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The chemical uptake form of potassium is K+. In plants, potassium is primarily absorbed from the soil in its ionic form (K+), which is a monovalent cation. This positively charged ion is crucial for several physiological functions within the plant, including enzyme activation, osmoregulation, and the regulation of stomatal openings.

The other options represent different elements that also play important roles in plant nutrition but are not the correct forms for potassium uptake. For example, Ca2+ refers to calcium ions, which help in cell wall stability and signaling processes. Mg2+ represents magnesium ions, vital for chlorophyll synthesis and as a cofactor for many enzymatic reactions. Na+ stands for sodium ions, which can sometimes play a role in certain plant physiological processes but does not replace the essential functions of potassium. Thus, K+ is uniquely identified as the form in which potassium is taken up by plants.