Is organic food radioactive?

Of course it is. All living things contain potassium, and the minor isotope 40K has a 1.25 Ga half life. But pedantry is not the purpose of this blog entry. The real question is whether or not some organic food is more radioactive than it ought to be- especially compared to equivalent non-organic crops.
Organic food is defined as food that is grown without the aid of synthetic fertilizers, herbicides, or pesticides (the definition of synthetic is left as an exercise to the pedants). On the coop farm down the road from my folk’s house, this means improving the soil with chicken poop and bringing in school kids to pull weeds in a rousing synergy of outdoor education and child labor. But chicken shit is not the only source of vital nutrients such as K, N, and P.
There are a few evaporite mines where nitrates are mined, but for the most part, nitrogen fertilizer is fixed from air by industrial processes. Potassium and phosphorous, on the other hand, are generally dug up as rocks, and then processed into a bioavailable form. In all these cases, the processing includes some industrial process which excludes the products from being considered organic. Thus the use of chicken shit or other biological fertilizers, which fix K, P, and N through biological pathways.
Trouble is, this isn’t the only organic solution. According to Manning (2008), some organic farms simply use raw phosphorite rock as a natural fertilizer. Since it isn’t processed, it doesn’t qualify as synthetic, which is great for classification but lousy for whoever has to eat the food. Here’s why.
Most phosphate rocks readily concentrate uranium. Igneous phosphates incorporate U (and Th) into their crystal structures as the magma solidifies, while sedimentary phosphates gradually accumulate uranium from groundwater as it flows through the rock unit. While this is handy for geologists who like to find phosphate deposits using their scintillometers, it does mean that phosphate rocks can be significant sources of ionizing radiation. And the number one rule of radiation safety is, “Do not eat or inhale”.
In theory, the processing of phosphate ore into fertilizer products is supposed to remove the uranium. In practice, the last time I analyzed a retail bag of superphosphate, it had about 50 ppm U. And the process of crushing and reacting the rock with acids is going to release the more volatile or soluble parts of the decay chain (e.g. Radon) even if the U removal is ineffective or ignored. So the radioactivity per phosphorous atom in a synthetic phosphate fertilizer is almost certainly going to be lower than that of the natural phosphate rock. Thus, organic farmer who chose to use natural phosphorite rocks as fertilizer are almost certainly blasting their fields with a higher dosage of radiation than farmers who use an equivalent amount of synthetic fertilizer.
Of course, chickens are not radioactive, and neither is their shit. So the organic classification system lumps low dose chicken shit farmers together with the radioactive phosphate rock hotheads. So a person trying to reduce his dosage needs to know how the food was grown; looking at the certification is insufficient.
Classification is no substitute for knowledge.

Manning D.A.C. 2008 Phosphate minerals, environmental pollution, and sustainable agriculture. Elements, 4, 105-108