Even before the use of tracers, agronomists realized the inefficiency of spreading fertilizer uniformly over a seed-bed. They know the fertilizer should be placed somewhere near the seed, but where? Above? Below? Beside? Below and beside? How far away? They had conducted some research, but the methods were slow and tedious.

Using tracers, the researchers confirmed earlier findings that roots within two or three days reached fertilizer placed less than two inches directly below seeds, but the roots tended to congregate there. When the fertilizer was two inches below and two inches to the side, roots reached it within a week and a better root system developed. With three inches between seeds and fertilizer, the desired seedling “boost” was delayed three or four weeks. (See Fig. 1.)

Do Fertilizers Move Fast in Plants?

The movement of radioactive phosphorus from root to leaf was found to be remarkably fast, sometimes requiring less than twenty minutes. (See Fig. 2.)

What Else Do Radioisotopes Tell Us?

Some plants take in chemicals that the plant probably cannot use: for example, the so-called locoweeds accumulate enormous amounts of selenium. With tracer techniques, we can see that the root uptake process has poor powers of discrimination.

Fig. 1—Soil tests tell how much of each fertilizer element is needed but not where to put it to give seedlings the much-needed “push.” With tracers it is found that:

(A) Fertilizer mixed throughout the soil gives the least benefit to seedlings.

(B) If placed in a band below and beside the seeds, the fertilizer gives high uptake and good root distribution.

(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.

When only chemical tests were available, food manufacturing in green leaves had to progress for hours before scientists could measure the products. But with tracers and other new techniques they have narrowed the experimental time to minutes and finally to seconds. Today they know that a green leaf has formed sugars more complex than fructose, “fruit sugar,” after exposure to light for only one second!

When the incredible complexities of photosynthesis are finally unraveled, radioactive tracers, especially radioactive carbon-14, will have provided the significant clues.

Plant Diseases and Weeds

How Can We Combat Plant Diseases?

At one time to stop epidemic spread of plant diseases was virtually impossible; farmers had to abandon fields and crops. Such catastrophes caused by microbes have changed the course of history. For example, the Irish famines of the 1840’s resulted from the potato blight and caused mass emigrations from Ireland.

In this country today plant diseases result in losses estimated at $3 billion a year. So far, the most economical means of reducing the ravages of plant diseases has been to breed resistant plant varieties. Although such a variety may cost $100,000 to develop, its cost is usually repaid within a year or two.

But the victory is only temporary. Although plants are bred to resist the pathogen (fungus) of the moment, Nature is constantly changing the microbial population by mutation and hybridization. Within a few years virulent strains of fungi which can attack the “resistant” variety increase to such an extent that the new variety must be replaced.

For crops that provide high per-acre income such as some vegetables and vine and tree fruits, chemical control of fungous diseases is economically possible; in fact, it is a real necessity. But such treatment is too costly for most field crops, unless some cheap seed treatment or fertilizer additive can be found.

A general breakthrough in control of plant diseases is yet to come. Because of thousands of pathogenic species, with hundreds of strains, it does not seem possible that the following questions could be answered about each one. What is the life cycle of the microbe? What conditions of temperature and humidity encourage it to spread? What plant species does it attack? How does it enter? What chemical changes within the cells of the plant determine whether they resist or succumb to the invader? How long can germs remain potent? How far can they travel by wind or water? What combination of resistant varieties, cultural methods, and chemical treatment will control the disease?

With tracers it is possible for the first time to measure chemical uptake in single spores and to follow chemicals through the plant. Perhaps the most enlightening information from such studies is that some fungicides are 10,000 times less effective per unit of “body weight” than are other chemicals used to destroy weeds and insects. Obviously the breakthrough in chemical control of plant diseases is yet to come.

Why Do Chemicals Destroy Some Plants?

Weeds cost this country an estimated $5 billion annually, which is more than the loss to either plant diseases or insects. Selective chemical weed killers such as “2, 4-D” have become so widely used that more than $135 million worth was sold in the United States in 1959. In proper