Excess potassium (K) is also a farmer's problem and is associated with manure applications. Most of the potassium in the feed (hay & silage) is returned to the fields in the manure (particularly urine or mixed manure). Excess K can also interfere with the absorbsion of other nutrients and micro-nutrients even though they may be present in the soil. The resulting problems seem to be one of balance (particularly Ca:K) rather than simply excess, and is accordingly treated by adding the deficient components. Since there is no practical way of adding elementals (single elements) to the soil, various compounds are used such as calcined magnesia (MgO), gypsum (CaSO4·2H2O), superphosphate (Ca(H2PO4)2 + 2CaSO4) and no doubt many others. MgO is typically used in pasture treatments as one of the effects of excess K is a reduction of Mg uptake leading to grass tetany in livestock, and is probably not relavent to your situation. As you can see from the formulas, gypsum will also add sulphur and superphosphate will add both sulphur and phosphorus in addition to the desired calcium. Gypsum is also a useful structural ammendment for some clay soils.
You will need some good specific advice; I suggest a commercial nursery supplier like http://www.growercentral.com/
who have an outlet in your area.