> I don't know of a rule of thumb. It depends on how many hydrogen ions the
> calcium ions need to displace (the "buffering capacity" or cation exchange
> capacity). The more clayey a soil is, the higher the CEC.
"New terms now command entry into
the eco-farmer's vocabulary - cation,
anion, exchange capacity, base saturation,
but all ultimately have to do with
the clay of the soil and the electricity of
nutrition, and how nutrients, or the lack
thereof, govern everything from crop
production to weed control."