pH difference in water vs. CaCl2

We went over this in our review session with Sandra but I found myself a little confused. It's regarding the soil order tables for Question 6 on Discussion 3, or Question 5 on the 2014 final exam. Here's what I've written in my notes, but I'm not sure if it's correct...

Why do we care about difference in pH when measured in H2O and CaCl2? If the difference is small, exchangeable acidity and thus buffering capacity will be high. High buffering capacity is good for soil organisms, but from a management perspective means we need to add more lime to achieve desired changes.

1. Do buffering capacity and exchangeable acidity depend on the difference, or do they just depend on the value of the pH in CaCl2, regardless of the pH value in H2O? I'm assuming it's the difference, but Sandra initially said "if the pH is large in CaCl2", which is a little unclear.
2. Does a smaller difference between pH in H2O and in CaCl2 indicate a higher or a lower buffering capacity?
3. Does a smaller difference between pH in H2O and in CaCl2 indicate a higher or a lower exchangeable acidity?
4. How much of a difference does there need to be for the buffering capacity to see a significant difference?

Thanks!

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)