Symptoms of Potassium Deficiency. Reduced Growth. If you see one plant or group of plants in your garden that are smaller than the others, then a potassium deficiency may be to blame. Yellowing of Old Leaves. Weak Roots. Small Stems. Defoliation. Visual Symptoms: Check the leaves for symptoms of potassium deficiency, such as yellowing, leaf curling, and necrosis. Timing: Potassium deficiency typically occurs later in the season when the plant's demand for potassium is highest. Fixing Potassium Deficiency sTPn92k.