The next compounds we shall take up are made of the elements mentioned last in our list: sulphur, phosphorus, iron, potassium, sodium, and calcium.
Calcium unites with sulphur and oxygen to form calcium sulphate, and with phosphorus and oxygen to form calcium phosphate. Sodium and potassium unite with oxygen and nitrogen to form sodium or potassium nitrates and so on with many other compounds.
Fortunately we do not have to learn to test for these separately. When found in organic tissue, they are usually grouped together and called "mineral matter " or " mineral salts," and the fact that they remain as ash, when organic matter is completely burned, is a sufficient test for these compounds at present.
Notice that all the elements except carbon and hydrogen may exist, combined as mineral compounds, in the soil where the plants can get them. Hydrogen is obtained from soil water and carbon from the carbon dioxide of the air. All the compounds mentioned so far, water, carbon dioxide, and numerous mineral salts, are inorganic substances.
One of the most important ways in which plants differ from animals is that they can use inorganic substances solely for food and recombine them into organic compounds, a thing which no animal can do. Nor can we imitate it in any laboratory experiment.
Though animals use water and some mineral salts, they depend for their life on the organic compounds made by the plants. Flesh-eating animals live on other animals, which in turn use plant food. The fact that plants can use inorganic food, while animals depend on plants for their inorganic nourishment, is one of the most important facts for us to remember.
Of course the plant forms these organic compounds for its own growth and food, to be stored away by the plant and used when necessary. Whenever we eat a loaf of bread or a piece of candy we are using material the wheat plant or sugar cane had assimilated and would have used as food for itself.
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