Blog

Soil health tests are a big conversation topic these days, but there’s a lot of conflicting information out there. Presented by Stacy Zuber, Ph.D., State Soil Health Specialist with USDA Natural Resources Conservation Service, this webinar discusses how soil health tests differ from measuring soil fertility, sampling and handling procedures and how to interpret your results.

Presenter: Stacy Zuber, Ph.D., State Soil Health Specialist with the USDA Natural Resources Conservation Service

Download Zuber's PowerPoint presentation.

• Soil health is the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans
• Soil health has many different functions, and soil health tests provide relative comparisons for biological, physical and chemical functions
• Soil health indicator effectiveness determined by:
  • Sensitivity to management
  • Short-term sensitivity
    • Within 1 to 3 years for significant management changes
  • Interpretable/related to soil function or process
  • Usefulness
    • Provides useful information towards assessing soil health status and addressing specific resource concerns
  • Production readiness
    • Ease of use
    • Cost effectiveness
  • Repeatability
  • Interpretable for agricultural management decisions
• Soil organic matter/carbon is a key indicator because of the relationship to almost every soil function
  • Soil organic matter (SOM) – 58% carbon
  • May take 3 to 5 years to show appreciable change
  • Could take longer in soils with higher inherent SOM
• Important to have aggregate stability in soil, which is strongly related to water infiltration and ability to resist erosion
• Readily available carbon food source
  • Microbial food source: Decomposing organic matter 33% to 50%
• Active carbon
  • Permanganate oxidizable carbon (POXC) is a useful indicator of
    long-term carbon sequestration
  • More sensitive to changes in crop and soil management than SOC
• Water extractable organic carbon
  • Cold-water extractant used in Haney Test
  • Rationale is that C that can dissolve in water is most available to microbes
• Microbial activity
  • Soil respiration
    • Rewet dried soil then incubate to measure carbon dioxide release over certain 24 or 96 hours
    • Indicates how much the microbes are eating and how active they are
• Bioavailable nitrogen
  • Acid citrate extractable (ACE) soil protein
    • Majority of N in soil organic matter is in proteins
    • Indicates amount of organic N cycling through microbial biomass and may be released in plant-available forms
  • Water extractable organic N
    • Part of Haney Test
• Microbial and functional diversity
  • Enzyme activities reflect potential for microbes to convert and cycle nutrients
    • Enzymes are the metabolic protein keys for breaking up larger molecules
      • Cellulose, DNA, lipids, chitin, lignin, proteins
  • Phospholipid fatty acids (PLFA)
    • Biochemical marker in cell membranes unique to broad classifications in soil biology
    • Indicates relative abundance of microbial groups and total microbial biomass
• Soil health test packages are becoming more widely available from commercial soil testing labs
• Haney Test is available from many commercial labs
  • Soil health measurements
    • Water extractable organic carbon and nitrogen
    • 24-hour soil respiration/carbon dioxide burst
• Soil health tests have few or no thresholds; must have results from another field for a relative comparison
  • Almost all tests are more is better, but limited by soil’s potential for improvement
• How to start soil health testing
  • Decide on sampling strategy – comparison or monitoring
    • Comparison
      • Make sure soil types and landscape position are similar
    • Monitoring
      • Sample every 3 to 5 years and consider evaluating different zones or taking a reference sample when taking your baseline
  • May only want to sample 1 to 3 locations per field to monitor
    • Soil health tests ($50 to $120) are more expensive than fertility tests (<$10) per sample
  • Slice method is recommended, especially for aggregate stability, or soil probe with 1-inch or greater diameter
    • For both, select 15 to 20 soil cores, record GPS coordinates of sample locations and collect enough soil
      • Often 3 to 4 cups, but depends on analyses and lab
• Most soil health indicators closely relate soil biology and have a high amount of spatial variability
  • Soil microbes tend to be concentrated in hotspots and are very sensitive to soil environmental conditions
• Be CONSISTENT with sampling procedures and lab methods
• Most commercial lab tests use air-dried samples, but soil biological measurements may require next-day shipping or shipping on ice
• Reminders
  • Follow these recommendations to minimize variability and maximize usefulness or tests
  • These tests are more sensitive, but small changes may still not be detectable
  • Tests are expensive, make sure you take the time to do them right