Most studies of plant nutrition and metabolism pertain to the following
questions. What do plants need for their best growth? How do they take
in the materials they need? What things are absorbed by roots and what
things by foliage? How does the plant turn water and other simple
compounds into carbohydrates and proteins?
Specific problems that atomic energy has helped to solve are listed.
What Happens to Fertilizer in the Soil?
Early research indicated that only 10 to 12 per cent of phosphorus
fertilizers was taken up by plants in the first year; the rest was
“locked into” the soil or washed away. With radioactive phosphorus-32
scientists found that as much as 50 to 70 per cent of the phosphorus in
a plant came from the fertilizer during the first two or three weeks of
growth.
Do Plants Absorb Through Roots Only?
Fertilizer applied to soil is largely wasted because it is either bound
by soil particles or is washed out of the root zone. If chemical
elements could go directly into leaves and bypass the wastefulness of
soils, a tremendous saving would result.
Botanists have learned in recent years that the foliage of plants can
take in some nutrients much as roots can. With tracers they discovered
that many nutrients are readily taken up by foliage, including bark of
dormant trees, even at temperatures below freezing. As shown by isotopic
tracers, elements such as phosphorus, nitrogen, and potassium move both
up and down from the point of application at rates similar to those
following root absorption. Urea (a nitrogen compound) is now used as a
nutrient foliar spray for many fruit and vegetable crops in this
country.
Where Should Fertilizer Be Placed?
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:
[Illustration: (A) Fertilizer mixed throughout the soil gives the least
benefit to seedlings.]
[Illustration: (B) If placed in a band below and beside the seeds, the
fertilizer gives high uptake and good root distribution.]
[Illustration: (C) If the fertilizer is placed directly beneath the
seeds, highest uptake occurs but roots tend to “bunch”—a handicap to
later growth.]