text: The Chernobyl accident released a large amount of caesium isotopes which were dispersed over a wide area. Cs is an isotope which is of long-term concern as it remains in the top layers of soil.  Plants with shallow root systems tend to absorb it for many years. Hence grass and mushrooms can carry a considerable amount of Cs, which can be transferred to humans through the food chain.
  One of the best countermeasures in dairy farming against Cs is to mix up the soil by deeply ploughing the soil. This has the effect of putting the Cs out of reach of the shallow roots of the grass, hence the level of radioactivity in the grass will be lowered. Also the removal of top few centimeters of soil and its burial in a shallow trench will reduce the dose to humans and animals as the gamma rays from Cs will be attenuated by their passage through the soil.  The deeper and more remote the trench is, the better the degree of protection.
  Fertilizers containing potassium can be used to dilute cesium and limit its uptake by plants.
  In livestock farming, another countermeasure against Cs is to feed to animals prussian blue. This compound acts as an ion-exchanger. The cyanide is so tightly bonded to the iron that it is safe for a human to consume several grams of prussian blue per day. The prussian blue reduces the biological half-life (different from the nuclear half-life) of the caesium. The physical or nuclear half-life of Cs is about 30 years. Caesium in humans normally has a biological half-life of between one and four months. An added advantage of the prussian blue is that the caesium which is stripped from the animal in the droppings is in a form which is not available to plants. Hence it prevents the caesium from being recycled. The form of prussian blue required for the treatment of animals, including humans is a special grade.  Attempts to use the pigment grade used in paints have not been successful.