Our list of elements important to organic life will end with three similar ones sodium, potassium, and calcium. These are light, metallic substances which burn when put in water and are therefore very dangerous to handle. Potassium compounds must be in the soil if plants are to thrive, while sodium and calcium compounds are necessary for the blood and skeleton of animals.
Nitrogen, sulphur, phosphorus, iron, sodium, potassium, and calcium are all obtained from their mineral compounds in the soil; animals use salt (a sodium compound) directly, while they get the other elements from plant foods. Plants in turn obtain them from the soil.
By themselves, all these elements are inorganic substances, but in the wonderful process of assimilation, plants and animals can combine them to form the living stuff of which their tissues are made. On the other hand, by the processes of oxidation, death, and decay, the complex organic compounds are broken up into simpler forms, and return to the soil or air as inorganic compounds or elements, to be used over again by organic things.
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