Potassium is vital for stem strength, root growth, and disease resistance. Many soils are naturally high in potassium, and it is readily available to plants. However, sandy and highly weathered soils can be deficient. Signs are irregular yellowing of lower leaves and poor root growth. Muriate of potash, greensand, and wood ash are good sources.
Calcium is important for cell-wall integrity and root and leaf growth.
If you're liming your soil regularly to keep the pH above 6, calcium deficiency would be unlikely. Low levels of calcium show as deformed new leaves and branches, and weak stems and roots. Magnesium is essential for chlorophyll and green leaf development. Pale leaves with green veins are a sign of deficiency. CEC measures the ability of soil particles to hold and release specific nutrients.