← Back to Blog

Soil Health Indicators: Beyond NPK Numbers

Soil Health · November 2025 · 5 min read

Standard soil tests measure nutrient levels and pH, but they tell you very little about the biological and physical condition of your soil. True soil health encompasses aggregate stability, water infiltration, microbial activity, and organic matter cycling. Newer tests like the Haney test and PLFA analysis give farmers practical tools to measure what has long been invisible.

Physical Indicators

Aggregate stability measures how well soil clumps hold together when exposed to water. The slake test is a simple in-field assessment: drop a dry soil clump into a jar of water and observe how quickly it falls apart. Stable aggregates indicate good soil structure, active biology, and resistance to erosion and crusting.

Water infiltration rate is another powerful field indicator. Insert a metal ring into the soil surface, pour in a measured amount of water, and time how long it takes to absorb. Healthy soils with good structure and active earthworm channels infiltrate water quickly, reducing runoff and improving drought resilience.

Biological Activity Tests

The CO2 burst test (also called the Solvita test) measures microbial respiration by incubating a dried soil sample and measuring the carbon dioxide released over 24 hours. Higher CO2 levels indicate a larger, more active microbial population that is cycling nutrients and building soil structure. This test is included in the Haney soil health assessment.

• The Haney test combines CO2 burst, water-extractable nutrients, and organic C:N ratio
• It generates a soil health score from 0 to 50+, with higher numbers indicating better health
• PLFA (phospholipid fatty acid) analysis identifies and quantifies microbial community groups
• Track biological indicators over time to measure the impact of management changes

NRCS Soil Health Assessment

The NRCS Soil Health Assessment framework combines physical, chemical, and biological indicators into a comprehensive evaluation. It includes both field tests (infiltration, compaction, aggregate stability) and laboratory analyses. Your local NRCS office can help you conduct an assessment and interpret the results.

The most valuable aspect of soil health testing is tracking change over time. A single test provides a snapshot, but annual or biennial testing on the same fields reveals whether your management practices are improving or degrading soil function. Pair soil health testing with practices like cover cropping, reduced tillage, and diverse rotations to measure real-world results.