Potassium (K) is measured as potash (K2O), similar to the way P is measured as P2O5. Crop need for K varies considerably as can be seen in Table 4. It is important that the soil K plus the applied K is enough to meet crop needs. However, excessive levels should be avoided because K can interfere with the uptake of Ca and Mg (see Cation Exchange Capacity and Base Saturation under Fundamentals of Soil Health and Soil Fertility). K is subject to leaching on sandy soils low in organic matter, so if high amounts are needed, split applications should be used.  Very high application rates of K are also known to suppress Mg uptake, and when soil test Mg levels are low, may cause Mg deficiency.