The Hidden Variable in Your Soil Test: Drying Temperature Effects on Micronutrients

While micronutrients might seem secondary to NPK, they’re crucial for diagnosing deficiencies, understanding soil chemistry, and making precise management decisions. The effects of drying on these parameters are often more dramatic than what we see with organic carbon, making it essential to understand what’s happening to your samples.

Micronutrients: The Dramatic Responders

While primary nutrients showed moderate responses to drying, micronutrients often exhibit dramatic changes. Understanding these shifts is crucial for accurate deficiency diagnosis.

Manganese: The Most Sensitive Indicator

Manganese shows perhaps the most dramatic response to drying of any nutrient:

The numbers tell a striking story:

• Portneuf silt loam (DTPA extraction):
  • Air-dried = 4.2 mg/kg → 40-45°C = 6.1 mg/kg → 105°C = 15.2 mg/kg
  • Nearly 4x increase from air-dried to high-temperature drying
• Even gentle drying matters: The 45% increase from air-drying to 40-45°C could be the difference between diagnosing deficiency or sufficiency

Why this happens: Manganese exists in various oxidation states in soil. Drying promotes the reduction of manganese oxides, converting them to more soluble Mn²⁺ forms. The process accelerates with temperature.

Iron: Following Manganese’s Lead

Iron behaves similarly to manganese, though usually with less dramatic changes:

Notable findings:

• DTPA-extractable Fe shows moderate increases with gentle drying
• Total extractable Fe (HCl method) shows less sensitivity to drying temperature
• The effect varies significantly by soil type and extraction method

Zinc and Copper: The Modest Responders

These micronutrients generally show smaller changes than iron and manganese:

Key patterns:

• Zinc: Generally increases with drying, but changes are often within analytical error at 40-45°C
• Copper: Shows the least sensitivity to drying among micronutrients
• Both nutrients: Effects are most pronounced at temperatures above 60°C

Secondary Nutrients: Calcium, Magnesium, and Sulfur

The secondary nutrients show varied responses to drying, largely dependent on soil type:

Calcium and Magnesium

Observations:

• Mehlich-3 extraction: Generally shows good stability for Ca and Mg at 40-45°C
• Ammonium acetate extraction: May show slight increases with drying
• Calcareous soils: More stable than acidic soils

Sulfur: The Overlooked Variable

While not shown in all studies, sulfate-sulfur can increase significantly with drying due to:

• Mineralization of organic sulfur compounds
• Oxidation of reduced sulfur forms
• Release from soil aggregates

Putting It All Together: A Practical Framework

After examining all these parameters, clear patterns emerge:

Minimal Impact at 40-45°C:

• Calcium and magnesium
• Copper

Moderate Impact at 40-45°C:

• Iron (10-30% increase typical)
• Zinc (5-20% increase typical)
• Calculated values (CEC, base saturation)

Significant Impact at 40-45°C:

• Manganese (30-50% increase common)
• Sulfur (when measured)
• Phosphorus (as discussed in Part 1)

Practical Interpretation Guidelines

When reviewing your soil test results, consider these factors:

Micronutrients:

1. Deficiency diagnosis: Be cautious with marginal values, especially for Mn and Fe
2. Sufficiency levels: May need adjustment based on drying method
3. Trend monitoring: Consistent methodology allows valid comparisons over time
4. Field verification: Tissue testing can help confirm suspected deficiencies

Calculated Values:

1. CEC: Focus on relative differences between soils
2. Base saturation: Useful for general guidance but not precision management
3. Ratios: Can be more affected than individual values

The Complete Picture: Why Standardization Matters

Across Parts 1 and 2 of this series, we’ve seen that soil drying affects nearly every parameter we measure, but not equally. The key insights:

1. Organic carbon is remarkably stable at 40-45°C for most soils
2. Primary nutrients show predictable patterns that can be understood and managed
3. Micronutrients are more sensitive, particularly manganese and iron
4. pH changes are usually minor at moderate drying temperatures

The beauty of standardized drying at 40-45°C is not that it eliminates all effects—it doesn’t. Instead, it provides:

• Consistency across time and laboratories
• Predictability in how values change
• Practicality for high-throughput testing
• Reliability for the most critical measurements

Moving Forward: Using This Knowledge

Understanding these drying effects empowers you to:

1. Better interpret your soil tests: Know which values might be elevated or suppressed
2. Make more informed decisions: Adjust interpretations based on parameter sensitivity
3. Track trends confidently: Consistent methodology enables valid comparisons
4. Communicate with your lab: Ask about their drying protocols and QA/QC procedures

The Bottom Line

Perfect accuracy in soil testing is an impossible goal, soil is too complex, too variable, and too alive. What we can achieve is something arguably more valuable: consistent, standardized measurements that allow us to monitor soil health, diagnose problems, and track our progress over time.

The 40-45°C drying protocol represents a scientifically sound compromise that preserves the integrity of our most important measurements while enabling practical, timely analysis. By understanding how this “hidden variable” affects our results, we can use soil testing more effectively as a tool for building soil health and farming success.

Remember: the goal isn’t perfect measurements, it’s actionable information. When we all use the same ruler, even if it’s slightly bent, we can still build something remarkable together.

Full list of references available upon request.