Dorothy Vaught, age 11, of Newport Beach, California for her question:

Can plants really live on air and water?

We are told that the green plant world uses sunlight to manufacture its basic food from air and ground water. This is true. But it is not the whole story. Far from it. Plants, like all living things are com¬plex chemical factors. In order to thrive and multiply, they need hundreds of different chemicals to perform hundreds of chemical processes.

In our wondrous world of nature, plenty of miracles go on behind the scenes. The growth of a green plant is a multi miracle, involving a whole multitude of chemical activities. Some of them we understand, most of them are still mysteries. But we do know that a plant needs much more than air and water.

Perhaps its greatest miracle is photosynthesis, the sunshine recipe. During the daylight hours, small bodies called chloroplasts use the energy from sunlight to synthesize, or put together, molecules of simple sugar    which is a basic plant food. The ingredients in this complicated recipe are carbon dioxide taken from the air and ground water absorbed through the roots.

But obviously plants are not made entirely of sugar. They contain oils and starches, assorted proteins and various vitamins in a multitude of different forms. Numerous chemical reactions are involved in building and rebuilding these many molecules. Obviously they need more than the atoms of hydrogen and carbon in sugar molecules. And the energy for this teeming activity is supplied by oxygen, absorbed from the air.

So, in addition to air and water, plants need a complex balanced diet of other chemical ingredients. These are dissolved in the soil and absorbed by the roots in liquid form. For example, fields of corn use up tons of these liquid plant foods as they grow. When the corn is harvested, the soil is left poorer. Fertilizers containing the balanced diet must be added to provide food for next year's crop.

The liquid diet varies from plant to plant. Some need more of this and less of that and some need a small trace of something extra. Nobody knows all the secret preferences of the plant world. But we do know that all plants need large amounts of six chemical elements    calcium and magnesium, potassium and nitrogen, sulphur and phosphorus. They also need smaller helpings of many other elements. This list includes traces of boron, iron and manganese, chlorine and copper and others.

These ingredients are chemical elements, however most of them must be served as chemical compounds. For example, plants must have nitrates because they cannot use nitrogen. Their sulphur and magnesium may be served in the form of magnesium sulphate and their iron as ferrous sulphate.

Nature provides the essential plant foods by recycling. Waste organic materials are decomposed and their chemicals returned to the soil to feed new plants. Man made fertilizers try to include the essentials. But plants get a more complete balanced diet from recycled wastes, composted from all sorts of organic material. These recycled fertilizers contain all the known and also the unknown ingredients.