This calculation provides some indication of the sign of the electrical charge currently dominant in the soil and applies particularly to acidic soils with variable-charge characteristics (Uehara and Gillman 1981).
Δ pH = [pHKCl – pHw]
where pHKCl refers to Method 4C1, 4C2, 4C3 or 4C4 and pHw refers to Method 4A1
Report Δ pH, including the –ive or +ive sign.
If Δ pH is positive, the soil pH is probably less than the point of zero charge (pH0) and the soil carries a net positive surface charge. When Δ pH is negative, the soil is above pH0 and carries a net negative surface charge. Dominance of a soil by variable-charge colloids can generally be assumed when Δ pH is a small negative (less than –0.5), zero, or positive value.
When Δ pH has a large negative value, nothing can be deduced about whether the surface charge is permanent or variable. Acid soils with Δ pH values near zero or positive can be expected to contain low levels of extractable Al (Uehara and Gillman 1980) but are likely to be calcium deficient.