Hi Tom,
I agree with George. It's much too simplistic to assume that a K deficiency can be detected by any one symptom, even those listed in the various deficiency charts. Unlike Carbon and the other macronutrients, K is not integrated into plants' cellular structure which would cause an obvious structural failure. There are few, if any direct K related algal dependencies. The role of K is to act as a catalyst in various reactions. K is very mobile but
mostly resides as the cation K+ in the cytasol, which is liquid enclosed by the cell membrane. K+ does not bind itself to organic molecules such as sugars but instead is more of an electrolyte similar to the liquid in a car battery, generating voltages across the cell membrane and thus attracting or repelling other ions into and out of the cell.
The efficiency with which plants execute their various functions is related to K+ levels. For example, K plays a role in the transfer of nitrate from the roots to the shoots and leaves. Without adequate K, nitrate accumulates in the roots and a feedback mechanism to the root cells stops further nitrate uptake. This is especially relevant to hobbyists who use predominantly root based dosing such as in the ADA scheme. So one may observe the yellowing associated with N deficiency in such a case.
Energy conversion, carbohydrate formation/translocation, N metabolism and other metabolic processes in plants are controlled by enzyme systems, at least 50 of which are activated by K. K activates the movement of water and solutes, it regulates the stomata opening and thus, intake of carbon dioxide, the plants' carbon source and it affects the pH of plant cells by neutralizing the excess of negative charges on proteins and nucleic acids.
So as a result of the indirect effects, it's very difficult to tell whether one has a K deficiency or whether a a direct deficiency of the other macronutrients is present. This is the primary reason for the "add a bit more of everything" EI philosophy which tends to drive the lean dosers mad.
Cheers,