(C) If the fertilizer is placed directly beneath the seeds, highest uptake occurs but roots tend to “bunch”—a handicap to later growth.
Fig. 2—Radioactive plant nutrients injected in soil roots
have not reached above ground parts in 5 minutes
but have, as indicated by Geiger counter, reached these parts in 20 minutes.
Tracer experiments reveal that roots cannot distinguish potassium (needed in large amounts) from other elements which are chemically similar but quite different in size. Once inside the plant, only potassium can be metabolized and similar but heavier elements (rubidium, cesium) are useless. This is like an absentminded builder who buys brick, boulders, and gravel indiscriminately for his wall and then finds he can use only part of his materials.
The process called photosynthesis whereby green plants use energy from the sun to convert simple compounds from air and soil into complex, energy-rich substances has been termed the most important chemical reaction in the world. It is the basis for man’s entire food supply and, except for nuclear energy, all significant fuel as well. Tracer techniques have multiplied the research efforts on photosynthesis tremendously.