pH difference in water vs. CaCl2

Mitchell, see similar posts below IF the difference between pH in water and pH in CaCl₂ is small, then the exchangeable acidity is small (i.e. buffer capacity is low). If the pH difference between pH in water and pH in CaCl₂ is large (i.e. around 1 pH unit) then the exchangeable acidity is large. Buffer capacity is measured a different way, but the difference between pH in water and CaCl₂ gives you an indication of buffer capacity.

You are correct that a high buffer capacity (ability of the soil to resist a change in pH) is desirable for soil organisms (i.e. they have a preferred pH range). And you are correct that a high buffer capacity can make the manipulation of soil pH challenging (i.e. a significant amount of lime may need to be added before a change in pH occurs).

1. Exchangeable acidity is measured by the pH in CaCl₂
2. The difference between pH in water and pH in CaCl₂ gives you an indication of the buffering capacity
3. A smaller difference between pH in water and pH in CaCl₂ indicates low exchangeable acidity
4. A difference of 0.5 to 1 pH units is significant

Consider the factors which contribute to the buffer capacity of a soil (clay and SOM)