Sodium ( Na+ ) is very abundant on Earth and occurs in crystalline and sedimentary rocks ( sands, clays, evaporites ).  Halite, or rock-salt, is common salt ( NaCl ), highly soluble in water.

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In contrast, sodium does not normally occur in carbonated rocks.

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Clays can be saturated in Na+ by the process of base exchange.

 

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Potassium ( K+ ) is quite abundant on Earth but not in water.

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It is easily absorbed and recombined in the soil ( particularly in clays ).

 

Its origin is crystalline rocks ( but in less susceptible to weathering than those that contain sodium), evaporites ( sylvinite ) and clays.

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• AEP standards for sodium and potassium are 150 mg/l Na and 12 mg/l K.

• WHO recommends < 200 mg/l Na.