107

Stories About Chemistry

INDEX

104. What is Phosphorus For?

Justus Liebig held that a plant could absorb atmospheric nitrogen, and that the soil had to be fertilized only with potassium and phosphorus. But he was out of luck with these elements. His �patent fertilizer� which an English firm undertook to produce failed to boost harvests. Only many years later did Liebig understand and appreciate his error. He had used insoluble phosphate salts out of fear that the soluble ones would soon be washed out of the soil by rain. But it turned out that plants could not assimilate phosphorus from insoluble phosphates. And man was obliged to provide a sort of �semi-finished product� for the plants.

Each year the harvests of the world carry off about 10 million tons of phosphoric acid from the fields. What do plants need phosphorus for? It is a constituent of neither fats nor hydrocarbons, nor do most protein molecules, especially the simplest ones, contain phosphorus. Still, without phosphorus none of these compounds would form.

Photosynthesis is not simply a process of hydrocarbon synthesis from carbon dioxide and water, which the plant can accomplish �with its little finger� it is a very complex process.

Photosynthesis takes place in what is known as the chloroplasts of the plant cells, these being special �organs� for the purpose. Chloroplasts contain a great deal of phosphorus compounds. Crudely chloroplasts can be compared to the stomach of an animal in which the food is digested and assimilated, because it is they that deal directly with the �building bricks� of the plant, namely, carbon dioxide and water.

The plant absorbs carbon dioxide from the air with the aid of phosphorus compounds. The inorganic phosphates transform the carbon dioxide into carbonate ions from which the complex organic molecules are subsequently built.

Of course, this does not exhaust the role of phosphorus in the vital activities of plants. Nor can it be said as yet that its importance for plants is wholly understood. However, even what is known already shows that the part it plays is very important.