Nitrogen is considered the plant nutrient most widely deficient in the world's soils. Various agricultural practices have therefore been developed to increase its concentration in the soil. These practices include incorporating legume varieties in pasture and applying various nitrogen-rich fertilisers (urea, sulfate of ammonia, blood and bone) to crops. Such practices sometimes cause plants grown in these soils to have nitrate levels above safe limits, resulting in livestock poisonings. Certain weeds, various root crops, cereal hays, and even immature cereal crops can also cause these poisonings. More than 80 specific plants are known to cause nitrate/nitrite poisoning.