There is a place in North America called the valley of death. Long ago, it was a good place with good weather and fertile land, which attracted countless European immigrants to move to this valley. From then on, there was a noisy voice in the silent valley. Where people cultivate wasteland and grow crops, they can get a good harvest every year. But it didn't last long. A few years later, a terrible disaster came to the valley. All kinds of strange diseases suddenly prevailed among the residents in the valley. Some were blind, some lost their hair, and more people died inexplicably. No one knows what the reason is. It is like an invisible plague. All humans and even wild animals entering the valley cannot be spared. From then on, the formerly vibrant Valley returned to its former desolation and loneliness.
With the passage of time, the valley of death gradually faded from people's memory. It was not until geologists came here to explore in the 1950s that they solved the secret of the valley of death. Originally, the stratum and soil in the valley contain a lot of selenium, while the sulfur required for plant growth is very rare. The plants in the valley cannot obtain enough sulfur. In order to maintain normal growth, they can only absorb selenium with properties close to sulfur. Therefore, the plants in the valley contain high concentrations of selenium. For humans and animals, only a small amount of selenium is needed in the body. If it exceeds a certain amount, it will cause poisoning. Of course, the residents in the valley do not know the mystery. Eating the grain and vegetables cultivated in the valley every day is equivalent to eating far more selenium than the normal demand every day. Over time, selenium accumulates more and more in the residents' bodies, eventually leading to various strange diseases and deaths.
The mystery of the valley of death has been solved, which provides scientists with an important enlightenment. Since plants can absorb selenium, can plants be used to "mine" selenium ore? Therefore, people established a unique mine in this valley area. They did not need to dig mines or mining machinery, but planted milkvetch on a large scale in the mining area. Milkvetch is a leguminous plant. Its ability to absorb selenium is much higher than that of other plants. In this way, when the milk vetch is about to blossom and bear fruit every year, that is, when the output is the highest, people begin to harvest, dry and roast, and finally extract selenium from the ashes roasted by the milk vetch. With the "plant miner" of milk vetch, it not only greatly reduces the mining cost and will not pollute the environment, but also has high efficiency. Each hectare of milk vetch can extract 2500 grams of selenium.