roots, that is– A healthy soil does not constrain roots, its structure allows plant roots to grow vertically and laterally. When roots look stunted or turn at odd angles, it is likely that the soil is compacted or has a plow layer that obstructs root growth because it lacks good structure and aeration for a crop. Stubby, deformed, discolored, or rotten roots can also indicate the presence of parasitic nematodes, plant‐feeding insect pests, or pathogenic microbes in the soil, none of which is desirable for a healthy soil.
Cologne– A healthy soil has a fragrant, earthy aroma, indicative of the many aerated biological processes happening. A soil that has a sour or rotten‐egg odor is poorly aerated, probably because of poor structure and poor drainage, and is not likely to be a hospitable environment conducive to plant root development or beneficial microbes.
What Do Researchers Need, and Can They Reach Consensus?
The Five C’s of Soil Health may be useful to a farmer, and they can use them to consider modifications to production practices that could push the soil toward more desirable characteristics. For research purposes, however, these indicators are insufficiently quantitative, repeatable, and explanatory for statistical analyses and hypothesis testing about soils at different locations, under different production systems, or subjected to different management practices. For those needs, measurements that are highly repeatable and based on standardized protocols and techniques within research laboratories are needed.
At this other extreme, a new set of challenges arises– how to get a representative sample, how to handle and store it before it can be analyzed, which properties to measure, which measurement method to use, how to report the data, how to develop recommendations from those data. Just as a physician cannot adequately describe the health of a human patient with a small number of measurements or distillation of many measurements into a single number, scientists must rely on multiple different indicator measurements to provide a scientifically meaningful assessment of a soil’s health. Preferences regarding specific measurements to make and methods to use are no doubt numerically equal to the number of scientists wanting to assess soil health. Reaching consensus in the community has been and continues to be a difficult task.
Currently, there are two integrated and coordinated efforts to identify suitable soil health indicator measurement protocols and to assess their utility throughout the country. One, led by the U.S. Department of Agriculture– Natural Resources Conservation Service (USDA‐NRCS) Soil Health Division (SHD), is Soil Health Technical Note No. 450–03 (Stott, 2019) entitled “Standard Indicators and Laboratory Procedures to Assess Soil Health.” The other is a research project led by the Soil Health Institute (SHI), which is evaluating the utility
