Agricultural Uses
In some elements, atoms are not stable, and they
spontaneously emit radiation of various types - a
phenomenon Henri Becquerel discovered in 1896.
Marie Curie used the term 'radioactivity' to describe
the emission of ionizing radiation by some of the
heavier elements; it is measured in Becquerels
(1 Bq = 1 disintegration/sec.)
Radiation causes damage to biological molecules, and
is a health hazard. Radioactive substances emit three
types of radiation: (a) Alpha radiation (a)composed of
helium (He
2+)ions, also called alpha particles, (b) Beta
radiation (β) consisting of electrons/positrons emitted by
a radioactive substance, also called B-particles, and (c)
Gamma radiation (γ) consisting of highly energized,
short wavelength electromagnetic waves resembling x-rays.
The phenomenon of radioactivity is based on the
principle that atoms contain still simpler parts and are not
indestructible particles. Radioactivity can be induced in
many nuclides by bombarding them with neutrons or
other particles. The rates of decay of radioactive
substances are unaffected by chemical changes, pressure,
temperature or electromagnetic fields, and each nuclide
has a decay constant or half-life. A radioactive nuclide
decays to a further radioactive nuclide, followed by a
series of transformations which end with the formation of
a stable, daughter nucleus.
Radionuclides are unstable elements that
disintegrate, emit radiation and form stable elements.
There are two main sources of radioactive nuclides
associated with fertilizers, which are (a) %, a naturally
occurring isotope of potassium, and (b) uranium,
thorium and their daughter elements present as
constituents in phosphate rocks. Potassium contains
0.012% of the radioactive isotope potassium (
40K) that
emits B and y radiations with a half-life of 1.8 x l09 years.
Potassium is an essential element for all living beings; an
adult body contains 0.2% potassium of its body weight.
The amount of potassium in the topsoil varies between
0.2 and 3.3%, giving activity concentrations of about 50
to 1000 Bq/kg soil.
The main concerns related to radioactive nuclides in
fertilizers are: (a) increased exposure of workers to the
radiation, and (b) their accumulation in soil leading to an
increased uptake in food crops.
The radioactivity level in some fertilizers ranges from
less than 1,000 to 10,000 Bq/kg weight of fertilizer,
depending on the source of the phosphate rock and the
content of phosphorus and potassium in the fertilizer.
Their accumulation in the top soil and uptake by plants
depend upon (a) the input from fertilizers, (b) the
duration of the fertilizer application, (c) soil conditions in
terms of the pH, moisture, organic matter, clay content,
etc., and (d) such factors as soil erosion, leaching, etc.
Assuming an application of 30 kg phosphorus/ha
mixed in the top 10 cm and non-removal of uranium from
phosphorus fertilizers, uranium accumulates at about
0.25% of the soil, annually. However, if P is
incorporated into the deeper soil layers or if uranium is
removed at the process site, its accumulation will be low.
Long-term data indicates that the risk of radioactivity
accumulation in soil arising out of P application is low and
does not pose any significant threat to human health.
