The Hidden Variable in Your Soil Test: Drying Temperature and pH
Soil pH influences nearly every chemical and biological process in your soil, yet it’s surprisingly sensitive to drying. The changes aren’t always intuitive, and they vary by soil type.
Soil pH: The Master Variable Gets Complicated
Key observations from the data:
- Acidic soils (like Caribou fine-loamy at pH 4.9): Show slight acidification with drying
- Field moist = 4.90 → Air-dried = 4.76
- Neutral to alkaline soils (like Delco loam at pH 7.7): Show remarkable stability
- Minimal change across all drying temperatures
- 40-45°C (7.6) actually shows a slight decrease compared to other temperatures
The Science Behind pH Changes
Research by Dowding et al. (2005) revealed that during drying, soil particle surfaces can become extremely acidic, dropping to pH values below 2 in some cases. This surface acidification drives many of the chemical changes we observe in other nutrients, particularly the increased solubility of metals like iron and manganese.
However, when we measure bulk soil pH, these extreme surface effects are diluted. The net result depends on complex interactions between:
- Organic matter decomposition
- Salt concentration effects
- CO₂ loss
- Surface chemistry changes
Practical takeaway: For most soils, pH changes at 40-45°C are minor (typically <0.2 units) and within the range of normal field variation.
Practical Interpretation Guidelines
- Small changes (<0.2 units) are within normal variation
- Lime recommendations: Usually not significantly affected
- Precision agriculture: May need to account for drying effects in zones
Soil pH is more sensitive to drying than many realize. So, we need to consider the effects of drying temperature and pH. Depending on soil type, drying can cause pH to shift in unexpected ways. These shifts are influenced by factors such as organic matter decomposition, salt concentration, CO₂ loss, and surface chemistry. Fortunately, most pH changes from drying at 40–45°C are minor—typically under 0.2 units—and usually don’t impact lime recommendations.
Full list of references available upon request.