When land managers change their practices to improve soil health, it can be difficult to measure the impact of those changes. According to Cathy Zabinski, professor of plant and soil ecology at Montana State University’s Department of Land Resources and Environmental Sciences, “There is no optimum standard for soil health in the same way that there is an optimal body temperature for human health.”
Soil health varies by type and climate, prompting Zabinski and former graduate student Zane Ashford to study a site-specific indicator called the “soil health gap.” This gap measures the difference in soil health characteristics between managed or disturbed land—such as cultivated, hayed, or grazed fields—and minimally disturbed land. The gap is expressed as a percentage; a larger number indicates a greater difference between the two soils.
Ashford compared cultivated and uncultivated fields near Forsyth and Fort Benton. He found that several indicators of soil health, including soil organic matter concentration, showed a 40% to 60% gap. The uncultivated sites were mainly intact perennial plant communities, while the cultivated fields used conservation farming methods such as no-till, no summer fallow, and crop rotation with perennials.
Clain Jones, MSU Extension soil fertility specialist and professor in the same department, said: “We would expect the soil health gap to be even larger in a cropping system that uses tillage and fallow largely because those practices offer less soil cover and return less residue to the soil.”
To measure the soil health gap, land managers can compare samples from managed areas with those from similar nearby undisturbed sites. Jones recommended matching slope, elevation, and aspect for accurate comparisons.
Zabinski and Ashford used biological indicators like enzyme activity as well as chemical indicators such as organic matter content. For more detailed assessments, additional tests are available but may become costly. As Jones explained: “If the land manager wants a detailed evaluation, they can test for several additional biological, chemical and physical soil health indicators, but this can quickly get expensive.”
Jones’ MSU Extension program offers online reference sheets titled The Soil Health Gap and Evaluating Soil Quality and Health that discuss various characteristics used in these assessments.
Tracking changes in the soil health gap every five years over 15-20 years could help identify management strategies that benefit soils. Jones noted that increasing plant residue returned to cropland—by switching annual crops to perennials or replacing fallow with crops—can improve conditions. For pasture or hay fields, addressing factors like poor fertility or overgrazing may also help.
Further information on evaluating soils is available through MSU Extension’s fact-sheet series The Soil Scoop.
