Soil health and salinity management have emerged as critical focuses for agricultural and ecological sustainability as scientific communities work to define measurable indicators for soil productivity. Experts emphasize that understanding the living ecosystem of soil and the mechanisms by which plants manage salt stress is vital for long-term land stewardship.
Defining Soil Health
Soil health is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. According to Utah State University Soils Specialist Grant Cardon, the concept has emerged as the educational face of efforts to protect and improve the quality and productivity of this life-sustaining resource. This focus follows decades of debate within the scientific communities of soil, environmental, agronomy, horticulture, and ecology regarding the sustainability and conservation of soil.
In 2015, coinciding with the International Year of Soil, the USDA-NRCS established its Soil Health Division. In 2016, this agency, working with non-profit organizations like the Soil Health Institute and the Noble Foundation, conducted focus group discussions with scientists. The goal of these discussions was to define soil health and determine measurable indicators of its status, which will eventually form the basis for a nationwide baseline assessment to track changes in soil conditions over time.
Key Indicators of Soil Quality
The 2016 focus group discussions identified five leading indicators of soil health. These include microbial diversity, general biological activity, active carbon content, and organic nitrogen availability. Chief among these is soil aggregate stability, which is considered the integrator of all other indicators. Soil structure is viewed as a direct function of the interplay between soil minerals, nitrogen and carbon cycling, and diverse, active microbial communities.
Managing Soil Salinity
In addition to general soil health, managing soil salinity is a significant concern throughout the western United States. Salts introduced into a plant's root zone create an osmotic restriction that prevents water movement into the plant. To combat this, certain plants utilize two primary biological mechanisms: osmoregulation or salt exclusion.
Osmoregulation involves a plant's ability to regulate the concentration of solutes inside the root to partially or fully overcome osmotic restriction. Salt exclusion is the ability of a plant to take up salt with water and then either purge the excess salt or compartmentalize it internally to reduce metabolic interference. Both processes require energy that would otherwise be used for growth, which can lead to stunted plant development even at low to moderate salt levels.
Plant Tolerance and Measurement
Plant tolerance to salinity varies widely between different species and even among different cultivars within a single species. Scientists measure salinity tolerance using two metrics: a salinity threshold value, which is the point at which a plant begins to suffer salt stress, and a slope factor, which indicates how severely plant performance decreases for every additional increase in salinity above that threshold. Salinity is measured in deci-Siemens per meter (dS/m), which tracks the electrical conductivity of a soil's saturation extract.
Specific crops demonstrate different sensitivities to these levels. For example, sweet corn is considered a moderately sensitive plant with a threshold value of 1.7 dS/m and a slope value of -11.5, meaning its production decreases by 11.5% for every unit of salinity increase above the threshold. In contrast, beets are relatively tolerant, possessing a higher threshold of 5.3 dS/m and a smaller slope value of -7.3.
Soil Reclamation Strategies
When soil salinity levels become too high for desired plant types, the soil must be reclaimed through leaching, which involves washing soluble salts out of the root zone. This requires flushing large quantities of low-salinity water through the soil. As a rule of thumb, applying six inches of clean water over the soil's saturation point can remove 50% of soluble salts from the top foot of soil. Doubling that amount can achieve an 80% reduction.
For trees and ornamental plants with deeper root systems, leaching may require flushing the top two feet of soil. To maintain low salinity levels, adding 15% to 20% excess water with each irrigation can be effective. Successful leaching also requires sufficient free drainage. This often requires improving infiltration by amending soil with organic matter to improve aggregation and soil structure. Additionally, the water table must remain deep enough so that leach water does not raise the local water table within 6 to 8 feet of the surface, which would otherwise cause salts to wick back up into the root zone.
Best Practices for Land Stewardship
To promote soil health, the NRCS and other agencies are working to provide specific guidelines tailored to regional climate and soil constraints. Identified beneficial practices include the use of cover crops, crop rotation, tillage minimization, crop residue management, and integrated management of irrigation, pests, and nutrients.
Several states have already begun implementing soil health scorecards to objectively rate current conditions and identify areas for improvement based on the established indicators. For those experiencing salinity issues—which may manifest as leaf burning or chronically stunted plants—experts recommend soil testing to determine the appropriate plant selections or necessary remediation steps.
