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Each year, as October comes to a close, I receive calls from producers asking about the best time to analyze soil biology using the Soil Health Assessment (SHA) or Phospholipid Fatty Acid (PLFA) test packages. These tests provide key insights into soil microbial activity and overall soil health:
- SHA measures microbial biomass, water-extractable carbon and nitrogen, plant-available nutrients, and water-stable aggregates percentage.
- PLFA quantifies total microbial biomass and classifies it into four microbial groups by percentage.
The SHA is applicable to all cropping systems, including conventional tillage, no-till, and cover crops. In contrast, the PLFA test is best suited for researchers and producers who have cultivated a well-functioning, healthy soil over time.
Understanding Soil Temperature
A general guideline for biological soil testing is to stop sampling when the average daily soil temperature at a 3–4-inch depth drops below 50-55°F. In Central Nebraska, the historical average date for reaching this threshold is November 1.
For other regions, the Soil Temperature Climatology Tool, developed by the Midwestern Regional Climate Center (MRCC) in collaboration with the USDA Midwest Climate Hub, provides historical soil temperature data for the north-central United States (1991–2020). This tool helps determine when soil temperatures are optimal for sampling.
Additionally, anyone can measure soil temperature manually using a soil thermometer. To do so:
- Insert the thermometer’s four-inch metal probe into the soil until the head touches the surface.
- Wait about five minutes.
- Pull out the probe and read the temperature.
- Take measurements around 10–11 AM for the most accurate reading.
Spring Sampling Considerations
The same temperature guidelines apply to biological soil testing in the spring. In the Corn Belt, a practical reference is you see the corn planters heading to the field. Farmers typically begin planting when the average daily soil temperature reaches 50-52°F.
According to NRCS, the best time to analyze soil biology is when the soil temperature is 65-86°F (the range in which plants thrive). However, testing must be done before planting to inform fertility and management decisions. Therefore, we recommend waiting until soil temperatures reach at least 50°F before sampling.
Soil Temperature’s Effect on Microbial Activity
When soil temperatures drop below 50-55°F, microbial activity slows or ceases:
- Some microbes become dormant.
- Others die off.
- A small number of cold-tolerant microbes remain active but rely on consuming dormant or dead microbes due to the lack of root exudates, carbon-rich solutions secreted by actively growing roots.
Sampling at temperatures below 50°F leads to unreliable results. The PLFA test will show reduced microbial biomass, and the SHA CO₂ 24-hour soil respiration measurement will underestimate biological activity. Since respiration values factor into additional calculations, inaccurate readings distort the overall soil health assessment.
Proper Sampling for Meaningful Results
Biological soil testing is an investment. To ensure accurate, meaningful results that truly reflect soil health, waiting for warm soil conditions is essential. Testing when soil temperatures are above 50°F provides the most precise measurement of microbial biomass, helping producers make informed management decisions.
