Fertilizers can contain elevated levels of the uranium series, the thorium series and potassium-40. One factor that impacts the amount of radioactivity in the fertilizer is its nitrogen, phosphorous and potassium content. This is usually specified as three sequential numbers representing the %N, %P, %K, e.g., 20-27-5 as in the case of the starter fertilizer shown here.
Nitrogen (20% in this example)
The nitrogen component, derived from an ammonia-related product, has no impact of the radioactivity of the fertilizer.
Phosphorous (27% in this example)
The phosphorous component of fertilizers is derived from phosphate rock. Since the latter, depending on its source, can be associated with uranium, and to a lesser extent thorium deposits, the fertilizer’s phosphorous content can impact the levels of the various members of the uranium series.
In most situations, the radionuclides of concern in fertilizers are the various members of the uranium series. Hence the interest in the source of phosphate.
The measurements provided in NCRP Report No.95 indicate that the raw phosphate materials (ammonium phosphate, superphosphate, diammonium phosphate, phosphoric acid, etc.) incorporated into fertilizer contain:
U-238: 22 – 140 pCi/g (800 - 5,200 mBq/g)
Th-230: 5.4 - 430 pCi/g (200 - 16,000 mBq/g)
Ra-226: 0.7 - 24 pCi/g (25 – 900 mBq/g).
Th-232: 0.14 to 4.6 pCi/g (5 to 170 mBq/g) of Th-232.
Note that the uranium series is more or less in equilibrium down to Th-230, but radium-226 and its decay products are present at much lower levels. This is due to the chemical processing of the material.
In the production of lawn and garden fertilizers, these raw materials are blended with low phosphate materials. As a result of this blending, the concentrations of U-238, Th-230, Ra-226 and Th-232 in the final product are 10 to 50% of the aforementioned concentrations in the raw material.
Potassium (5% in this example)
The potassium component is usually derived from potash. The source of the potassium is irrelevant. The higher the level of potassium in the fertilizer, the higher the concentration of potassium-40. Slightly elevated levels of rubidium-87, a pure beta emitter, have also been reported in potash – rubidium and potassium have similar chemistries.
Fertilizer Use in the U.S.
Millions of tons of agricultural fertilizer are used in the United States each year (e.g., 46 million in 1974). Guimond (1978) calculated that such use effectively represented the transfer of 1000 curies per year of U-238 and Th-230 from ore bodies to the “surface plow layer.” It has been speculated that the long-term application of fertilizer can result in a uranium concentration that is several times higher (e.g., 2-3 times) higher than normal. For some crops such as potatoes, tomatoes and tobacco, as much as 200 pounds of phosphate are applied per acre each year.
It has also been reported that the surface runoff has increased the uranium concentrations in river water (Spalding and Sachett).
There are two pathways by which fertilizers can contribute to the radiation exposure of the public: the external exposures to the gamma rays emitted by the fertilizer, and the internal exposures due to the consumption of food grown on land that had been fertilized. Since the levels of potassium in the body are under tight metabolic control, the amount of K-40 in food does not affect an individual's dose.
The exposure rates to the general public as a result of gamma ray emissions are extremely low. Nevertheless, it is sometimes possible to detect an increase in the exposure rate near such fertilizers using a simple survey meter (e.g., the radioactivity in the example shown here is easily detected - the high phosphorous content means that the uranium levels are also high). Of more concern is the potential doses to warehouse workers and those involved in the processing of the phosphate raw materials. The EPA (Windham et al) has estimated that the gamma ray exposures to workers in the phosphate industry might range from 30 to 300 mrem per year, and that the inhalation of dust might result in a dose equivalent to the lungs as high as 5 rem per year.
How the use of uranium containing fertilizers affects
the radioactivity of food crops is unclear. Nevertheless, NCRP Report 95
estimated that the resulting exposure to an average member of the
Eisenbud, M., Gesell, T. Environmental Radioactivity. Academic Press. 4th edition; 1997.
Guimond, R.J. The Radiological Aspects of Fertilizer Utilization. in NUREG/CP-0001. Radioactivity in Consumer Products: 380-393; 1978.
NCRP. Radiation Exposure of the
Spalding, R.F., Sachett, W.M. Uranium in Runoff from
the Gulf of
Copyright 1999, Oak Ridge Associated Universities