Interpretation of Soil Health Tests

The concept of soil health has gained traction among farmers and other ag professionals who are interested in sustainable crop production as well as environmental quality. In response to growing awareness, several laboratories have been established that provide soil health testing. There is some debate among soil scientists and within the agricultural community as to the usefulness, sensitivity, repeatability, and interpretation of soil health tests. In general, soil health tests cost more than traditional soil fertility tests, so it is important to understand what soil health entails, how it is measured, and factors that can influence the interpretation of soil health tests, especially biological assessments.

Soil health is commonly defined as "the capacity of soil to function as a vital living system, within an ecosystem and land-use boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and promote plant and animal health."  Although there are several variations of this definition, most emphasize the site-specific, sustained ability of soil to provide services arising from the complex interactions among the chemical, physical, and biological characteristics of a soil, and the connection of these interactions to soil function and management.

Soil health tests usually include typical measures of soil fertility (chemical characteristics).  Where soil health tests differ is the inclusion of measures of soil physical and biological health.  For example, assessment of physical health can include bulk density and compaction, and assessment of soil biological health can include measures of "active" carbon (fraction of soil organic matter that is readily usable as a resource to support soil organisms) and soil respiration ("flush of CO₂ on rewetting").  Soil respiration is considered an indicator of soil microbial biomass, microbial activity, and soil carbon availability.  Active C is different from soil respiration because it measures easily accessible C regardless of the current microbial activity in the sample.  This article will focus on factors that can affect the results and interpretation of soil biological assessments.

Biological Indicators for Soil Health Testing

Soil microbes are of critical importance for long-term fertility of soils and are considered sensitive indicators for the impact of management practices on soil health. A healthy soil harbors an abundant, active, diverse microbial community which is key to many soil functions that impact soil productivity, including nutrient cycling, decomposition of organic matter, erosion control and water regulation by the formation of water stable aggregates and soil structure, and pest and disease regulation through predation, antagonisms, and competition with harmful organisms.  Because measurements of the actual abundance, diversity, and function or identity of soil microbes would be prohibitively expensive and time consuming, soil health tests usually include "black-box" proxies for microbial biomass and their activity (e.g., CO₂ flush) soil respiration.

What makes a good soil health indicator?

To assess the impacts of and guide management practices, a useful soil health indicator must be based on mechanistic linkages between the indicator and soil functions or ecosystem services (e.g., crop yield). The indicator must respond to differences in management with sensitivity and repeatability and have consistency of interpretation. If measurements of soil health are inconsistent for soil management, then management recommendations are likely to be inconsistent. Above all, the indicator must be able to differentiate among the impacts of various soil management practices on diverse soils.

It has been especially difficult to identify reliable biological indicators because microbial communities are very dynamic – many environmental factors influence microbe abundance, species, and activities in space and time. This is important because the results of biological soil health tests can vary widely according to environmental and other factors and this variability can interfere with the interpretation of overall soil health scores.  For example, a recent study on farms and on research stations by agronomists in North Carolina found both analytic variability (results of soil health tests on the same soils submitted to different testing services did not always agree with one another), as well as indicator variability (soil biological indicators did not distinguish between long-term management considered beneficial for biological soil health and management considered detrimental to biological soil health). The researchers also found no relationship between the ability of long-term soil conservation practices to achieve acceptable soil health scores and greater crop yields.  The researchers concluded that there may be substantial variability in soil properties regardless of agronomic management that determine biological indicator scores, or that the soil health indicators may not be sensitive enough to differentiate among soil management effects on properties of soils with different compositions.

Listed below are some of the factors that research has found can affect the results and interpretation of soil biological health tests.

Analytical Variability: Differences among Soil Health Laboratories

Currently, there are no standard operating procedures among soil health test labs to reduce differences in results. Analytical variability is likely soil-specific and can result in unreliable and inconsistent recommendations. Examples of sources of analytical variability that can affect test results among soil health testing labs include: 

• Sieve mesh size used to process soil
• Soil water content 
• Direction of rewetting dry soil before carbon flush tests (adding water to the top of sample vs. from the bottom through capillary action)
• Different extractants and methodologies

Environmental Variability: Differences in Biological Indicators due to Space and Time

• Soil parent material
• Region and climate zone 
• Topography
• Hydrology
• Soil texture (fraction of sand, silt, clay), type, and moisture
• Time of year in which sample is collected time (e.g., seasonal fluctuations of microbial populations and activity can be greater than those due to management)
• Time of sampling within a season (e.g., there are biological "hot moments" when soil microbes are especially abundant and active, such as when crops are actively growing and root exudates are abundant)
• Temperature, precipitation, and other weather factors
• Plant species and proximity of soil samples to plants (in-row vs. between-row; row crops vs. sod crops)
• Land use (arable, orchards, and grassland) 
• Plant life cycle (annual vs. perennial) 
• Location in the field (i.e., soil biology is not uniform, there are "hotspots" such as in earthworm tunnels or in the root zone (rhizosphere) of plants)
• Area of root zone sampled
• Plant growth stage

Conclusions and Recommendations

The growing interest in soil health is an extremely positive development because it raises awareness and guides management that can reduce the environmental and economic consequences arising from degraded and unhealthy soils.  However, the inherent complexity among and within soil systems complicates soil health evaluations because environmental factors can interact with intrinsic soil properties in variable ways. These effects are not equal for all soils in all places. Numerous soil health indicators have been and are being developed, but the effectiveness of combining analyses of different soil indicators into a comprehensive and universal interpretation of soil health, while a worthwhile goal, remains elusive.  Evidence suggests the need for the development of region- or soil-specific scoring functions, interpretations, and recommendations.

Compared to conventional soil fertility testing, soil health testing is in an early stage of development. Scientists and practitioners continue to identify new and to evaluate and refine existing biological indicators, and to improve interpretations of soil health scores by building a better understanding of how environmental, sampling, and processing factors impact them. If growers or other ag professionals choose to use soil health tests to guide and assess management, it is important to collect and handle samples consistently and uniformly and submit them to a single laboratory. Caution must be exercised in the interpretation and use of the results due to sources of variation among labs, soils, and environments.  A recent meta-analysis of published research reporting the accuracy and repeatability of biological indicators of soil health found that active carbon (permanganate oxidizable carbon) was the most sensitive, repeatable, and consistent indicator, and was the most predictive of overall soil health scores and had the lowest variability within soil textural groups. Current research suggests that for representative test results, samples should be collected to a uniform depth (whole plow layer) during a period in early autumn after incorporation of plant residues to reduce the influence of plant growth and incorporated plant residues, but before soil biological activity diminishes in late fall.  In some climate zones, such as those with short windows between harvest and winter conditions, timing to achieve meaningful biological measures may be challenging.